Computational Study of the C-H∙∙∙H-C Contacts in Valine-Methane Complexes.

A series of complexes between neutral Valine and methane that feature potential homopolar C-H∙∙∙H-C contacts were located on the MP2/aug-cc-pVTZ potential energy hypersurface. In order to better estimate the strength of this contacts, the interaction energies were improve by single-point calculations at different levels of theory (MP2, CCSD(T), SAPT2, SAPT2+3) together with Dunning’s basis sets (aug-cc-pVXZ; X=T,Q,5). Topological analysis of the electron density within the QTAIM framework, NCI plots and energy decomposition within the SAPT framework were used to discuss the nature of this interactions. The complexes whose monomers only interact though C-H∙∙∙H-C contacts indicate that these interactions are entirely due to dispersion forces, are not directional and are much stronger than expected (the interaction energies of the complexes range from -0.7 to -1.0 kcal/mol). This large value is remarkable considering the small size of the interacting groups herein considered (methane, and one or two Valine’s methyl groups), and indicates that in biological systems, where those interactions can be very numerous in the presence of multiple aliphatic amino acids, if those interactions are not properly model, magnitudes as ligand-receptor affinities, protein-protein interaction energies and protein stabilities might be grossly misestimated. Finally, since some of the computed complexes also include stronger interactions than homopolar C-H∙∙∙H-C contacts, we analyzed if the potential C-H∙∙∙H-C contacts in these complexes are really contributing to stabilize the complexes or are just a geometrical artifact arising from the maximization of stronger interactions.