First principle computational study on the full conformational space of L-proline diamides

Ab initio molecular orbital computations were carried out at three levels of theory: RHF/3-21G, RHF/631G(d), and WLYP/6-31G(d), on four model systems of the amino acid proline, HCO-Pro-NH2 [I], HCOPro-NH-Me [II], MeCO-Pro-NH2 [III], and MeCO-Pro-NH-Me [IV], representing a systematic variation in the protecting N- and C-terminal groups. Three previously located backbone conformations, gamma(L), epsilon(L), and alpha(L), were characterized together with two ring-puckered forms syn (gauche(+) = g(+)) or DOWN and anti (gauche(-) = g(-)) or UP, as well as trans-trans, trans-cis, cis-trans, and cis-cis peptide bond isomers. The topologies of the conformational potential energy cross-sections (PECS) of the potential energy hypersurfaces (PEHS) for compounds [I]-[IV] were explored and analyzed in terms of potential energy curves (PEC), and HCO-Pro-NH2 [I] was also analyzed in terms of potential energy surfaces (PESs). Thermodynamic functions were also calculated for HCO-Pro-NH2 [I] at the CBS-4M and G3MP2 levels of theory. The study confirms that the use of the simplest model, compound [I] with P-N = P-C = H, along with the RHF/3-21G level of theory, is an acceptable practice for the analysis of peptide models because only minor differences in geometry and stability are observed.