Abstract
In the dye-binding method, the absorbance increase caused by a protein error of a pH indicator is observed only in a restricted pH range. However, this pH range in the presence of a detergent has not yet been examined. Thus, the author investigated the pH (pHUL) where the absorbance increase becomes zero by a calculation based on the chemical equilibrium of a protein error of a pH indicator, and by experiments using four sulfonephthalein dyes. The pHUL value changed only with the detergent concentration, but did not change at all due to the dye, buffer solution or protein concentrations. Although the pHUL value was different according to the kind of dye used, it correlated well with the pKD values (dissociation constant) of BPB, BCG, BCP and BTB. The characteristics of pHUL in the reactions of the four dyes indicated good agreement with that obtained by a calculation.
