HPLC Characterization of the Gramicidin A Dimer-Monomer Conformational Equilibrium in Ethanol and Study of the Effect of Calcium Ion

Direct analysis of gramicidin double helices using packed column supercritical fluid chromatography

Direct analysis of the monomeric and four double helical dimeric conformations of gramicidin has been achieved using packed column supercritical fluid chromatography (pSFC). Using a PRP-1 polymeric column and typical conditions of 40 degrees C column temperature, 25 MPa column pressure, and 35% n-pentanol modifier addition, all of the gramicidin conformers were readily separated. To evaluate the method, the dynamic characteristics of the monomer and dimer species were monitored as a function of solvent type, incubation time, solvent temperature, and initial concentration. The findings agree with those previously obtained by other methods but also yield new information about the relative amounts of two closely related dimers (species 1 and 2) as well as the simultaneous changes in the full dimer/monomer distribution. Results indicate that the developed pSFC method can be an informative complimentary tool for readily monitoring changes in the full profile of gramicidin species present in different environments.

Size-exclusion high-performance liquid chromatography in the study of the autoassociating antibiotic gramicidin A in micellar milieu

Gramicidin A (gA) is a polypeptide antibiotic which forms dimeric channels specific for monovalent cations in biological membranes. It is a polymorphic molecule that adopts several different conformations, double-stranded (ds) helical dimers (pore conformation) and single-stranded beta-helical dimers (channel conformation). This study investigated the conformational adaptability of gramicidin A when incorporated into micelles as membrane-mimetic model system. Taking advantage of our reported, versatile, size-exclusion high-performance liquid chromatography (SE-HPLC) strategy that allows the separation of double-stranded dimers and monomers, we have quantitatively characterized the conformational transition undergone by the peptide in the micellar milieu. The importance of both hydrophobic/hydrophilic moieties of the amphipaths in the stabilization of concrete conformational species is demonstrated using detergents with different hydrocarbon chain length and/or polar head. SE-HPLC is a valuable, rapid, accurate technique for the structural characterization of hydrophobic autoassociating peptides that work in lipid environments such as biological membranes.

High-performance liquid chromatographic separation of modified and native melittin following transglutaminase-mediated derivatization with a dansyl fluorescent probe

The 26-amino acid linear, amphiphilic peptide melittin was enzymatically modified with the fluorescent probe monodansylcadaverine using guinea pig liver transglutaminase and a fluorescent derivative of stoichiometry 1:1 was obtained. Reversed-phase and size-exclusion high-performance liquid chromatographic modes were tested in order to resolve the labelled peptide and native species. The influence of several operational variables was analysed and the elution conditions were optimized so that a satisfactory resolution could be achieved in both instances in a rapid, easy manner. Both chromatographic modes offer the possibility of accurate monitoring of the time course of the enzyme-mediated conversion and more interestingly, can be applied to the semi-preparative purification of the labelled peptide.

HPLC study on the 'history' dependence of gramicidin A conformation in phospholipid model membranes

A novel HPLC methodology for the study of gramicidin A reconstituted in model membranes has been tested in comparison with circular dichroism data. It is shown that this chromatographic technique not only corroborates most of the recent spectroscopic results but allows one to explain them in terms of mass fractions of different actual conformational species of GA in the phospholipid assemblies. In particular, the dependence of the inserted peptide configuration on the organic solvent and other parameters involved in the 'history' of the sample preparation and handling has been analyzed by HPLC in two phospholipid model systems: small unilamellar vesicles and micelles. Moreover, a slow conformational transition of GA towards a beta 6.3-helical configuration, accelerated by heat incubation, has been also chromatographically visualized and quantitatively interpreted.

New high-performance liquid chromatography-based methodology for monitoring the conformational transitions of self-associating hydrophobic peptides, incorporated into liposomes

A new high-performance size-exclusion chromatographic strategy is reported for the analysis of the hydrophobic self-associating peptide gramicidin A, incorporated into artificial phospholipid vesicles (liposomes). The method is based on the direct injection of a few microlitres of the gramicidin A-containing liposome suspension into the column, which is eluted with a non-polar solvent, such as tetrahydrofuran. The type and amount of information which can be derived from this methodology have been evaluated. Using this chromatographic approach, a correlation has been unambiguously shown to exist between the organization of the peptide in the vesicles and a number of variables involved in the method of preparation of liposomes. Finally, a gramicidin A conformational transition has been monitored in the phospholipid vesicles which proved to be dependent on the class of phospholipid present in the liposome.

Analysis of the binding of Ca2+ to gramicidin A in ethanol in terms of the dimer-monomer conformational equilibrium

This study reports the first direct observation of the binding of Ca2+ to gramicidin A in ethanol, analysed in terms of the polypeptide dimer-monomer conformational equilibrium. High performance size-exclusion chromatography has been successfully used to elucidate the binding mechanism and to determine the rate constants as well as the stoichiometry of the processes involved. In addition, fluorescence intensity and anisotropy measurements have revealed a dependence of the number of accessible Ca2+-binding sites on the peptide concentration.