The study of the interactions between surfactants and proteins is of significant scientific and technological importance[1]. Proteindetergent interactions highlight the multifarious ways in which proteins respond to changes in their environment . Ionic detergents can denature proteins by strong binding to charged and hydrophobic side chains at millimolar concentrations, unlike chemical denaturants such as guanidinium chloride or urea, which are only effective at molar concentrations, presumably due to weak binding to the protein backbone[2]. The present work is an attempt to study the Effect of anionic surfactant SDS on the dissociation constants of the amino acid tryptophan. Tryptophan solution titration was carried out by HCl and NaOH in the absence and presence of different concentrations of SDS (1, 2, 5 and 10 mM). The results show that the effect of SDS on pKa1values is different at concentrations below and above the cmc، which indicates that SDS increases the acidity of the amino group at the concentrations below the cmc, whereas it decreases the acidity of this group in the concentrations above the cmc. However, the effect of SDS on pKa2 is similar in both concentration ranges (below and above the cmc) i.e. SDS increases the acidity of the amino group.

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