[The variation in the quantum yield and fluorescence decay of tyrosine as a function of pH].
1971; National Institutes of Health; Volume: 229; Issue: 1 Linguagem: Inglês
Autores Tópico(s)
Photochromic and Fluorescence Chemistry
ResumoAbstract In this work, we studied the variation in the quantum yield and fluorescence decay of tyrosine in aqueous solution as a function of pH. In the pH range 4–8, the quantum yield is constant and the decay is exponential with a lifetime τ1 = 3.6 nsec. For pH > 8, the quantum yield decreases while the lifetime remains constant and equal to τ1. This result can be explained by tyrosine O ion formation, the fluorescence of which is negligeable. The quantum yield decreases also for acid pH near the pK1 = 2.2 corresponding to the carboxyl protonation, but the equilibrium in the ground state between the carboxylate ion and the protonated carboxyl, cannot completely explain, the quantum yield variation and the shape of the decay curves. We observed that the quantum yield decreases faster with pH than predicted by the mass law. In addition the fluorescence excited by a very short flash, decreases faster than predicted by the simple stochiometry. We are able to account very satisfactorily for these experimental results by taking into account protonation and deprotonation reactions, in the excited state. We determined the protonation rate constant equal to 3.6·1010 sec−1·mole−1 and that of deprotonation equal to 5.8·10 8 sec −1 ·p K 1 ∗ in the excited state is 1.8 which is relatively close to that of the ground state.
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