A computational study of partially modified retro-inverso valine dipeptides:: Effect of the side chain on the conformational preferences of malonyl and gem-diamino residues

A high-level study of the partially modified retro-inverso compounds derived from valine dipeptide, i.e., CH3NHCOCHRCONHCH3 and CH3CONHCHRNHCOCH3 where R = -CH(CH3)(2), is presented. Minimum energy conformations were obtained at the HF/6-31G(d) and MP2/6-31G(d) levels from a systematic conformational analysis. The effects of electron correlation and basis set on relative energies were investigated by performing single point calculations at the HF/6-311G(d,p) and MP2/6-311G(d,p) levels on both HF/6-31G(d) and MP2/6-31G(d) geometries. Furthermore, the influence of polar solvents, i.e., water and methanol, on the conformational preferences of the two compounds was determined using the polarizable continuum model. A comparison with results previously obtained for other partially modified retro-inverso dipeptides promoted an understanding of how the side chain affects the conformational preferences of the malonyl and gem-diamino residues.