Comparative Analysis of Sulfonium-pi, Ammonium-pi, and Sulfur-pi Interactions and Relevance to SAM-Dependent Methyltransferases

We report the measurement and analysis of sulfonium-pi, thioether-pi, and ammonium-pi interactions in a beta-hairpin peptide model system, coupled with computational investigation and PDB analysis. These studies indicated that the sulfonium-pi interaction is the strongest and that polarizability contributes to the stronger interaction with sulfonium relative to ammonium. Computational studies demonstrate that differences in solvation of the trimethylsulfonium versus the trimethylammonium group also contribute to the stronger sulfonium-pi interaction. In comparing sulfonium-pi versus sulfur-pi interactions in proteins, analysis of SAM- and SAH-bound enzymes in the PDB suggests that aromatic residues are enriched in close proximity to the sulfur of both SAM and SAH, but the populations of aromatic interactions of the two cofactors are not significantly different, with the exception of the Me-pi interactions in SAM, which are the most prevalent interaction in SAM but are not possible for SAH. This suggests that the weaker interaction energies due to loss of the cation-pi interaction in going from SAM to SAH may contribute to turnover of the cofactor.