STUDIES ON THE CONFORMATION OF BOC-PROTECTED (S)-(-ISOVALINE HOMOPEPTIDE METHYL-ESTERS IN THE SOLID-STATE AND IN SOLUTION())

X-Ray diffraction analyses of the fully protected peptides Boc-[(S)-Iv a](n)-OMe (n = 3, 4, 6) reveal two independent molecules in the asymme tric unit. The structures of these can be described as beta-turns or 3 (10) helices (depending on the length of the oligopeptide) of alternat ing screw sense (M and P) in a head to tail alignment. This structure is stabilized by hydrogen bonds between the N-H(1) of the (M)-helix an d the O=C(omega-1) of the (P)-helix and the N-H(2) (M) and the ester c arbonyl group (P). Low temperature H-1-NMR spectra of the hexamer in C D2Cl2 solution show two interchanging species in a ratio of 4:1; NOESY experiments prove that these are the two helical conformers found in the crystal (P:M, 4:1). The NOESY spectrum at -90 degrees C indicates the pairing of (P) and (M) helices. Thermodynamic and kinetic paramete rs for the helix transformation P reversible arrow M (unfolding/foldin g) are presented.