HPLC in the characterisation of conformational species of linear gramicidins

High-performance size-exclusion chromatography (HPSEC) has proved to be a highly simplified and rapid procedure to investigate the conformational behaviour of gramicidin A inserted in different model membrane systems (lipid dispersions, liposomes, micelles) based on the separation of double-stranded dimers and monomers present in the lipid assemblies. The HPSEC approach has been extended to the characterisation of acylated analogs of gramicidin (palmitoyl- and oleoyl-derivatives) and a series of peptide analogs where tryptophan residues were chemically modified or replaced by less polar residues (phenylalanine or naphtylalanine) lacking H-bonding ability. The Chromatographic results indicate that, in fresh preparations, the acylation modifies the dimer/monomer relationship also depending on the peptide/lipid molar ratio and membrane composition. The alteration of tryptophan residues either by chemical modification or substitution by more hydrophobic amino acids causes an appearance of double-stranded dimers, more significant as the number of tryptophan-substitutions increases and is also influenced by the specific location of the tryptophan residues in the sequence.