Selenocysteine derivatives I. Sidechain conformational potential energy surface of N-acetyl-L-selenocysteine-N-methylamide (MeCO-L-Sec-NH-Me) in its beta(L) backbone conformation

Selenocysteine is expected to have 92=81 conformations [in the backbone: y(g+,a,g-)¥f(g+,a,g-); 32=9 and in the sidechain: c1(g+,a,g-)¥c2(g+,a,g-); 32=9]. All torsional modes of the sidechain (c1: rotation about the Ca–Cb and c2: rotation about the Cb–Se bonds) were investigated in the relaxed bl backbone [(y,f), (a,a)] conformation. The relaxed potential energy surface (PES) obtained at the RHF/3-21G level of theory contained seven out of nine possible minima of the sidechain. All minima were re-optimized at the RHF/6-31G(d) and the B3LYP/6-31G(d) levels of theory. Two of the minima (g+a) and (g-g+) located at RHF/3-21G level of theory were annihilated when optimized at higher levels of theory. The frequency calculations for the found minima were used to construct the thermodynamic functions. The relative energies of the –CH2–SeH sidechain conformations have been compared with the relative energies of the analogous –CH2–SH and –CH2–OH sidechain conformers. Oxidative dimerization energies were also estimated