PEPTIDE MODELS .19. SIDE-CHAIN CONFORMATIONAL ENERGY SURFACE E=F((CHI-1,CHI-2)) AND AMIDE-I VIBRATIONAL FREQUENCIES OF N-FORMYL-L-PHENYLALANINAMIDE (FOR-PHE-NH2) IN ITS GAMMA(L) OR GAMMA(INV) OR C-7(EQ) BACKBONE CONFORMATION

In a study of cross sections of the E = f(chi(1),chi(2)) side-chain co nformational potential energy surface of the gamma(L) or C-7(eq) backb one conformation of For-L-Phe-NH2, it was found that there are three c onformations (g +, a and g -) due to rotation about the C-alpha x C-be ta bond. It should be emphasised that the gamma(L) backbone conformati on is conserved during rotation about chi(1). However, there is only o ne unique conformation along the rotation about the C-beta chi 2 Ph bo nd. The -CH2-Ph group showed greater stabilisation, with respect to hy drogen (Gly), than the -CH3 (Ala) or -CH2-OH (Ser) substituents. The h ydrogen-bonded C=O (amide 1) vibrational frequency is split into two b ands due to the coupling of the C=O stretching and -NH2 scissoring mod es of motion. The other carbonyl, not involved in hydrogen bonding, ha s a characteristic single IR band with a relatively high frequency. Th e orientation of the -Ph group has no appreciable effect on these vibr ational frequencies.