Biomedical subjects

R S Hodges

Publications and source records attributed to R S Hodges.

At least 19 recordsLinked to original sources

PubMed2002

Conformation and interaction of the cyclic cationic antimicrobial peptides in lipid bilayers.

To investigate the role of peptide-membrane interactions in the biological activity of cyclic cationic peptides, the conformations and interactions of four membrane-active antimicrobial peptides [based on Gramicidin S (GS)] were examined in neutral and negatively charged micelles and phospholipid vesicles, using CD and fluorescence spectroscopy and ultracentrifugation techniques. Moreover, the effects of these peptides on the release of entrapped fluorescent dye from unilamellar vesicles of phosphatidylcholine (PC) and phosphatidylethanolamine/phosphatidylglycerol (PE/PG) were studied. The cyclic peptides include GS10 [Cyclo(VKLdYP)2], GS12 [Cyclo(VKLKdYPKVKLdYP)], GS14 [Cyclo(VKLKVdYPLKVKLdYP)] and [d-Lys]4GS14 [Cyclo(VKLdKVdYPLKVKLdYP)] (underlined residues are d-amino acids), were different in their ring size, structure and amphipathicity, and covered a broad spectrum of hemolytic and antimicrobial activities. Interaction of the peptides with the zwitterionic PC and negatively charged PE/PG vesicles were distinct from each other. The hydrophobic interaction seems to be the dominant factor in the hemolytic activity of the peptides, as well as their interaction with the PC vesicles. A combination of electrostatic and hydrophobic interactions of the peptides induces aggregation and fusion in PE/PG vesicles with different propensities in the order: [d-Lys]4GS14 > GS14 > GS12 > GS10. GS10 and GS14 are apparently located in the deeper levels of the membrane interfaces and closer to the hydrophobic core of the bilayers, whereas GS12 and [d-Lys]4GS14 reside closer to the outer boundary of the interface. Because of differing modes of interaction of the cyclic cationic peptides with lipid bilayers, the mechanism of their biological activity (and its relation to peptide-lipid interaction) proved to be versatile and complex, and dependent on the biophysical properties of both the peptides and membranes.

Antimicrobial Cationic Peptides↗

PubMed2002

A possible mechanism for partitioning between homo- and heterodimerization of the yeast homeodomain proteins MATa1 and MATalpha2.

The yeast Saccharomyces cerevisiae has three cell types distinguished by the proteins encoded in their mating-type (MAT) loci: the a and alpha haploids, which express the DNA-binding proteins a1, and alpha1 and alpha2, respectively, and the a/alpha diploid which expresses both a1 and alpha2 proteins. In a/alpha cells, a1-alpha2 heterodimers repress haploid-specific genes and MATalpha1, whereas alpha2 homodimers repress a-specific genes, indicating dual regulatory functions for alpha2 in mating-type control. We previously demonstrated that the two leucine zipper-like coiled-coil motifs, called alpha2A and alpha2B, in the alpha2 N-terminal domain are important to a1-alpha2 heterodimerization. A unique feature of alpha2B is the occurrence of three atypical amino acid residues at a positions within the hydrophobic core. We have conducted mutational analyses of alpha2B peptides and the full-length protein. Our data suggest that these residues may play a critical role in partitioning of the alpha2 protein between heterodimerization with a1 and homodimerization with itself.

Amino Acid Sequence↗

PubMed2002

Determination of stereochemistry stability coefficients of amino acid side-chains in an amphipathic alpha-helix.

We describe here a systematic study to determine the effect on secondary structure of d-amino acid substitutions in the nonpolar face of an amphipathic alpha-helical peptide. The helix-destabilizing ability of 19 d-amino acid residues in an amphipathic alpha-helical model peptide was evaluated by reversed-phase HPLC and CD spectroscopy. l-Amino acid and d-amino acid residues show a wide range of helix-destabilizing effects relative to Gly, as evidenced in melting temperatures (DeltaTm) ranging from -8.5 degrees C to 30.5 degrees C for the l-amino acids and -9.5 degrees C to 9.0 degrees C for the d-amino acids. Helix stereochemistry stability coefficients defined as the difference in Tm values for the l- and d-amino acid substitutions [(DeltaTm' = TmL and TmD)] ranging from 1 degrees C to 34.5 degrees C. HPLC retention times [DeltatR(XL-XD)] also had values ranging from -0.52 to 7.31 min at pH 7.0. The helix-destabilizing ability of a specific d-amino acid is highly dependent on its side-chain, with no clear relationship to the helical propensity of its corresponding l-enantiomers. In both CD and reversed-phase HPLC studies, d-amino acids with beta-branched side-chains destabilize alpha-helical structure to the greatest extent. A series of helix stability coefficients was subsequently determined, which should prove valuable both for protein structure-activity studies and de novo design of novel biologically active peptides.

