Large scale conformational transitions in β-structural motif of gramicidin A: kinetic analysis based on CD and FT-IR data.

Gramicidin A (gA) is a polypeptide antibiotic, which forms dimeric channels specific for monovalent cations in artificial and biological membranes. It is a polymorphic molecule that adopts a unique variety of helical conformations, including antiparallel double-stranded ↑↓β5.6 or ↑↓β7.2 helices (number of residues per turn) and a single-stranded β6.3 helix (the 'channel form'). The behavior of gA-Cs(+) complex in the micelles of TX-100 was studied in this work. Transfer of the complex into the micelles activates a cascade of sequential conformational transitions monitored by CD and FT-IR spectroscopy: [Formula: see text] At the first step after Cs(+) removal, the RH ↑↓β5.6 helix is formed, which has been discussed so far only hypothetically. Kinetics of the transitions was measured, and the activation parameters were determined. The activation energies of the ↑↓β5.6 → β-helical monomer transition in dioxane and dioxane/water solutions were also measured for comparison. The presence of water raises the transition rate constant ~10(3) times but does not lead to crucial fall of the activation energy. All activation energies were found in the 20-25 kcal/mol range, i.e. much lower than would be expected for unwinding of the double helix (when 28 H-bonds are broken simultaneously). These results can be accounted for in the light of local unfolding (or 'cracking') model for large scale conformational transitions developed by the P. G.Wolynes team [Miyashita O, Onuchic JN, Wolynes PG. Proc. Natl. Acad. Sci. USA 2003; 100: 12570-12575.].