Biomedical subjects

R N Lewis

Publications and source records attributed to R N Lewis.

At least 55 records · Page 3Linked to original sources

PubMed1994

Components of the carbonyl stretching band in the infrared spectra of hydrated 1,2-diacylglycerolipid bilayers: a reevaluation.

Previous vibrational spectroscopic studies of solid acyl-alkyl and diacyl phosphatidylcholines suggested that the sn1- and sn2-carbonyl stretching modes of 1,2-diacylglycerolipids have different absorption maxima. To address the assignment of sn1- and sn2-carbonyl stretching modes of hydrated 1,2-diacylglycerolipids, aqueous dispersions of 1-palmitoyl-2-hexadecyl phosphatidylcholine (PHPC), 1-hexadecyl-2-palmitoyl phosphatidylcholine (HPPC), 1,2-dipalmitoylphosphatidylcholine (DPPC), as well as hydrated samples of unlabeled, sn1-13C=O-labeled, sn2-13C=O-labeled, and doubly 13C=O-labeled dimyristoylphosphatidylcholine (DMPC) were examined by Fourier transform infrared spectroscopy. The ester carbonyl stretching (nu C=O) bands of HPPC and PHPC each exhibit maxima near 1726 cm-1 and appear to be a summation of three subcomponents with maxima near 1740 cm-1, 1725 and 1705-1711 cm-1. In contrast, the nu C=O band of DPPC exhibits its maximum near 1733 cm-1 and appears to be a summation of two components centered near 1742 and 1727 cm-1. Thus the ester carbonyl group of the acyl-alkyl PCs appears to reside in a more polar environment than the ester carbonyl groups of their diacyl analogue. This observation implies that the polar/apolar interfaces of hydrated bilayers formed by PHPC and by HPPC are significantly different from that of DPPC and raises the question of whether the acyl-alkyl PCs are suitable models of their diacyl analogue. The absorption maximum of the nu C=O band of the doubly 13C=O-labeled DMPC occurs near 1691 cm-1 and those of its subcomponents occur near 1699 and 1685 cm-1. These frequencies are consistent with a 12C=0/13C0 'isotopic shift' of 42-43cm-1. snl - and snY2-13C0O-labeled DMPC each exhibit well resolved 12C and 13C vc-0 bands with absorption maxima near 1734 and 1692 cm-1, respectively. With both specifically 13C=O-labeled lipids, the 12C and 13C vo bands each seem to be a summation of subcomponents with absorption maxima near 1742 and 1727 cm-1 (12C vc=o) and 1699 and 1685 cm-1 (13C VC_o),regardless of whether the 13C=O-labeled fatty acyl chain is esterified at the snl - or sn2- positions of the glycerol backbone.We conclude that in hydrated 1,2-diacyl PC bilayers, the patterns of infrared absorption exhibited by ester carbonyl groups located at the primary and secondary positions of the glycerol backbone are similar. Also, the resolvable subcomponents of their v0 bands are each a summation of comparable contributions from both ester carbonyl groups and therefore cannot be attributed to the inequivalent locations of the two ester carbonyl groups. This result differs from that of the vibrational spectroscopic studies alluded to above and raises the question of whether data obtained in studies of dry (or poorly hydrated) lipids are applicable to fully hydrated lipid bilayers. To address questions of why the results of the two studies differ, we have also examined the vc=o bands of solid samples of DPPC, HPPC, and PHPC. We find that the vc-0 bands of all solid lipids studied differ from those of the hydrated samples. Moreover, with solid lipids the vc=o bands vary with the enantiomeric configuration,enantiomeric purity and thermal thermal history as well as with the way in which the sample was prepared. Also, although the vc=o bands of solid HPPC and PHPC vary significantly with sample preparation methodology, samples of PHPC and HPPCprepared by the same method exhibit very similar vC-0 absorption bands. We conclude as far as the organization of lipid polar/apolar interfaces is concerned, solid lipids are not good models of hydrated lipid bilayers and suggest that this may be largely responsible for the different conclusions drawn in this work and in previously published studies.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1994

Enigmatic thermotropic phase behavior of highly asymmetric mixed-chain phosphatidylcholines that form mixed-interdigitated gel phases.