Amino Acid Sequence↗

PubMed2001

A polyalanine-based peptide cannot form a stable transmembrane alpha-helix in fully hydrated phospholipid bilayers.

The conformation and amide proton exchangeability of the peptide acetyl-K(2)-A(24)-K(2)-amide (A(24)) and its interaction with phosphatidylcholine bilayers were examined by a variety of physical techniques. When dissolved in or cast from methanol as a dried film, A(24) is predominantly alpha-helical. In aqueous media, however, A(24) exists primarily as a mixture of helical (though not necessarily alpha-helical) and random coiled structures, both of which allow rapid H-D exchange of all amide protons. When incorporated into phospholipids in the absence of water, A(24) also exists primarily as a transmembrane alpha-helix. However, upon hydration of that system, rapid exchange of all amide protons also occurs along with a marked change in the amide I absorption band of the peptide. Also, when dispersed with phosphatidylcholine in aqueous media, the conformation and thermal stability of A(24) are not significantly altered by the presence of the phospholipid or by its gel/liquid-crystalline phase transition. Differential scanning calorimetric and electron spin resonance spectroscopic studies indicate that A(24) has relatively minor effects on the thermodynamic properties of the lipid hydrocarbon chain-melting phase transition, that it does not abolish the lipid pretransition, and that its presence has no significant effect on the orientational order or rates of motion of the phospholipid hydrocarbon chains. We therefore conclude that A(24) has sufficient alpha-helical propensity, but insufficient hydrophobicity, to maintain a stable transmembrane association with phospholipid bilayers in the presence of water. Instead, it exists primarily as a dynamic mixture of helices and other conformers and resides mostly in the aqueous phase where it interacts weakly with the bilayer surface or with the polar/apolar interfacial region of phosphatidylcholine bilayers. Thus, polyalanine-based peptides are not good models for the transmembrane alpha-helical segments of natural membrane proteins.

Calorimetry, Differential Scanning↗

PubMed2001

Mapping the interacting regions between troponins T and C. Binding of TnT and TnI peptides to TnC and NMR mapping of the TnT-binding site on TnC.

Muscular contraction is triggered by an increase in calcium concentration, which is transmitted to the contractile proteins by the troponin complex. The interactions among the components of the troponin complex (troponins T, C, and I) are essential to understanding the regulation of muscle contraction. While the structure of TnC is well known, and a model for the binary TnC.TnI complex has been recently published (Tung, C.-S., Wall, M. E., Gallagher, S. C., and Trewhella, J. (2000) Protein Sci. 9, 1312-1326), very little is known about TnT. Using non-denaturing gels and NMR spectroscopy, we have analyzed the interactions between TnC and five peptides from TnT as well as how three TnI peptides affect these interactions. Rabbit fast skeletal muscle peptide TnT-(160-193) binds to TnC with a dissociation constant of 30 +/- 6 microm. This binding still occurs in the presence of TnI-(1-40) but is prevented by the presence of TnI-(56-115) or TnI-(96-139), both containing the primary inhibitory region of TnI. TnT-(228-260) also binds TnC. The binding site for TnT-(160-193) is located on the C-terminal domain of TnC and was mapped to the surface of TnC using NMR chemical shift mapping techniques. In the context of the model for the TnC.TnI complex, we discuss the interactions between TnT and the other troponin subunits.

Animals↗

PubMed2001

Are trigger sequences essential in the folding of two-stranded alpha-helical coiled-coils?

