The effect of temperature and lipid on the conformational transition of gramicidin A in lipid vesicles

The present study investigated the effect of temperature and lipid/peptide molar ratio on the conformational changes of the membrane peptide gramicidin A from a double-stranded helix to a single-stranded helical dimmer in 1,2-dimyristoyl-glycerol-3-phosphochloine (DMPC) vesicles. Tryptophan fluorescence spectroscopy results suggested that the conformational transition fitted a three-state (two-step) "folding" model. Rate constants, k₁ and k₂, were determined for each of the two steps. Since k₁ and k₂ increased with an increase in temperature, we hypothesized that the process corresponded to the breakage and formation of the backbone hydrogen bonds. The k₁ was from 10 to 45 folds faster than k₂, except for lipid/peptide molar ratios above 89.21, where k₂ increased rapidly. At molar ratios below 89.21, k₂ was insensitive to changes in lipid concentration. To account for this phenomenon, we proposed that while the driving interaction at high molar ratios is between the indole rings of the tryptophan residues and the lipid head groups, at low molar ratios there may be an intermolecular interaction between the tryptophan residues that causes gramicidin A to form an organized aggregated network. This aggregated network, caused by the tryptophan-tryptophan interaction, may be the main effect responsible for the slow down of the conformation change.