Photoalkylation of aromatic amino acids as a model of photochemical modification of protein with chloroacetamide.

Abstract

On irradiation of tryptophan and tyrosine in aqueous solutions with the 2537 A light in the presence of chloroacetamide, photoalkylations of both amino acids proceeded with the same quantum yield of 0.10. Quenching rate constants (kq) of the fluorescences of tryptophan and tyrosine with chloroacetamide were determined to be 4.0-4.5×109 and 4.0×109 M-1sec-1, respectively. Almost no difference between both kq can explain the same reactivity of tryptophan and tyrosine. In an equimolar mixture, the reactivity of tryptophan was about 7 times that of tyrosine, and this reflects the ratio of extinction coefficients of both amino acids at 2537 A. In a mixture of several amino acids, if the photolysis was stopped when tryptophan disappeared less than 20%, all amino acids except tryptophan were recovered almost quantitatively. Wavelength dependence of this photolysis was finally examined. On irradiation of the 303 nm light, quantum yield for disappearance of tryptophan was also 0.10, whereas that of tyrosine was almost nil.