The amino acid residues comprising the interface between strands of the coiled-coil motif are usually hydrophobic and make a major contribution to coiled-coil folding and stability. However, in some cases the presence of excellent hydrophobic residues at the coiled-coil interface is insufficient for folding. It has been proposed that a "consensus trigger sequence" exists that is necessary within the coiled-coil domains of various proteins to trigger folding. Therefore, in this study we designed a 31-residue hybrid sequence based on sequences from the two-stranded parallel coiled-coil domains of the yeast transcriptional activator GCN4 and the actin-bundling protein Dictyostelium discoideum cortexillin I. The hybrid and its analogs were studied by CD spectroscopy and analytical ultracentrifugation. The hybrid had stable residues in the core "a" and "d" positions in the 3-4 hydrophobic repeat, denoted (abcdefg)n, but did not have a consensus trigger sequence and did not possess appreciable secondary structure as determined by CD spectroscopy. The substitutions in the parent peptide were introduced at positions other than "a" and "d", altering a variety of interactions including alpha-helical propensity, interchain and intrachain electrostatics, and hydrophobicity. Although the substitutions did not bring the overall sequence in closer agreement to the consensus trigger sequence, they increased coiled-coil folding and stability. Therefore, our results suggest that the combination of stabilizing effects along a protein sequence is a more general indicator of protein folding in coiled-coils than the identification of a specific trigger sequence. We propose that surpassing a critical threshold stability value using any type or combination of stabilizing effects will allow coiled-coils to fold, in the absence of a specific trigger sequence per se.

Amino Acid Sequence↗

PubMed2001

Cholesterol attenuates the interaction of the antimicrobial peptide gramicidin S with phospholipid bilayer membranes.

We have investigated the effect of the presence of 25 mol percent cholesterol on the interactions of the antimicrobial peptide gramicidin S (GS) with phosphatidylcholine and phosphatidylethanolamine model membrane systems using a variety of methods. Our circular dichroism spectroscopic measurements indicate that the incorporation of cholesterol into egg phosphatidylcholine vesicles has no significant effect on the conformation of the GS molecule but that this peptide resides in a range of intermediate polarity as compared to aqueous solution or an organic solvent. Our Fourier transform infrared spectroscopic measurements confirm these findings and demonstrate that in both cholesterol-containing and cholesterol-free dimyristoylphosphatidylcholine liquid-crystalline bilayers, GS is located in a region of intermediate polarity at the polar--nonpolar interfacial region of the lipid bilayer. However, GS appears to be located in a more polar environment nearer the bilayer surface when cholesterol is present. Our (31)P-nuclear magnetic resonance studies demonstrate that the presence of cholesterol markedly reduces the tendency of GS to induce the formation of inverted nonlamellar phases in model membranes composed of an unsaturated phosphatidylethanolamine. Finally, fluorescence dye leakage experiments indicate that cholesterol inhibits the GS-induced permeabilization of phosphatidylcholine vesicles. Thus in all respects the presence of cholesterol attenuates but does not abolish the interactions of GS with, and the characteristic effects of GS on, phospholipid bilayers. These findings may explain why it is more potent at disrupting cholesterol-free bacterial than cholesterol-containing eukaryotic membranes while nevertheless disrupting the integrity of the latter at higher peptide concentrations. This additional example of the lipid specificity of GS may aid in the rational design of GS analogs with increased antibacterial but reduced hemolytic activities.

Anti-Bacterial Agents↗

PubMed2001

A differential scanning calorimetric and 31P NMR spectroscopic study of the effect of transmembrane alpha-helical peptides on the lamellar-reversed hexagonal phase transition of phosphatidylethanolamine model membranes.

