Biomedical subjects

M R Morrow

Publications and source records attributed to M R Morrow.

28 records · Page 2Linked to original sources

PubMed1993

Glycosphingolipid acyl chain orientational order in unsaturated phosphatidylcholine bilayers.

The glycosphingolipid, galactosyl ceramide (GalCer), was studied by 2H nuclear magnetic resonance (NMR) in fluid phospholipid bilayer membranes, with regard to arrangement of its acyl chain. For this purpose, species with perdeuterated 18-carbon fatty acid (18:0[d35]GalCer) or with perdeuterated 24-carbon fatty acid (24:0[d47] GalCer) were dispersed in bilayers of the 18-carbon phospholipid, 1-stearoyl-2-oleoyl-phosphatidylcholine (SOPC). For 18:0[d35] GalCer, smoothed profiles of the order parameter, SCD, were found to be very similar to one another over the range of glycolipid concentration, 5-40 mol%. In addition, they were very similar to orientational order parameter profiles well known from the literature on phospholipid and glycolipid acyl chains (which deals in general with membranes of homogeneous chain length in the range 14-18 carbons). Corresponding order parameter profiles for the long-chain species, 24:0[d47] GalCer, were also similar to one another for glycolipid concentrations between 5 and 40 mol%. Their shapes, however, were distinctly different from those of the shorter chain analogues. SCD profiles for the two species were quantitatively similar to a membrane depth of C15. SCD values at C16 and C17 were approximately 20 and 30%, respectively, higher for the long-chain glycosphingolipid than for its short-chain analogue in SOPC. Nitroxide spin labels attached rigidly to C16 of the long-chain glycolipid in SOPC gave electron paramagnetic resonance (EPR) order parameters that were twice as high as for a spin label at C16 on the shorter chain glycolipid. Comparison was made between spectra of 24:0[d47] GalCer in SOPC and fully hydrated bilayers of the pure 24:0[d47] GalCer, a system that is considered to be partially interdigitated in fluid and gel phases. The resultant 2H NMR order parameter profiles displayed similar features, indicating that related organizational properties exist in these fluid systems. Effective chain length of 24:0[d47] GalCer within the SOPC membrane was calculated using the method of Schindler and Seelig (1975. Biochemistry, 14:2283-2287). The result suggested that the long-chain fatty acid should protrude roughly one third of the host matrix chain length across the bilayer midplane. However, a treatment of the same order parameters making very few assumptions about chain conformation indicated a high degree of orientational flexibility for the "extra" length of the long chain fatty acid. It seems likely that a realistic treatment of the long-chain fatty acidin a shorter chain fluid host matrix considers interdigitation as a subset of the conformational possibilities, many of which are rapidly interconverting on the NMR timescale of 10-4_10-5 s and longer lived on the EPR timescale of 10-8_10-9 s.

Biophysical Phenomena↗

PubMed1992

Glycosphingolipid phase behaviour in unsaturated phosphatidylcholine bilayers: a 2H-NMR study.

2H-NMR was employed to consider the arrangement of a glycosphingolipid, N-(lignoceroyl-d47)galactosylceramide, in bilayers of the mono-unsaturated phospholipid, 1-stearoyl-2-oleoylphosphatidylcholine. The deuterated glycolipid prepared by partial synthesis was incorporated at concentrations ranging from 5 mol% to 53 mol% into unsonicated liposomes, and its spectra were recorded from +76 degrees C to -10 degrees C. First spectral moments were plotted as a function of temperature for each sample composition and, along with inspection of the spectra, were employed to infer a phase diagram describing glycolipid behaviour in the unsaturated phospholipid host matrix. It was possible to refine the result using 2H-NMR difference spectroscopy. The phase diagram obtained was indicative of peritectic behaviour. At glycolipid concentrations exceeding about 20 mol% there was considerable tendency to glycolipid phase separation--as indicated by coexistence of fluid phospholipid-enriched and gel phase glycolipid-enriched domains over a wide range of temperatures, and by coexistence of distinct ordered phase domains at lower temperature. In contrast, at lower glycolipid concentrations reflective of many biological membranes, the lipid components were miscible in both the liquid crystal and gel phases, with only a narrow temperature range of fluid and ordered phase coexistence. For the fluid phase at low glycolipid concentrations, spectra of the deuterated glycolipid 24-carbon fatty acid suggest that orientational order is low for a number of methylene groups near the methyl end of the chain.

Deuterium↗

PubMed1992

Chain-length dependence of lipid bilayer properties near the liquid crystal to gel phase transition.

