Conformational species of gramicidin A in non-polar solvent. A kinetic and thermodynamic treatment in the absence and presence of phosphatidylcholine as studied by high-performance liquid chromatography.

A kinetic and thermodynamic study has been carried out to characterize quantitatively the conformational equilibrium of gramicidin A (GA) in tetrahydrofuran at different peptide concentrations in the absence and presence of egg yolk phosphatidylcholine by using size-exclusion high-performance liquid chromatographic analysis. In the absence of lipid, the experimental data fit a simple dimer-monomer equilibrium, the rate and equilibrium constants for the dissociation process being (1.6 +/- 0.7) X 10(-7) s-1 and (8.5 +/- 0.3) X 10(-6) M, respectively. A higher extent of monomerization and a decrease in the time required for reaching equilibrium are detected in the presence of phospholipid, the kinetic and thermodynamic effects depending on both lipid and GA concentrations. In order to account for these observations a cyclic equilibrium mechanism is proposed which is analysed in terms of four conformational species, namely, free monomer, free dimer, lipid-bound monomer and lipid-bound dimer. The results obtained are discussed in relation to recent literature data on lipid-protein interactions.