We have investigated the effects of the model alpha-helical transmembrane peptide Ac-K(2)L(24)K(2)-amide (L(24)) on the thermotropic phase behavior of aqueous dispersions of 1,2-dielaidoylphosphatidylethanolamine (DEPE) to understand better the interactions between lipid bilayers and the membrane-spanning segments of integral membrane proteins. We studied in particular the effect of L(24) and three derivatives thereof on the liquid-crystalline lamellar (L(alpha))-reversed hexagonal (H(II)) phase transition of DEPE model membranes by differential scanning calorimetry and (31)P nuclear magnetic resonance spectroscopy. We found that the incorporation of L(24) progressively decreases the temperature, enthalpy, and cooperativity of the L(alpha)-H(II) phase transition, as well as induces the formation of an inverted cubic phase, indicating that this transmembrane peptide promotes the formation of inverted nonlamellar phases, despite the fact that the hydrophobic length of this peptide exceeds the hydrophobic thickness of the host lipid bilayer. These characteristic effects are not altered by truncation of the side chains of the terminal lysine residues or by replacing each of the leucine residues at the end of the polyleucine core of L(24) with a tryptophan residue. Thus, the characteristic effects of these transmembrane peptides on DEPE thermotropic phase behavior are independent of their detailed chemical structure. Importantly, significantly shortening the polyleucine core of L(24) results in a smaller decrease in the L(alpha)-H(II) phase transition temperature of the DEPE matrix into which it is incorporated, and reducing the thickness of the host phosphatidylethanolamine bilayer results in a larger reduction in the L(alpha)-H(II) phase transition temperature. These results are not those predicted by hydrophobic mismatch considerations or reported in previous studies of other transmembrane alpha-helical peptides containing a core of an alternating sequence of leucine and alanine residues. We thus conclude that the hydrophobicity and conformational flexibility of transmembrane peptides can affect their propensity to induce the formation of inverted nonlamellar phases by mechanisms not primarily dependent on lipid-peptide hydrophobic mismatch.

Calorimetry, Differential Scanning↗

PubMed2001

Peptide models of the helical hydrophobic transmembrane segments of membrane proteins: interactions of acetyl-K2-(LA)12-K2-amide with phosphatidylethanolamine bilayer membranes.

High-sensitivity differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to study the interaction of a synthetic alpha-helical hydrophobic transmembrane peptide, acetyl-Lys(2)-(Leu-Ala)(12)-Lys(2)-amide [(LA)(12)], and members of a homologous series of n-saturated diacylphosphatidylethanolamines (PEs). In the lower range of peptide mole fractions, the DSC endotherms exhibited by the lipid/peptide mixtures consist of two components. The temperature and cooperativity of the sharper, higher temperature component are very similar to those of pure PE bilayers and are almost unaffected by variations in the protein/lipid ratio. However, the fractional contribution of this component to the total enthalpy changes decreases with increases in peptide concentration, and this component completely disappears at higher protein mole fractions. The other component, which is less cooperative and occurs at a lower temperature, predominates at higher protein concentrations. These two components of the DSC endotherm have been assigned to the chain-melting phase transitions of peptide-nonassociated and peptide-associated PE molecules, respectively. Although the temperature at which the peptide-associated PE molecules melt is progressively decreased by increases in (LA)(12) concentration, the magnitude of this downward shift is progressively greater as the length of the PE hydrocarbon chain decreases. As well, mixtures of (LA)(12) with the longer chain PEs exhibit unusual biomodal enthalpy variations, suggesting peptide immiscibility in thicker gel state bilayers. Moreover, the enthalpy of the chain-melting transition of the peptide-associated PE does not decrease to zero even at high peptide concentrations, indicating that (LA)(12) attenuates but does not abolish the cooperative gel/liquid-crystalline phase transition of the lipids with which it is in contact. Our FTIR spectroscopic data indicate that (LA)(12) remains in a predominantly alpha-helical conformation in liquid-crystalline PE bilayers of various hydrophobic thickness but that the helical conformation is altered in gel-state PE bilayers generally, probably due to peptide lateral aggregation. These data also suggest that (LA)(12) significantly disorders the hydrocarbon chains of adjacent PE molecules in both the gel and liquid-crystalline states, relatively independently of lipid hydrocarbon chain length. Many aspects of PE/(LA)(12) interactions exhibit a different dependence on the hydrophobic thickness of the host bilayer than was observed in our previous study of (LA)(12)-phosphatidylcholine (PC) model membranes [Zhang et al. (1995) Biochemistry 34, 2362-2371]. The differing effects of (LA)(12) incorporation on PE and PC bilayers is ascribed primarily to the much stronger lipid polar headgroup interactions characteristic of the former system. Finally, the considerable differences observed in the behavior of (LA)(12) and the related polyleucine-based peptide P(24) in both PC and PE bilayers indicate that the structure of the hydrophobic core of alpha-helical transmembrane peptides can affect their conformational plasticity and state of aggregation and thus the nature of their interactions with different phospholipid bilayers.

Alanine↗

PubMed2001

Self diffusion and spectral modifications of a membrane protein, the Rubrivivax gelatinosus LH2 complex, incorporated into a monoolein cubic phase.