The temperature dependence of the mean orientational order parameter in the vicinity of the liquid crystal to gel phase transition is obtained from the first moment M1 of deuterium nuclear magnetic resonance spectra for bilayers of chain perdeuterated phosphatidylcholines with acyl chains of 12, 14, 16, and 18 carbons. The data clearly show an increasing temperature dependence of the orientational order parameter in the vicinity of the transition, with the effect becoming more pronounced with decreasing chain length. Assuming a linear relationship between the mean orientational order parameter and the extension of the acyl chain, estimates of the change in area of the membrane at the transition are shown to be consistent with those obtained from other measurements. It is shown that the transition may be modeled in terms of a Landau expansion of the free energy involving a small number of phenomenological parameters. From this it is shown that the behavior of these systems in the temperature range of interest is, in large part, controlled by the close proximity of a spinodal to the transition temperature.

Biophysical Phenomena↗

PubMed1991

The effect of changes in gramicidin conformation on bilayer lipid properties.

The effects of two different gramicidin conformations on lipid phase behaviour and dynamics are compared. Samples of chain-perdeuterated dimyristoylphosphatidylcholine containing gramicidin were first prepared with gramicidin in a state having a circular dichroism spectrum generally identified as corresponding to the non-channel conformation. The effects, on bilayer lipid properties, of gramicidin in this conformation were then determined using deuterium nuclear magnetic resonance measurements of acyl chain orientational order and transverse relaxation times as a function of temperature. These samples were then incubated at 65 degrees C to convert the gramicidin to a state with a circular dichroism spectrum of the type generally identified with the channel conformation. The nuclear magnetic resonance measurements were then repeated. In the gel phase, it was found that transverse relaxation time and chain orientational order of the lipid were insensitive to gramicidin conformation. In the liquid crystalline phase, gramicidin in the channel conformation was found to have a slightly larger effect on transverse relaxation and orientational order than gramicidin in the non-channel conformation. The perturbation of the phase behavior by gramicidin was found to be relatively insensitive to gramicidin conformation.

Circular Dichroism↗

PubMed1990

Interaction between perdeuterated dimyristoylphosphatidylcholine and low molecular weight pulmonary surfactant protein SP-C.

A low molecular weight hydrophobic protein was isolated from porcine lung lavage fluid using silicic acid and Sephadex LH-20 chromatography. The protein migrated with an apparent molecular weight of 5000-6000 on SDS-PAGE under reducing and nonreducing conditions. Gels run under reducing conditions also showed a minor band migrating with a molecular weight of 12,000. Amino acid compositional analysis and sequencing data suggest that this protein preparation contains intact surfactant protein SP-C and about 30% of truncated SP-C (N-terminal leucine absent). The surfactant protein was combined with perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) in multilamellar vesicles. The protein enhanced the rate of adsorption of the lipid at air-water interfaces. The ability of the protein to alter normal lipid organization was examined by using high-sensitivity differential scanning calorimetry (DSC) and 2H nuclear magnetic resonance spectroscopy (2H NMR). The calorimetric measurements indicated that the protein caused a decrease in the temperature maximum (Tm) and a broadening of the phase transition. At a protein concentration of 8% (w/w), the enthalpy change of transition was reduced to 4.4 kcal/mol compared to 6.3 kcal/mol determined for the pure lipid. NMR spectral moment studies indicated that protein had no effect on lipid chain order in the liquid-crystal phase but reduced orientational order in the gel phase. Two-phase coexistence in the presence of protein was observed over a small temperature range below the pure lipid transition temperature. Spin-lattice relaxation times (T1) were not substantially affected by the protein. Transverse relaxation time (T2e) studies suggest that the protein influences slow lipid motions.

Adsorption↗

PubMed1990

Transverse nuclear spin relaxation in phosphatidylcholine bilayers containing gramicidin.

Deuterium nuclear magnetic resonance has been used to study transverse relaxation in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, perdeuterated and specifically deuterated at the alpha position of the chains, containing the polypeptide gramicidin at concentrations of 0, 1, and 4 mol%. For 4 mol% gramicidin, the bilayer is thought to undergo a continuous phase change rather than a phase transition proceeding via two phase coexistence. Information is obtained regarding lipid dynamics in the continuous phase change region of the phase diagram. In the presence of gramicidin, the transverse relaxation time measured by the quadrupole echo technique, T2e, passes through a minimum in the gel phase. The gramicidin concentration dependence of T2e suggests that the polypeptide reduces the temperature sensitivity of the correlation time responsible for the minimum. The polypeptide also increases the sensitivity of the first spectral moment, M1, to the quadrupole echo pulse separation. This behavior is attributed to a polypeptide-induced enhancement of the spread in T2e along the acyl chains. Quadrupole Carr-Purcell-Meiboom-Gill experiments are used to separate contributions to the observed behavior from fast and slow motions.

Deuterium↗

PubMed1988

A phenomenological model for lipid-protein bilayers with critical mixing.

