Predictor for sulfur-aromatic interactions in globular proteins.

The predictor, Y = 2.54 (formula: see text), where Y is the number of sulfur-aromatic interactions in a globular protein and N is the total number of amino acids in the protein, accounts for 75% of the variation in this type of interaction that occurs in 22 globular proteins whose three-dimensional structure has been determined. We find that S-pi interactions are not random events but rather are the outcome of a competition between the dipolar groups, amides and sulfur-containing, for ring neighbors. This outcome is strongly weighted in favor of S-pi interactions by the presence in the protein of positively charged side-chains; it is not affected by the number of strictly non-polar side-chains.