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

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 pK(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, beta-yr, beta-Hty, Phg, Tic(OH)) have a minor influence on the value of pK(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 alpha-hydrogen atom in Dopa by alpha-methyl group caused increase of pK(a) to the value observed for tyrosine. The pK(a) values of acetyl group of amino acid studied are in the range from 0.3 to 0.6. (C) 2002 Elsevier Science B.V. All rights reserved.