Micro-ionization constants for tyrosine

Abstract

Macro-ionization constants for tyrosine at 25°C (9.21; 10.46), 60°C (8.51; 10.09) and 90°C (8.06; 10.00) have been determined from pH titration data. The use of spectrophotometric data over the pH range 9–10.5 in conjunction with the titration data has provided micro-ionization, constants at these temperatures. Representing the acidic groups in order of ionization —COOH (1), –OH (2), –NH⁺₃(3), then (at 25°C) pk^T₁₂= 9.75, pk^T₁₃= 9.36, pk^T₁₂₃= 9.92, pk^T₁₃₂= 10.31, (60°C) 9.20, 8.61, 9.40, 9.99 and (90°C) 9.02, 8.11, 9.04, 9.95. The errors involved in the method are discussed and a comparison made with the results at 25°C obtained by other workers. The use of the data in considerations of metal-dye-aminoacid formation constants is indicated.