Peptide models XXVII. An exploratory ab initio study on the 21(st) amino acid side-chain conformations of N-formyl-L-selenocysteinamide (For-L-Sec-NH2) and N-acetyl-L-selenocysteine-N-methylamide (Ac-L-Sec-NHMe) in their gamma(L) backbone conformation

Selenocysteine is expected to have 9 x 9 = 81 conformations [3 x 3 = 9 back bone: psi (g(+),a,g(-)) x phi (g(+),a,g(-)) and 3 x 3 = 9 side-chain: chi(1 ) (g(+),a,g(-)) x chi(2) (g(+),a,g(-))]. In the present study, all the tors ional modes of the side-chain (chi(1): rotation about the C-alpha-C-beta an d chi(2): rotation about the C-beta-Se bonds) were investigated in the rela xed gamma(L) backbone [(phi,psi); (g(-),g(+))] conformation. Seven out of t he nine expected minima were found at the RHF/3-21G level of theory for N-f ormyl-L-selenocysteinamide (For-L-Sec-NH2) and N-acetyl-L-selenocysteine-N- methylamide (Ac-L-Sec-NHMe). The stabilization energy exerted by the -CH2-S eH side-chain has been compared with that of -CH2-SH and -CH2-OH. Relative energies of the various conformers were also obtained via single point calc ulations at the B3LYP/6-31G(d,p) level of theory. Topological analysis of t he electron density has been performed by Bader's Atoms in Molecule (AIM) a pproach using the results. The structures were also optimized at the B3LYP/ 6-31+G(d,p) level of theory.