A Landau expansion of free energy in terms of area/lipid has been used to obtain protein-lipid phase diagrams with critical mixing and a maximum phase separation concentration. Simulations using this model indicate that differential scanning calorimetry scan shapes and transition enthalpies observed for lipid-synthetic peptide mixtures are consistent with this type of phase diagram. The critical mixing point and the homogeneous mixture critical point are distinguished.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1988

Differential scanning calorimetry and 2H NMR studies of the phase behavior of gramicidin-phosphatidylcholine mixtures.

The extents of two-phase coexistence in the phase diagrams of mixtures of gramicidin with 1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine (DPPC-d62) and with 1,2-bis(perdeuteriomyristoyl)-sn-glycero-3-phosphocholine (DMPC-d54) mixtures have been explored with differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H NMR). For both systems, increased gramicidin content causes a decrease in transition enthalpy and a broadening of the peak in excess heat capacity at the transition. In DMPC-d54-based mixtures, the broadening is roughly symmetric about the pure lipid transition temperature. Addition of gramicidin to DPPC-d62 extends the excess heat capacity peak on the low-temperature side, resulting in a slightly asymmetric scan. Deuterium NMR spectra showing a superposition of gel and liquid-crystalline components, observed for both mixtures, indicate the presence of two-phase coexistence. For the DPPC-d62-based mixtures, two-phase coexistence is restricted to an approximately 2 degrees C temperature range below the pure transition temperature. For DMPC-d54-based mixtures, the region of two-phase coexistence is even narrower. For both mixtures, beyond a gramicidin mole fraction of 2%, distinct gel and liquid-crystal contributions to the spectra cannot be distinguished. Along with the broad featureless nature of the DSC scan in this region, this is taken to indicate that the transition has been replaced by a continuous phase change. These results are consistent with the existence of a closed two-phase region having a critical concentration of gramicidin below 2 mol%.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1986

Studies on the interaction of human erythrocyte band 3 with membrane lipids using deuterium nuclear magnetic resonance and differential scanning calorimetry.

Human erythrocyte band 3, reconstituted into large unilamellar phospholipid vesicles, has been used as a model system for studying the interactions between membrane lipids and large transmembrane glycoproteins. Both 2H-nuclear magnetic resonance (2H-NMR) and differential scanning calorimetric techniques have been used to probe dimyristoylphosphatidylcholine-band 3 interactions over the temperature range 4-32 degrees C. Analysis of 2H-NMR spectra allowed the assignment of liquid crystal, gel phase and two-phase regions for several protein/lipid mole fractions in the range (1-20) X 10(-4). Sample size was limited by the amount of available glycoprotein and this precluded exact determination of the phase boundaries for this system. The sharp discontinuity in the spectral first moment, M1, seen at the phase transition of the pure phospholipid is progressively diminished by addition of protein, and at the highest protein concentration the first moment varies smoothly between the two phases. For T greater than 26 degrees C or less than 16 degrees C, the moments are relatively insensitive to protein concentration, while between 20 and 26 degrees C the moments increase with protein concentration up to the boundary of the two-phase region. Beyond this boundary, they remain constant or decrease slightly with increasing amount of protein. A preliminary phase diagram for band 3 in this lipid system is presented, based on 2H-NMR data. Differential scanning calorimetry (DSC) showed that addition of glycoprotein dramatically alters the scan shape and tends to extend the coexistence of two phases to higher temperatures.

Anion Exchange Protein 1, Erythrocyte↗

PubMed1985

Simultaneous modeling of phase and calorimetric behavior in an amphiphilic peptide/phospholipid model membrane.

Differential scanning calorimetry (DSC) experiments have been performed on the amphiphilic peptide/1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine system for which partial phase diagrams have been measured by deuterium nuclear magnetic resonance. The solute concentration dependence of the transition enthalpy in such systems is often interpreted in terms of an annulus of lipid withdrawn, by the solvent, from participation in the transition while the bulk lipid melts with its fully enthalpy. This idea is equivalent to postulating ideal mixing between the lipid and the peptide/lipid complex, and there is little justification for such an assumption. Adaptation of regular solution theory to this system demonstrates that the peptide concentration dependence of the transition enthalpies can be incorporated into a thermodynamic model which reproduces the observed phase behavior fairly well without postulating that a complexing annulus of lipid around the peptide be withdrawn from participating in the chain-melting transition. The model parameters determined by simultaneous fitting of the phase behavior and transition enthalpies are used to simulate the DSC scan shapes. The asymmetry of the calorimetric scans for chi 2 less than or equal to 0.02 is reproduced by the model, but a broad component observed for higher concentration is not. In light of the results presented here, previous analyses of the calorimetric behavior of two-component systems in terms of symmetric transitions which do not account for the possible extent of a region of two-phase equilibrium must be questioned.

Calorimetry, Differential Scanning↗