Twelve saturated mixed-chain phosphatidylcholines have been identified for which the thermotropic phase behavior observed upon cooling from the L alpha phase is dependent upon the thermal history of the sample in the gel phase. If fully hydrated samples of these lipids are cooled and soon thereafter examined by differential scanning calorimetry, one observes a single highly cooperative endotherm (the chain-melting phase transition) upon heating, and on subsequent cooling, a single exotherm that may occur at temperatures as much as 4-6 degrees C below that of the single endotherm observed upon heating. In contrast, if the samples are incubated in the gel state at low temperatures for prolonged periods of time, one observes a single heating endotherm as before, but two sharp exotherms upon cooling. The latter transitions occur at temperatures close to that of the single endotherm observed upon heating and the single cooling exotherm observed prior to incubation in the gel state. The combined enthalpy of the two cooling exotherms is the same as that of the single heating endotherm or the single cooling exotherm initially observed. Infrared spectroscopic and X-ray diffraction studies indicate that the structural conversions characteristic of liquid-crystalline/gel phase transitions occur at both of those cooling exotherms. Of the 12 lipids that exhibit this unusual behavior, nine fulfill the previously defined structural requirements for the formation of the so-called mixed-interdigitated gel phase, and there is evidence in the literature that one of the three remaining lipids also forms such a structure. Infrared spectroscopic studies of the other two lipids indicate that their gel phases exhibit spectroscopic features that closely resemble those of lipids that meet the previously defined structural criteria for the formation of mixed-interdigitated gel phases and that differ markedly from those of both saturated symmetric-chain and saturated mixed-chain phosphatidylcholines that do not normally form mixed-interdigitated gel phases. Also, electron density reconstructions based on small-angle X-ray diffraction studies of the gel phases of those two lipids indicate that the thickness of their gel phase bilayers is consistent with their forming mixed-interdigitated gel phases. Thus the unusual thermotropic phase behavior described here may be a general characteristic of phosphatidylcholines that form mixed-interdigitated gel phases. This unusual behavior is not associated with any major change in any of several physical properties of these lipid bilayers but may arise from an alteration of the size and/or structure of microdomains present in the liquid-crystalline phase.

Gels↗

PubMed1994

Comparative differential scanning calorimetric and FTIR and 31P-NMR spectroscopic studies of the effects of cholesterol and androstenol on the thermotropic phase behavior and organization of phosphatidylcholine bilayers.

We have investigated the comparative effects of the incorporation of increasing quantities of androstenol and cholesterol on the thermotropic phase behavior of aqueous dispersions of members of a homologous series of linear saturated diacyl PCs1 using high sensitivity DSC. We have also employed FTIR and 31P-NMR spectroscopy to study the comparative effects of androstenol and cholesterol incorporation on the organization of the host PC bilayer in both the gel and liquid-crystalline states. The effects of androstenol and cholesterol incorporation on the thermotropic phase behavior of shorter chain PCs like 14:0 PC are generally similar but not identical. The incorporation of either sterol progressively decreases the temperature and enthalpy, but not the cooperativity, of the pretransition and completely abolishes it at sterol concentrations above 5 mol%. Moreover, at sterol concentrations of 1 to 20-25 mol%, both androstenol and cholesterol incorporation produce DSC endotherms consisting of superimposed sharp and broad components, the former due to the hydrocarbon chain melting of sterol-poor and the latter to the melting of sterol-rich 14:0 PC domains. The temperature and cooperativity of the sharp component are reduced slightly with increasing concentration of androstenol or cholesterol, and the enthalpy of the sharp component decreases progressively and becomes zero at 20-25 mol% sterol. As well, at cholesterol or androstenol concentrations above 20-25 mol%, the enthalpy of the broad component also decreases linearly with increasing sterol incorporation and becomes zero at sterol levels of about 50 mol%. However, whereas cholesterol incorporation progressively increases the temperature of the broad component of the DSC endotherm, androstenol incorporation decreases the temperature of this component. In contrast, the effects of androstenol and cholesterol incorporation on the thermotropic phase behavior of the intermediate and longer chain PCs studied here are considerably different. Although the incorporation of cholesterol increases the main phase transition temperature of 16:0 PC slightly and decreases the phase transition of 18:0 PC and 21:0 PC, androstenol incorporation decreases the main phase transition temperatures of all three PCs rather markedly. Moreover, androstenol is less effective in reducing the enthalpy and cooperativity of the broad component of the DSC endotherm of 16:0 PC and especially 18:0 PC bilayers in comparison to cholesterol. Androstenol incorporation (> 5 mol%) also results in the appearance of a second, low temperature endotherm in the DSC traces of the intermediate and longer chain PC dispersions that is not observed in similar cholesterol/PC dispersions. FTIR and 31P-NMR results suggest that this endotherm arises from a temperature-induced dissolution of androstenol in the gel phase PC bilayers. This second endotherm occurs at lower androstenol concentrations and increases in area at a given androstenol level as the chain length of the host PC bilayer increases. We ascribe the increasing immiscibility of androstenol in both the gel and liquid-crystalline states of PC bilayers of increasing thickness to an increasing degree of hydrophobic mismatch between the androstenol molecule and the host phospholipid bilayer.

Androstenols↗

PubMed1994

Studies of the thermotropic phase behavior of phosphatidylcholines containing 2-alkyl substituted fatty acyl chains: a new class of phosphatidylcholines forming inverted nonlamellar phases.

