Surfactant Effects on Tautomeric and Microscopic Equilibria of Tryptophan: Experimental and Theoretical Studies

Surfactant molecules are used as interesting tools to study the structure, function and stability of proteins. Protonation states of amino acids may be changed in the presence of surfactants. In this work, using experimental observations and molecular dynamic simulation, the effects of sodium dodecyl sulfate on the acid dissociation constants of tryptophan was examined. The acid–base equilibrium of tryptophan molecules was examined in the aqueous solution and in the presence of different concentrations of SDS. Different concentrations of SDS have diverse effects on the values of pKa1. However, the effect of SDS on pKa2 is similar in all concentration ranges. Furthermore, the microconstants related to the same equilibria (k11, k22, k12, and k21) have different values in the presence and absence of SDS. These results show the different protonation states of tryptophan molecules in the presence and absence of surfactant. Molecular dynamics simulation showed that in the absence of SDS, tryptophan molecules form molecular aggregates similar to the result of stacking. But in the presence of SDS, stacking between tryptophan molecules is disrupted by hydrophobic and hydrogen bonding interactions of surfactant.