The observed V3 torsional barriers reported for nine methyl groups attached alpha to peptide bond linkages in five gas phase biomimetics are interpreted at the HF/6311++G(d,p) level of theory in terms of the natural bonding orbitals and the natural resonance structures of the peptide bond. This decomposition has revealed that delocalization of the nitrogen lone pair electrons into anti-bonding orbitals of the carbonyl group is principally responsible for the V3 torsional barrier and lowest energy staggered or anti conformation of the carbonyl methyl group. In contrast, the minimum energy configuration and V3 barriers of the amide methyl groups are dominated by Lewis-like steric interactions that lead to syn preferences. The Lewis vs non-Lewis energies are sufficiently well balanced that low barriers (<0.5 kcal/mole) result in both methyl group classes. These results reveal that a linear correlation exists between the barriers to internal rotation of attached methyl groups and the relative importance of the two principal resonance structures that contribute to the peptide bond.
Citation: Journal of Physical Chemistry A
Pub Type: Journals
Lewis structures, peptide bonds, torsional barriers