We have synthesized a number of 1,2-diacyl phosphatidylcholines with hydrophobic substituents adjacent to the carbonyl group of the fatty acyl chain and studied their thermotropic phase behavior by differential scanning calorimetry, 31P-nuclear magnetic resonance spectroscopy, and x-ray diffraction. Our results indicate that the hydrocarbon chain-melting phase transition temperatures of these lipids are lower than those of the n-saturated diacylphosphatidylcholines of similar chain length. In the gel phase, the 2-alkyl substituents on the fatty acyl chains seem to inhibit the formation of tightly packed, partially dehydrated, quasi-crystalline bilayers (Lc phases), although possibly promoting the formation of chain-interdigitated bilayers. In the liquid-crystalline state, however, these 2-alkyl substituents destabilize the lamellar phase with respect to one or more inverted nonlamellar structures. In general, increases in the length, bulk, or rigidity of the alkyl substituent result in an increased destabilization of the lamellar gel and liquid-crystalline phases and a greater tendency to form inverted nonlamellar phases, the nature of which depends upon the size of the 2-alkyl substituent. Unlike normal non-lamella-forming lipids such as the phosphatidylethanolamines, increases in the length of the main acyl chain stabilize the lamellar phases and reduce the tendency to form nonlamellar structures. Our results establish that with a judicious choice of a 2-alkyl substituent and hydrocarbon chain length, phosphatidylcholines (and probably most other so-called "bilayer-preferring" lipids) can be induced to form a range of inverted nonlamellar structures at relatively low temperatures. The ability to vary the lamellar/nonlamellar phase preference of such lipids should be useful in studies of bilayer/nonbilayer phase transitions and of the molecular organization of various nonlamellar phases. Moreover, because the nonlamellar phases can easily be induced at physiologically relevant temperatures and hydration levels while avoiding changes in polar headgroup composition, this new class of 2-alkyl-substituted phosphatidylcholines should prove valuable in studies of the physiological role of non-lamella-forming lipids in reconstituted lipid-protein model membranes.

Biophysical Phenomena↗

PubMed1993

Calorimetric and spectroscopic studies of the interaction of Manduca sexta apolipophorin III with zwitterionic, anionic, and nonionic lipids.

The nature of the interaction of apolipophorin III (apoLp-III) from the insect Manduca sexta with a variety of zwitterionic and anionic phospholipids and with several nonionic glycolipids was investigated by differential scanning calorimetry (DSC) and 31P-NMR spectroscopy. Monoglucosyldiacylglycerol, phosphatidylethanolamine, and phosphatidic acid (pH 7.2) appear to exhibit the weakest interaction with apoLp-III. DSC studies revealed that the gel/liquid-crystalline phase transition of these lipids is little affected by the binding of apoLp-III. Diglucosyldiacylglycerol, phosphatidylcholine (PC), phosphatidylserine, and phosphatidic acid (pH congruent to 8.8) seem to exhibit somewhat stronger interactions with apoLp-III. The binding of apoLp-III to these lipids induces the formation of lipid domains which melt less cooperatively and at higher temperatures than do the pure lipid dispersions, while having little effect on the melting enthalpy of lipid hydrocarbon chains. Phosphatidylglycerol (PG) and phosphatidic acid (pH > 9.3) appear to exhibit the strongest interactions with apoLp-III. The binding of apoLp-III to these lipids severely disrupts their bilayer structure, resulting in marked reductions in the cooperativity and enthalpy of the gel/liquid-crystalline phase transition of the lipids. Studies of binary mixtures of PC and PG indicate that such bilayer-disrupting interactions only occur in the presence of nonphysiologically high concentrations of PG. The binding of apoLp-III to binary mixtures of diacylglycerol and zwitterionic phospholipid has little effect on the chain-melting or the bilayer/nonbilayer phase transitions of these lipids, but it appears to promote the retention of water at the surface of the lipid aggregate. Our results indicate that the binding of apoLp-III to lipid bilayers is mediated primarily through polar and/or ionic interactions at the lipid bilayer surface. Our results also suggest that the interaction of apoLp-III with lipid bilayers promotes the hydration of their surfaces, a property which is consistent with the proposed in vivo functions of this protein.

Animals↗

PubMed1993

Evidence for two pools of cholesterol in the Acholeplasma laidlawii strain B membrane: a deuterium NMR and DSC study.

