Spectrophotometric titration of ionisable groups in proteins: a theoretical study

Abstract

A simple theoretical model is proposed for evaluation of the optical titration behaviour of tyrosyl and carboxyl residues in proteins. The p K values involved in the model are computed using the semi-empirical method. The titration curves are calculated using the values of the molar absorption differences for tyrosyl residues in the ultraviolet (UV) region at 245 and 295 nm, and for carboxyl residues in the infrared (IR) region at 1565 and 1707 cm −1, respectively. The theoretical tyrosyl titration curves are compared with the experimental data for lysozyme, myoglobin and chymotrypsinogen (available in the literature). This approach provides a good tool for distinguishing between the ionisation and the conformational changes in the alkaline range. The quantitative evaluation of the change of molar extinction coefficients as a function of pH in the case of carboxyl titration for lysozyme, trypsin and cytochrome c shows a good agreement with the experimental titration data.