The light-harvesting complex LH2 from a purple bacterium, Rubrivivax gelatinosus, has been incorporated into the Q230 cubic phase of monoolein. We measured the self-diffusion of LH2 in detergent solution and in the cubic phase by fluorescence recovery after photobleaching. We investigated also the absorption and fluorescence properties of this oligomeric membrane protein in the cubic phase, in comparison with its beta-octyl glucoside solution. In these experiments, native LH2 and LH2 labeled by a fluorescent marker were used. The results indicate that the inclusion of LH2 into the cubic phase induced modifications in the carotenoid and B800 binding sites. Despite these significant perturbations, the protein seems to keep an oligomeric structure. The relevance of these observations for the possible crystallization of this protein in the cubic phase is discussed.

Diffusion↗

PubMed2001

Contribution of translational and rotational motions to molecular association in aqueous solution.

Much uncertainty and controversy exist regarding the estimation of the enthalpy, entropy, and free energy of overall translational and rotational motions of solute molecules in aqueous solutions, quantities that are crucial to the understanding of molecular association/recognition processes and structure-based drug design. A critique of the literature on this topic is given that leads to a classification of the various views. The major stumbling block to experimentally determining the translational/rotational enthalpy and entropy is the elimination of vibrational perturbations from the measured effects. A solution to this problem, based on a combination of energy equi-partition and enthalpy-entropy compensation, is proposed and subjected to verification. This method is then applied to analyze experimental data on the dissociation/unfolding of dimeric proteins. For one translational/rotational unit at 1 M standard state in aqueous solution, the results for enthalpy (H degrees (tr)), entropy (S degrees (tr)), and free energy (G degrees (tr)) are H (degrees) (tr) = 4.5 +/- 1.5RT, S (degrees) (tr) = 5 +/- 4R, and G (degrees) (tr) = 0 +/- 5RT. Therefore, the overall translational and rotational motions make negligible contribution to binding affinity (free energy) in aqueous solutions at 1 M standard state.

Bacterial Proteins↗

PubMed2001

Membrane-bound structure and alignment of the antimicrobial beta-sheet peptide gramicidin S derived from angular and distance constraints by solid state 19F-NMR.

The antimicrobial properties of the cyclic beta-sheet peptide gramicidin S are attributed to its destabilizing effect on lipid membranes. Here we present the membrane-bound structure and alignment of a derivative of this peptide, based on angular and distance constraints. Solid-state 19F-NMR was used to study a 19F-labelled gramicidin S analogue in dimyristoylphosphatidylcholine bilayers at a lipid:peptide ratio of 80:1 and above. Two equivalent leucine side chains were replaced by the non-natural amino acid 4F-phenylglycine, which serves as a highly sensitive reporter on the structure and dynamics of the peptide backbone. Using a modified CPMG multipulse sequence, the distance between the two 19F-labels was measured from their homonuclear dipolar coupling as 6 A. in good agreement with the known backbone structure of natural gramicidin S in solution. By analyzing the anisotropic chemical shift of the 19F-labels in macroscopically oriented membrane samples, we determined the alignment of the peptide in the bilayer and described its temperature-dependent mobility. In the gel phase, the 19F-labelled gramicidin S is aligned symmetrically with respect to the membrane normal, i.e., with its cyclic beta-sheet backbone lying flat in the plane of the bilayer, which is fully consistent with its amphiphilic character. Upon raising the temperature to the liquid crystalline state, a considerable narrowing of the 19F-NMR chemical shift dispersion is observed, which is attributed the onset of global rotation of the peptide and further wobbling motions. This study demonstrates the potential of the 19F nucleus to describe suitably labelled polypeptides in membranes, requiring only little material and short NMR acquisition times.

Anti-Bacterial Agents↗

PubMed2001

Conformation and other biophysical properties of cyclic antimicrobial peptides in aqueous solutions.