Recent investigations have indicated that there exists a well-defined range of membrane hydrocarbon order compatible with good growth of the microorganism Acholeplasma laidlawii B [Monck, M., Bloom, M., Lafleur, M., Lewis, R. N. A. H., McElhaney, R. N., & Cullis, P. R. (1992) Biochemistry 31, 10037-10043]. Since cholesterol increases hydrocarbon order in membranes, it was of interest to examine the effect of cholesterol on the hydrocarbon order and growth characteristics of A. laidlawii B. Cholesterol is normally absent from A. laidlawii membranes since it is neither biosynthesized nor required for the growth or survival of the microorganism. However, cholesterol will be incorporated into the membrane if exogenously supplied to the A. laidlawii culture. For membranes prepared from cells grown in the presence of cholesterol, chemical determinations indicated cholesterol represented as much as 40 mol% of the total membrane lipid. However, 2H NMR order parameter measurements and DSC studies of the same membrane preparation suggested that cholesterol was present at significantly lower levels (approximately 10-15 mol%) in the membrane lipid bilayer. Further incorporation of cholesterol into the A. laidlawii lipid bilayer was found to occur with an increase in temperature or by lyophilization and rehydration at high temperatures, suggesting that sterol present in a separate pool in the membrane preparation could then gain access to the bilayer. 2H NMR spectra of A. laidlawii membrane preparations containing deuterium-labeled cholesterol indicate that the bulk of the cholesterol present in this separate pool is in a solid form.

Acholeplasma laidlawii↗

PubMed1993

Differential scanning calorimetric study of the effect of cholesterol on the thermotropic phase behavior of a homologous series of linear saturated phosphatidylcholines.

We have studied the effects of cholesterol on the thermotropic phase behavior of aqueous dispersions of a homologous series of linear saturated phosphatidylcholines, using high-sensitivity differential scanning calorimetry and an experimental protocol which ensures that broad, low-enthalpy phase transitions are accurately monitored. We find that the incorporation of small amounts of cholesterol progressively decreases the temperature and the enthalpy, but not the cooperativity, of the pretransition of all phosphatidylcholines exhibiting such a pretransition and that the pretransition is completely abolished at cholesterol concentrations above 5 mol % in all cases. The incorporation of increasing quantities of cholesterol also alters the main or chain-melting phase transition of these phospholipid bilayers in both hydrocarbon chain length-dependent and hydrocarbon chain length-independent ways. At cholesterol concentrations of from 1 to 20-25 mol %, the DSC endotherms of all phosphatidylcholines studied consist of a superimposed sharp and broad component, the former ascribed to the melting of cholesterol-poor and the latter to the melting of the cholesterol-rich phosphatidylcholine domains. The temperature and cooperativity of the sharp component are reduced only slightly and in a chain length-independent manner with increasing cholesterol concentration, an effect we ascribe to the colligative effect of the presence of small quantities of cholesterol at the domain boundaries. Moreover, the enthalpy of the sharp component decreases and becomes zero at 20-25 mol % cholesterol for all of the phosphatidylcholines examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry, Differential Scanning↗

PubMed1993

Studies of mixed-chain diacyl phosphatidylcholines with highly asymmetric acyl chains: a Fourier transform infrared spectroscopic study of interfacial hydration and hydrocarbon chain packing in the mixed interdigitated gel phase.

The mixed interdigitated gel phases of unlabeled, specifically 13C = O-labeled, and specifically chain-perdeuterated samples of 1-O-eicosanoyl, 2-O-lauroyl phosphatidylcholine and 1-O-decanoyl, 2-O-docosanoyl phosphatidylcholine were studied by infrared spectroscopy. Our results suggest that at the liquid-crystalline/gel phase transition temperatures of these lipids, there is a greater redistribution in the populations of free and hydrogen-bonded ester carbonyl groups than is commonly observed with symmetric chain n-saturated diacyl phosphatidylcholines. The formation of the mixed interdigitated gel phase coincides with the appearance of a marked asymmetry in the contours of the C = O stretching band, a process which becomes more pronounced as the temperature is reduced. This asymmetry is ascribed to the emergence of a predominant lipid population consisting of free sn1- and hydrogen-bonded (hydrated) sn2-ester carbonyl groups. This suggests that the region of the mixed interdigitated bilayer polar/apolar interface near to the sn1-ester carbonyl group is less hydrated than is the case with the noninterdigitated gel-phase bilayers formed by normal symmetric chain phosphatidylcholines. In the methylene deformation region of the spectrum, the unlabeled lipids exhibit a pronounced splitting of the CH2 scissoring bands. This splitting is significantly attenuated when the short chains are perdeuterated and collapses completely upon perdeuteration of the long chains, irrespective of whether the long (or short) chains are esterified to the sn1 or sn2 positions of the glycerol backbone. These results are consistent with a global hydrocarbon chain packing motif in which the zigzag planes of the hydrocarbon chains are perpendicular to each other and the sites occupied by long chains are twice as numerous as those occupied by short chains. The experimental support for this chain-packing motif enabled more detailed considerations of the possible ways in which these lipid molecules are assembled in the mixed interdigitated gel phase. Generally, our results are compatible with a previously proposed model in which the mixed interdigitated gel phase is an assembly of repeat units which consists of two phosphatidylcholine molecules forming a triple-chain structure with the long chains traversing the bilayer and with the methyl termini of the shorter chains opposed at the bilayer center. Our data also suggest that the packing format which is most consistent with our results and previously published work is one in which the hydrocarbon chains of each repeat unit are parallel to each other with the repeat units themselves being perpendicularly packed.

