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 conformational potential energy surface of the gamma(L) or C-7(eq) backbone conformation of For-L-Phe-NH2, it was found that there are three conformations (g +, a and g -) due to rotation about the C-alpha x C-beta bond. It should be emphasised that the gamma(L) backbone conformation is conserved during rotation about chi(1). However, there is only one unique conformation along the rotation about the C-beta chi 2 Ph bond. The -CH2-Ph group showed greater stabilisation, with respect to hydrogen (Gly), than the -CH3 (Ala) or -CH2-OH (Ser) substituents. The hydrogen-bonded C=O (amide 1) vibrational frequency is split into two bands due to the coupling of the C=O stretching and -NH2 scissoring modes of motion. The other carbonyl, not involved in hydrogen bonding, has a characteristic single IR band with a relatively high frequency. The orientation of the -Ph group has no appreciable effect on these vibrational frequencies.