As a step towards understanding the mechanism of the biological activity of cyclic antimicrobial peptides, the biophysical properties and conformations of four membrane-active cyclic peptide antibiotics, based on gramicidin S (GS), were examined in aqueous environments. These cyclic peptides, GS10 [cyclo(VKLdYP)2], GS12 [cyclo(VKLKdYPKVKLdYP)], GS14 [cyclo(VKLKVdYPLKVKLdYP)] and [d-Lys]4GS14 [cyclo(VKLdKVdYPLKVKLdYP)] (d-amino acid residues are denoted by d and are underlined) had different ring sizes of 10, 12 and 14 residues, were different in structure and amphipathicity, and covered a broad spectrum of hemolytic and antimicrobial activities. GS10, GS12 and [d-Lys]4GS14 were shown to be monomeric in buffer systems with ionic strength biological environments. GS14 was also monomeric at low concentrations, but aggregated at concentrations > 50 microm. The affinity of peptides for self-assembly and interaction with hydrophobic surfaces was related to their free energy of intermolecular interaction. The effects of variations in salt and organic solvent (trifluoroethanol) concentration and temperature on peptide conformation were also examined. Similar to GS, GS10 proved to have a stable and rather rigid conformation in different environments and over a broad range of temperatures, whereas GS12, GS14 and [d-Lys]4GS14 had more flexible conformations. Despite its conformational similarity to GS10, GS14 had unique physicochemical properties due to its tendency to aggregate at relatively low concentrations. The biophysical data explain the direct relation between structure, amphipathicity and hydrophobicity of the cyclic peptides and their hemolytic activity. However, this relation with the antimicrobial activity of the peptides is of a more complex nature due to the diversity in membrane structures of microorganisms.

Anti-Bacterial Agents↗

PubMed2001

Designing heterodimeric two-stranded alpha-helical coiled-coils: the effect of chain length on protein folding, stability and specificity.

The E/K coil, a heterodimeric coiled-coil, has been designed as a universal peptide capture and delivery system for use in applications such as biosensors and affinity chromatography. In this design, heterodimer formation is specified through the placement of charged residues at the e and g positions of the heptad repeat. The affinity and stability of the E/K coil has been modified in order to allow a greater range of conditions for association and dissociation by varying the chain length to obtain three, four and five heptad coiled-coils (21, 28 and 35 residues per polypeptide chain). The effect of chain length on stability and folding was examined by circular dichroism spectroscopy, guanidine hydrochloride denaturation, and redox equilibrium experiments. It was found that increases in chain length produced increases in the stability of heterodimeric coiled-coils, but in a nonlinear fashion. The resulting disulfide-bridged heterostranded molecules and reduced heterodimers span a wide range of stabilities (deltaG=3.3-11.9 kcal/mol), greatly expanding their scope for use in protein engineering and biomedical applications.

Amino Acid Sequence↗

PubMed2001

Antifreeze protein from shorthorn sculpin: identification of the ice-binding surface.

Shorthorn sculpins, Myoxocephalus scorpius, are protected from freezing in icy seawater by alanine-rich, alpha-helical antifreeze proteins (AFPs). The major serum isoform (SS-8) has been reisolated and analyzed to establish its correct sequence. Over most of its length, this 42 amino acid protein is predicted to be an amphipathic alpha-helix with one face entirely composed of Ala residues. The other side of the helix, which is more heterogeneous and hydrophilic, contains several Lys. Computer simulations had suggested previously that these Lys residues were involved in binding of the peptide to the [11-20] plane of ice in the <-1102> direction. To test this hypothesis, a series of SS-8 variants were generated with single Ala to Lys substitutions at various points around the helix. All of the peptides retained significant alpha-helicity and remained as monomers in solution. Substitutions on the hydrophilic helix face at position 16, 19, or 22 had no obvious effect, but those on the adjacent Ala-rich surface at positions 17, 21, and 25 abolished antifreeze activity. These results, with support from our own modeling and docking studies, show that the helix interacts with the ice surface via the conserved alanine face, and lend support to the emerging idea that the interaction of fish AFPs with ice involves appreciable hydrophobic interactions. Furthermore, our modeling suggests a new N terminus cap structure, which helps to stabilize the helix, whereas the role of the lysines on the hydrophilic face may be to enhance solubility of the protein.

Alanine↗

PubMed2000

Development of simultaneous purification methodology for multiple synthetic peptides by reversed-phase sample displacement chromatography.