Acylation↗

PubMed1993

Calorimetric and spectroscopic studies of the polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines.

The polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines was investigated by differential scanning calorimetry, 31P-nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Upon heating, aqueous dispersions of dried samples of the short- and medium-chain homologues (n < or = 17) exhibit single, highly energetic transitions from a dry, crystalline form to the fully hydrated, liquid-crystalline bilayer at temperatures higher than the lamellar gel-liquid-crystalline phase transition exhibited by fully hydrated samples. In contrast, the longer chain homologues (n > or = 18) first exhibit a transition from a dehydrated solid form to the hydrated L beta gel phase followed by the gel-liquid-crystalline phase transition normally observed with fully hydrated samples. The fully hydrated, aqueous dispersions of these lipids all exhibit reversible, fairly energetic gel-liquid-crystalline transitions at temperatures that are significantly higher than those of the corresponding phosphatidylcholines. In addition, at still higher temperatures, the longer chain members of this series (n > or = 16) exhibit weakly energetic transitions from the lamellar phase to an inverted nonlamellar phase. Upon appropriate incubation at low temperatures, aqueous dispersions of the shorter chain members of this homologous series (n < or = 16) form a highly ordered crystal-like phase that, upon heating, converts directly to the liquid-crystalline phase at the same temperature as do the aqueous dispersions of the dried lipid. The spectroscopic data indicate that unlike the n-saturated diacyl phosphatidylcholines, the stable crystal-like phases of this series of phosphatidylethanolamines describe an isostructural series in which the hydrocarbon chains are packed in an orthorhombic subcell and the headgroup and polar/apolar interfacial regions of the bilayer are effectively immobilized and substantially dehydrated. Our results suggest that many of the differences between the properties of these phosphatidylethanolamine bilayers and their phosphatidylcholine counterparts can be rationalized on the basis of stronger intermolecular interactions in the headgroup and interfacial regions of the phosphatidylethanolamine bilayers. These are probably the result of differences in the hydration and hydrogen bonding interactions involving the phosphorylethanolamine headgroup and moieties in the polar/apolar interfacial regions of phosphatidylethanolamine bilayers.

Biophysical Phenomena↗

PubMed1992

FTIR spectroscopic studies of the conformation and amide hydrogen exchange of a peptide model of the hydrophobic transmembrane alpha-helices of membrane proteins.

The conformation and amide hydrogen exchangeability of the hydrophobic peptide Lys2-Gly-Leu24-Lys2-Ala-amide were studied by Fourier transform infrared spectroscopy. In these studies information on the secondary structure of the peptide was obtained from an examination of the contours of both the amide I and amide II absorption bands. The conformationally sensitive amide I and amide II regions of the infrared spectra suggest that the peptide is predominantly alpha-helical and that it contains some non-alpha-helical structures which are probably in an extended conformation. Studies of the exchangeability of the amide protons of the peptide indicate that there are two populations of amide protons which differ markedly with respect to their exchangeability with the bulk solvent phase, whether the peptide is dissolved in methanol or dispersed in hydrated lipid bilayers. One population of amide protons is very readily exchangeable, and our data suggest that it arises primarily but not exclusively from the extended regions of the peptide. The other population exchanges very slowly with the bulk solvent and appears to originate entirely from the alpha-helical domain of the peptide. This latter population is virtually unexchangeable when the peptide is dispersed in hydrated phosphatidylcholine bilayers but can be largely exchanged when the peptide is solubilized with methanol. We suggest that this slowly exchanging population of amide protons arises from the central part of the hydrophobic polyleucine core which forms a very stable alpha-helix that would be deeply buried in the hydrophobic domain of hydrated lipid bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1992

Interaction of a peptide model of a hydrophobic transmembrane alpha-helical segment of a membrane protein with phosphatidylcholine bilayers: differential scanning calorimetric and FTIR spectroscopic studies.

