Some aspects of steady state and time-resolved fluorescence of tyrosine and related compounds

Abstract

The fluorescence characteristics decay kinetics of para-tyrosine and related compounds, e.g. ortho-tyrosine, meta-tyrosine, N-acetyl tyrosine, tyrosine methyl ester, N-acetyl tyrosine ethyl ester, tyramine and phenyl alanine, were studied in aqueous solution at various pH values. A dynamic analysis of the protonation-deprotonation equilibria provides values of pK_a^∗ of the COOH group. The fluorescence decay is monoexponential except at pH ≈ pK_a(COOH). Fluorescence from tyrosine methyl ester and N-acetyl tyrosine ethyl ester decays biexponentially, and is attributed to the existence of rotamers at room temperature. The difference in the fluorescence decay kinetics of the isomeric tyrosines suggests a possible method for detecting ortho-tyrosine in a mixture with para-tyrosine and phenyl alanine. Phosphate ions quench the fluorescence of all the compounds except phenyl alanine. The quenching rate constants obtained using steady state and time-resolved fluorescence methods compare well and confirm the presence of excited state interaction through the OH group. From studies on the effect of temperature on the fluorescence characteristics, values of t^O_F, k^O_NR and ΔE are obtained.