STUDY OF THE AMIDE-CENTER-DOT-CENTER-DOT-CENTER-DOT-ESTER HYDROGEN-BOND IN SMALL MOLECULES AND ITS INFLUENCE ON THE CONFORMATION OF POLYPEPTIDES AND RELATED POLYMERS

We present a theoretical investigation of the hydrogen bonding between formamide and (Z)-methyl acetate as a model system for the amide ester interaction that may occur in certain polypeptides and related polymers. We have performed a full-geometry optimization of the complex in various geometrical arrangements within the ab initio molecular orbital theory at the Hartree-Fock (HF) level with the 6-31G(d) basis set. The results have been extended by performing HF and MP2 calculations with the 6-31++G(d,p) and 6-31G(d) basis sets, respectively. Computed interaction energies revealed that the more favorable interaction occurs between the amide and the carbonyl oxygen of the ester group. Thus, the strongest form of the amide ester hydrogen bonding appeared to be of similar strength that amide amide interaction. Solvent effects on the amide ester hydrogen bonding have been investigated using self-consistent reaction-field (SCRF) calculations with the 6-31G(d) basis set. The results indicated that the amide ester interactions are greatly modulated by the solvent and display a poor tendency to form molecular associations in an aqueous medium. To understand the role played by amide ester interactions on the conformational preferences of polypeptides and related polymers, semiempirical calculations were carried out on poly(alpha-L-aspartate)s, poly(beta-L-aspartate)s and poly(alpha-L-glutamate)s. The intramolecular interactions between the amide and side chain ester groups proved to have a determinant influence on the preferred helix sense of the three families of compounds. For the poly(alpha-L-glutamate)s the right-handed beta-helix was the most favored conformation, whereas for poly(alpha-L-aspartate)s both right- and left-handed alpha-helices are feasible depending on the nature of the substituent. Regarding poly(beta-L-aspartate)s, the right-handed helix was preferred due to the presence of a C-5 interaction between the amide group of the main chain and the carbonyl oxygen of the ester group. These results are in excellent agreement with previous X-ray diffraction data. Finally, hydrogen-bond geometries of compounds determined experimentally have been obtained from the Cambridge Structural Data Base. The results have been analysed and compared with the theoretical models derived in this work.