High-sensitivity differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to study the interaction of a synthetic model hydrophobic peptide, Lys2-Gly-Leu24-Lys2-Ala-amide, and members of the homologous series of n-saturated diacylphosphatidylcholines. In the low range of peptide mole fractions, the DSC thermograms exhibited by the lipid/peptide mixtures are resolvable into two components. One of these components is fairly narrow, highly cooperative, and exhibits properties which are similar to but not identical with those of the pure lipid. In addition, the fractional contribution of this component to the total enthalpy change, the peak transition temperature, and cooperativity decrease with an increase in peptide concentration, more or less independently of acyl chain length. The other component is very broad and predominates in the high range of peptide concentration. These two components have been assigned to the chain-melting phase transitions of populations of bulk lipid and peptide-associated lipid, respectively. Moreover, when the mean hydrophobic thickness of the PC bilayer is less than the peptide hydrophobic length, the peptide-associated lipid melts at higher temperatures than does the bulk lipid and vice versa. In addition, the chain-melting enthalpy of the broad endotherm does not decrease to zero even at high peptide concentrations, suggesting that this peptide reduces but do not abolish the cooperative gel/liquid-crystalline phase transition of the lipids with which it is in contact. Our DSC results indicate that the width of the phase transition observed at high peptide concentration is inversely but discontinuously related to hydrocarbon chain length and that gel phase immiscibility occurs when the hydrophobic thickness of the bilayer greatly exceeds the hydrophobic length of the peptide. The FTIR spectroscopic data indicate that the peptide forms a very stable alpha-helix under all of our experimental conditions but that small distortions of its alpha-helical conformation are induced in response to any mismatch between peptide hydrophobic length and bilayer hydrophobic thickness. These results also indicate that the peptide alters the conformational disposition of the acyl chains in contact with it and that the resultant conformational changes in the lipid hydrocarbon chains tend to minimize the extent of mismatch of peptide hydrophobic length and bilayer hydrophobic thickness.

Amino Acid Sequence↗

PubMed1992

Influence of lipid composition on the orientational order in Acholeplasma laidlawii strain B membranes: a deuterium NMR study.

2H NMR techniques have recently been developed to determine the complete orientational order profile of lipid bilayers employing lipids containing perdeuteriated palmitic acid [Lafleur, M., Fine, B., Sternin, E., Cullis, P.R., & Bloom, M. (1989) Biophys. J. 56, 1037-1041]. In this work, these techniques have been applied to study order profiles in intact membranes derived from Acholeplasma laidlawii strain B. It is shown that complete orientational order profiles can be readily obtained from the intact membranes of A. laidlawii B grown on equimolar amounts of perdeuteriated palmitic acid and a nondeuteriated fatty acid of varying length and unsaturation. By variation of the fatty acid composition employing mixtures of perdeuteriated palmitic acid with myristic, elaidic, oleic, or linoleic acid, a range of hydrocarbon order compatible with high rates and extents of cell growth has been obtained where the average order parameter, mean value of S, varies over the range 0.140-0.176. This same variation in order is seen for liposomes derived from total lipids extracted from these intact membranes. 2H NMR studies on liposomes composed of individual species of the extracted lipids indicate that modulation of the membrane lipid headgroup composition has the potential to play an important role in maintaining the membrane order within this range.

Acholeplasma laidlawii↗

PubMed1992

Effect of the chirality of the glycerol backbone on the bilayer and nonbilayer phase transitions in the diastereomers of di-dodecyl-beta-D-glucopyranosyl glycerol.

We have studied the physical properties of aqueous dispersions of 1,2-sn- and 2,3-sn-didodecyl-beta-D-glucopyranosyl glycerols, as well as their diastereomeric mixture, using differential scanning calorimetry and low angle x-ray diffraction. Upon heating, both the chiral lipids and the diastereomeric mixture exhibit characteristically energetic L beta/L alpha phase transitions at 31.7-32.8 degrees C and two or three weakly energetic thermal events between 49 degrees C and 89 degrees C. In the diastereomeric mixture and the 1,2-sn glycerol derivative, these higher temperature endotherms correspond to the formation of, and interconversions between, several nonlamellar structures and have been assigned to L alpha/QIIa, QIIa/QIIb, and QIIb/HII phase transitions, respectively. The cubic phases QIIa and QIIb, whose cell lattice parameters are strongly temperature dependent, can be identified as belonging to space groups Ia3d and Pn3m/Pn3, respectively. In the equivalent 2,3-sn glucolipid, the QIIa phase is not observed and only two transitions are seen at 49 degrees C and 77 degrees C, which are identified as L alpha/QIIb and QIIb/HII phase transitions, respectively. These phase transitions temperatures are some 10 degrees C lower than those of the corresponding phase transitions observed in the diastereomeric mixture and the 1,2-sn glycerol derivative. On cooling, all three lipids exhibit a minor higher temperature exothermic event, which can be assigned to a HII/QIIb phase transition. An exothermic L alpha/L beta phase transition is observed at 30-31 degrees C. A shoulder is sometimes discernible on the high temperature side of the L alpha/L beta event, which may originate from a QIIb/L alpha phase transition prior to the freezing of the hydrocarbon chains. None of the lipids show evidence of a QIIa phase on cooling. No additional exothermic transitions are observed on further cooling to -3 degrees C. However, after nucleation at 0 degrees C followed by a short period of annealing at 22 degrees C, the 1,2-sn glucolipid forms an Lc phase that converts to an L alpha phase at 39.5 degrees C on heating. Neither the diastereomeric mixture nor the 2,3-sn glycerol derivative shows such behavior even after extended periods of annealing. Our results suggest that the differences in the phase behavior of these glycolipid isomers may not be attributable to headgroup size per se, but rather to differences in the stereochemistry of the lipid polar/apolar interfacial region, which consequently effects hydrogen-bonding, hydration, and the hydrophilic/hydrophobic balance.

