Contribution of cation-pi interactions to protein stability

Calculations predict that cation- interactions make an important contribution to protein stability. While there have been some attempts to experimentally measure strengths of cation-pi interactions using peptide model systems, much less experimental data are available for globular proteins. We have attempted to determine the magnitude of cation-pi interactions of Lys with aromatic amino acids in four different proteins (LIVBP, MBP, RBP, and Trx). In each case, Lys was replaced with Gln and Met. In a separate series of experiments, the aromatic amino acid in each cation-pi pair was replaced by Leu. Stabilities of wild-type (WT) and mutant proteins were characterized by both thermal and chemical denaturation. Gln and aromatic --> Leu mutants were consistently less stable than corresponding Met mutants, reflecting the nonisosteric nature of these substitutions. The strength of the cation-pi interaction was assessed by the value of the change in the free energy of unfolding [DeltaDeltaG(degrees) = DeltaG(degrees)(Met) - DeltaG(degrees)(WT)]. This ranged from +1.1 to -1.9 kcal/mol (average value -0.4 kcal/mol) at 298 K and +0.7 to -2.6 kcal/mol (average value -1.1 kcal/mol) at the Tm of each WT. It therefore appears that the strength of cation-pi interactions increases with temperature. In addition, the experimentally measured values are appreciably smaller in magnitude than calculated values with an average difference /DeltaG(degrees)expt - DeltaG(degrees)calc/av of 2.9 kcal/mol. At room temperature, the data indicate that cation-pi interactions are at best weakly stabilizing and in some cases are clearly destabilizing. However, at elevated temperatures, close to typical Tm's, cation-pi interactions are generally stabilizing.