The Ionization Constants of Organic Compounds. I. The Microscopic Ionization Constants of Tyrosine and Related Compounds

Abstract

Abstract A method has been formulated for the estimation of the microscopic ionization constants from spectroscopic data which measure the fractional ionization of one of the groups as a function of the pH value. This method has been applied at the ionic strength of 0.05 to the ionization of tyrosine (I), tyramine (II), 3-iodotyrosine (III), tyrosine methyl ester (IV), tyrosine ethyl ester (V), 3,5-diiodotyrosine (VI), and 3,5-dibromotyrosine (VII). The microscopic constants obtained are 9.55, 9.57, 10.08, 10.06 for I; 9.70, 10.09, 10.72, and 10.33 for II; 8.00, 8.70, 9.52 and 8.82 for III; 8.41, 7.27, 8.99 and 10.13 for IV, and 8.43, 7.31, 8.97 and 10.09 for V, in the order of pka, pkb, pkc, and pkd. For VI and VII, the pka values are 6.44 and 6.29, and the pkb values are 9.36 and 9.22, respectively. It has been illustrated that the method may be applied even in cases where ka<<kb. This approach can probably be extended to other Systems containing two dissociable protons, one of which affects the ultraviolet absorption and the other does not.