Biophysical Phenomena↗

PubMed1992

Structures of the subgel phases of n-saturated diacyl phosphatidylcholine bilayers: FTIR spectroscopic studies of 13C = O and 2H labeled lipids.

The subgel phases of unlabeled, specifically chain perdeuterated and specifically 13C = O labeled representative samples of the n-saturated diacylphosphatidylcholines were studied by Fourier-transform infrared spectroscopy. Our results indicate that the spectroscopic properties exhibited by the subgel phases of the longer chain homologues are not consistent with that of a pure phase and we suggest that this is because the observed spectrum is a summation of spectroscopic features arising from both their subgel and L beta gel phases. Using spectral subtraction techniques, we obtained a spectrum which we believe is more representative of the pure subgel phase and from it we suggest that the subgel phase of the long chain phosphatidylcholines is an ordered crystallike structure containing two vibrationally inequivalent populations of lipid molecules arranged with the zigzag planes of their hydrocarbon chains parallel. For dipalmitoylphosphatidylcholine, our data indicate that its stable subgel phase is generally similar to that of the longer chain homologues but it is a more ordered structure in which the polar/apolar interfacial region is probably less hydrated. With the medium chain (N = 13-15) compounds, two populations of vibrationally equivalent molecules are also present in the subgel phase, but unlike DPPC and the longer chain homologues, the zigzag planes of their sn1- and sn2- acyl chains are perpendicular to each other, and a sn1-ester C = O group of one of the populations is in relatively close contact with an sn2-ester C = O group of the other population. With the shorter chain (N = 10 - 12) compounds, our data is indicative of a very complex quasi-crystalline assembly in which there may be at least three vibrationally inequivalent populations of lipid molecules with rotationally disordered hydrocarbon chains. Moreover, the conformation of the glycerol backbone may well be very different from that usually expected of this class of phospholipids. With all of these lipids, the structural pictures which emerge from our studies of the various subgel phases are in many aspects incompatible with that deduced from the single crystal x-ray studies of dimyristoylphosphatidylcholine. We suggest that this is because under our experimental conditions, these lipids have effectively been crystallized from water, whereas the sample used for the single-crystal x-ray study was crystallized from organic solvents.

Carbon Isotopes↗

PubMed1990

Physical properties of glycosyldiacylglycerols: an infrared spectroscopic study of the gel-phase polymorphism of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols.

The thermotropic and barotropic gel-phase polymorphism of a homologous series of saturated, straight-chain beta-D-glucosyldiacylglycerols was studied by Fourier transform infrared spectroscopy. Three spectroscopically distinct lamellar gel phases were detected thermotropically. Upon cooling to temperatures below the gel/liquid-crystalline phase transition temperature, all of these lipids form a metastable L beta gel phase characterized by orientationally disordered all-trans acyl chains. The transformation of the metastable L beta phase to a stable crystalline (Lc2) phase first involves the formation of an intermediate which itself is an ordered crystal-like (Lc1) phase. In the intermediate Lc1 phase, the zigzag planes of the polymethylene chains are nearly perpendicular to one another, and one of the ester carbonyl oxygens is engaged in a strong hydrogen bond, probably to the 2-hydroxyl of the sugar headgroup. The transformation of the Lc1 phase to the Lc2 phase involves a reorientation of the all-trans hydrocarbon chains and is probably driven by the strengthening of the hydrogen bond between the carbonyl ester oxygen and its proton donors. Since a "solid-state" reorganization of the acyl chains is an integral part of that process, it tends to become more sluggish as the chain length increases and is not observed with the longer chain homologues (N greater than 16). The spectroscopic characteristics of the most stable gel phases of the odd- and even-numbered members of this homologous series of compounds exhibit only minor differences, indicating that the structures of these phases are generally similar. The barotropic phase behavior of the shorter and longer chain beta-D-glucosyldiacylglycerols is also different. Compression of the L beta phase of the shorter chain compounds results in immediate conversion to their stable lc phases, whereas compression of the L beta phase of the longer chains does not. Furthermore, compression of the longer chain compounds may result in the formation of chain-interdigitated bilayers, whereas this is not the case for the shorter chain homologues. We suggest that the gel phase formed by any given homologue at a given temperature or pressure is that which maximizes the sometimes competing requirements for the optimal packing of the sugar headgroups and the hydrocarbon chains.

Gels↗

PubMed1990

Physical properties of glycosyl diacylglycerols. 1. Calorimetric studies of a homologous series of 1,2-di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols.

