Acidity of carboxyl group of tyrosine and its analogues and derivatives studied by steady-state fluorescence spectroscopy

Abstract The acidity of the carboxyl group of tyrosine and its derivatives and analogues was studied by means of fluorimetric titration using a steady-state fluorescence method. The p K a value of carboxyl group of tyrosine, its analogues and derivatives with blocked amino or hydroxyl group or both determined from the fluorimetric titration curve indicates that the methylation of hydroxyl group of phenolic ring, as well as the position of carboxyl group with respect to the phenol ring (Tyr, β-Tyr, β-Hty, Phg, Tic(OH)) have a minor influence on the value of p K a . The conversion of a protonated amino group of tyrosine or its analogues to the N -acetyl derivatives or its removal result in major lowering of the acidity of carboxyl group. The introduction of an additional hydroxyl group into a phenolic ring (Dopa) slightly increased acidity of the carboxyl group compared to tyrosine, whereas the replacement of α-hydrogen atom in Dopa by α-methyl group caused increase of p K a to the value observed for tyrosine. The p K a values of acetyl group of amino acid studied are in the range from 0.3 to 0.6.