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:
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 ~103 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.]. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.
Transfer of Cs-gA complex into the TX-100 micelles and dioxane solution activates a cascade of sequential conformational transitions monitored by CD and FT-IR spectroscopy. Kinetics of the transitions was measured and the activation parameters determined. The results are discussed in the light of local unfolding, (or “cracking”) model of large scale conformational transitions.