EQUILIBRIUM AND NONEQUILIBRIUM CONFORMATIONS OF PEPTIDES IN LIPID BILAYERS

A synthetic, hydrophobic, 27-amino-acid-residue peptide 'K27', modelle d on the trans-membrane domain of the slow voltage-gated potassium cha nnel, IsK, has been incorporated into a lipid bilayer and its conforma tional properties studied using FT-IR spectroscopy. The conformation f ollowing reconstitution is found to be dependent on the nature of the solvent employed. When the reconstitution is conducted by solvent evap oration from a methanol solution, aggregates comprised of beta-strands are stabilised and their concentration is essentially invariant with time. By contrast, when trifluoroethanol is used, the initial conforma tion of the peptide is alpha-helical. This then relaxes to an equilibr ium state between alpha-helices and beta-strands. The alpha-helix-to b eta-strand conversion rate is relatively slow, and this allows the kin etics to be studied by FT-IR spectroscopy. The reverse process is much slower but again can be demonstrated by FT-IR. Thus, it appears that a true equilibrium structure can only be achieved by starting with pep tide in the alpha-helical conformation. We believe this result should be of general validity for hydrophobic peptide reconstitution. The imp lications for conformational changes in membrane proteins are discusse d. (C) 1997 Elsevier Science B.V.