In this paper we extend our theoretical studies dealing with the dependence of
relative proton and carbon chemical shifts (CSs) of protein backbone atoms on their
conformational position. In an earlier paper (A. Czajlik, I. Hudáky, A. Perczel, J Comp
Chem 2011, 32, 3362) we reported on a fair agreement between calculated and
observed backbone CSs as a function of backbone conformation. Applying the
polarizable continuum model (PCM) in this work, we compare relative CSs of fully
optimized alanine diamide conformers with gas phase calculations and experimental
results. Along a path on the Ramachandran surface, we collated calculated relative
CSs obtained with and without explicit water molecules, as well as with and without
considering the PCM reaction field. Furthermore, we traced the energetically relevant
reaction paths along the torsional angle ψ connecting the lowest energy minima
(helical, extended, polyproline II and inverse γ-turn) on the Ramachandran plot, with
the prospect to facilitate identifying them by their relative CSs. We found that
consideration of the solvent effect of the environment around a diamide model
improves the agreement with experimental findings on abundant conformers. This
agreement is of the level achieved previously by a thorough gas phase investigation
on considerably larger oligoalanine models. By relating DeltaδCα, DeltaδHα and DeltaδCβ values
of polyproline II and inverse γ-turn to the experimentally well characterized helical
and extended data, our calculations contribute to protein secondary structure
prediction based on nuclear magnetic CS