The polymorphic phase behavior of aqueous dispersions of a homologous series of 1,2-di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols was studied by differential scanning calorimetry. At fast heating rates unannealed samples of these lipids exhibit a strongly energetic transition, which has been identified as a lamellar gel/liquid crystalline (L beta/L alpha) phase transition (short- and medium-chain compounds) or a lamellar gel to inverted hexagonal (L beta/HII) phase transition (long-chain compounds) by X-ray diffraction studies (Sen et al., 1990). At still higher temperatures, some of the lipids that form lamellar liquid-crystalline phases exhibit an additional transition, which has been identified as a transition to an inverted nonbilayer phase by X-ray diffraction studies. The lamellar gel phase formed on initial cooling of these lipids is a metastable structure, which, when annealed under appropriate conditions, transforms to a more stable lamellar gel phase, which has been identified as a poorly hydrated crystal-like phase with tilted acyl chains by X-ray diffraction measurements (Sen et al., 1990). With the exception of the di-19:0 homologue, the crystalline phases of these lipids are stable to temperatures higher than those at which their L beta phases melt and, as a result, they convert directly to L alpha or HII phases on heating. Our results indicate that the length of the acyl chain affects both the kinetic and thermodynamic properties of the crystalline phases of these lipids as well as the type of nonbilayer phase that they form. Moreover, when compared with the beta-anomers, these alpha-D-glucosyl diacylglycerols are more prone to form ordered crystalline gel phases at low temperatures and are somewhat less prone to form nonbilayer phases at elevated temperatures. Thus the physical properties of glucolipids (and possibly all glycolipids) are very sensitive to the nature of the anomeric linkage between the sugar headgroup and the glycerol backbone of the lipid molecule. We suggest that this is, in part, due to a change in orientation of the glucopyranosyl ring relative to the bilayer surface, which in turn affects the way(s) in which the sugar headgroups interact with each other and with water.

Calorimetry, Differential Scanning↗

PubMed1990

Physical properties of glycosyl diacylglycerols. 2. X-ray diffraction studies of a homologous series of 1,2-Di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols.

X-ray diffraction methods were used to characterize the thermotropic polymorphism exhibited by aqueous dispersions of a homologous series of 1,2-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols. Upon cooling from temperatures at which the acyl chains of these lipids are melted, all of these compounds form structures that exhibit both low-angle and wide-angle diffraction patterns consistent with the formation of lamellar L beta gel phases. After a suitable protocol of low-temperature annealing, complex diffraction patterns consistent with the formation of highly ordered, lamellar, crystal-like phases are obtained. These patterns are similar for all of the compounds studied, suggesting that the unit cell structure is invariant. The assumption that the unit cell structure is invariant permits the assignment of phases to the diffraction orders, thereby making possible the construction of electron density profiles. These electron density profiles indicate that the crystal-like phases of these lipids are poorly hydrated structures with the hydrocarbon chains inclined at 35 degrees to the bilayer normal. The diffraction patterns of the crystal-like phases of these lipids changed abruptly at the calorimetrically determined phase transition temperatures to those characteristic of either lamellar liquid crystalline phases (N less than or equal to 17) or inverted nonbilayer phases. With these X-ray diffraction data we demonstrate that, at elevated temperatures, the shorter chain homologues (N less than or equal to 16) form cubic phases of the Pn3m space group, whereas the longer chain compounds form inverted hexagonal phases.

Diglycerides↗

PubMed1990

Subgel phases of n-saturated diacylphosphatidylcholines: a Fourier-transform infrared spectroscopic study.

The subgel phases of a homologous series of saturated straight-chain diacylphosphatidylcholines with hydrocarbon chains consisting of 10-18 carbon atoms were studied by Fourier-transform infrared spectroscopy. All of these lipids initially form a subgel phase which is spectroscopically similar to that obtained when fully hydrated multilamellar dispersions of dipalmitoylphosphatidylcholine are incubated at 0-4 degrees C for 2-4 days. However, further low-temperature incubation of those phosphatidylcholines with acyl chains of 16 or fewer carbon atoms results in the sequential formation of 1 or more additional, spectroscopically distinct subgel phases, with the number of such phases increasing as hydrocarbon chain length decreases. Our data indicate that the formation of all of these subgel phases involves both reorientation of the acyl chains and major changes in hydration and/or hydrogen-bonding interactions at the polar/apolar interfacial region of the lipid bilayer. We suggest that the driving force behind the formation of these Lc phases is the formation of an extended hydrogen-bonding network in the interfacial region of the bilayer and that the optimization of this network probably requires some distortion of the optimal packing of the acyl chains. As a result, an increase in acyl chain length makes the formation of these Lc phases less favorable and eventually prevents optimization of the hydrogen-bonding network at the bilayer polar/apolar interface.

Calorimetry↗