We have developed a low-pressure protocol, designed as a rapid, simple and cost-effective procedure for the efficient and parallel purification of multiple peptide mixtures. This was achieved through adaptation of our novel reversed-phase sample displacement chromatography (SDC) method, where the major separation process takes place in the absence of organic modifier, to modular solid-phase extraction (SPE) technology. Thus, crude peptide sample is applied at overload conditions to extraction columns consisting of SPE tubes containing silica-based reversed-phase packing. By applying a vacuum to draw the solution through the packing, product separation from hydrophobic and hydrophilic impurities is accomplished in a two-stage purification unit: a short pre-column functions as a trap for hydrophobic impurities, while a second, longer SPE column is used as a product isolation column. Thus, under ideal SDC conditions, washing with a 100% aqueous solvent will achieve retention of hydrophobic impurities on the trap, with displacement of product and hydrophilic impurities from the trap to the product isolation column; hydrophilic impurities are thus displaced off the product isolation to waste, leaving only product retained on the main column. In this initial evaluation, this purification system has demonstrated excellent separation of product, in good yield, from both hydrophilic and hydrophobic impurities over a wide range of peptide hydrophobicity and crude composition for model synthetic peptide systems representing crude peptide mixtures.

Amino Acid Sequence↗

PubMed2000

Trapping the monomeric alpha-helical state during unfolding of coiled-coils by reversed-phase liquid chromatography.

Reversed-phase liquid chromatography (RPLC) offers a unique opportunity to monitor the transition from the native state (N) to the structural intermediate state (I) for proteins whose secondary structure is comprised entirely of amphipathic helices, such as coiled-coils. During RPLC, the hydrophobicity of the stationary phase and mobile phase results in the unfolding of the tertiary/quaternary structure of coiled-coils but retains alpha-helical secondary structure and thus isolates the I state. A set of five peptides, alphaalpha-36, betabeta-36, alphabeta-36, gammadelta-36 and omegaomega-36, was generated by shuffling guest hydrophobes at equivalent sites in a symmetric host frame. In one of the peptides, omegaomega-36, all the alpha-glutamic residues in the host frame were replaced by gamma-glutamic residues. alphaalpha-36, betabeta-36, alphabeta-36, gammadelta-36 form two-stranded coiled-coils of identical helical content and unfold as a two-state transition during temperature denaturation while the fifth peptide, omegaomega-36, is a random coil and cannot be induced in to an alpha-helical structure even in the presence of a helix inducing solvent, 50% trifluoroethanol. By comparing the stability order of the four coiled-coils in the N-->I transition (measured by RPLC studies) with that in the N-->D (denatured state) transition (measured by calorimetry), it is concluded that there is a direct correlation between the relative stabilities of these peptides in these two unfolding transitions. This result supports a hierarchical folding mechanism for coiled-coils.

Amino Acid Sequence↗

PubMed2000

Diastereoisomeric analogues of gramicidin S: structure, biologicalactivity and interaction with lipid bilayers.

Analogues of a structurally equivalent version of theantimicrobial decameric cyclic peptide gramicidin S, GS10 [cyclo-(Val-Lys-Leu-d-Tyr-Pro)(2)], were designed to study theeffect of distortion in the beta-sheet/beta-turn structure of thecyclic peptide on its biological activity. In one approach, thehydrophobic nature of GS10 was conserved, and single amino acids in itsbackbone were replaced systematically with their correspondingenantiomers to give five diastereoisomeric analogues. In a relatedapproach, a more basic and hydrophilic analogue of GS10 [cyclo-(Lys-Val-Lys-d-Tyr-Pro(5)-Lys-Leu-Lys-d-Tyr-Pro(10))], together with two of itsmonosubstituted diastereoisomeric analogues (featuring d-Lys(1) or d-Val(2) respectively), weresynthesized. CD spectra were measured in a variety of environments,i.e. aqueous, aqueous trifluoroethanol and those containing SDSmicelles or phospholipid vesicles. In comparison with GS10 spectra, CDspectra of both groups of analogues in these environments exhibitedstructural distortion. Moreover, compared with GS10, antimicrobial andhaemolytic activities of the analogues were drastically decreased, implying the existence of a threshold minimum amphipathicity foreffective biological activity. However, in both groups of analogues,there was a correlation between amphipathicity and antimicrobial andhaemolytic activities. In the second group of analogues, bothelectrostatic and hydrophobic factors were related to theirantimicrobial and haemolytic activities. In order to gain an insightinto the nature of the biological activity of the two classes of cyclicpeptides, the relationship of their structure to interaction with lipidmembranes, and the implied mechanisms, were analysed in some detail inthe present study.

Amino Acid Sequence↗