The time-resolved fluorescence properties of phenol and straight-chained phenol derivatives and tyrosine and simple tyrosine derivatives are reported for the pH range below neutrality. Phenol and straight-chained phenol derivatives exhibit single exponential fluorescence decay kinetics in this pH range unless they have a titratable carboxyl group. If a carboxyl group is present, the data follow a two-state, ground-state, Henderson-Hasselbalch relationship. Tyrosine and its derivatives with a free carboxyl group display complex fluorescence decay behavior as a function of pH. The complex kinetics cannot be fully explained by titration of a carboxyl group; other ground-state processes are evident, especially since tyrosine analogues with a blocked carboxyl group are also multiexponential. The fluorescence kinetics can be explained by a ground-state rotamer model. Comparison of the preexponential weighting factors (amplitudes) of the fluorescence decay constants with the 1H NMR determined phenol side-chain rotamer populations shows that (1) tyrosine derivatives with a blocked or protonated carboxyl group have at least one rotamer exchanging more slowly than the radiative and nonradiative rates, and the fluorescence data are consistent with a slow-exchange model for all three rotamers, (2) the shortest fluorescence decay constant is associated with a rotamer where the carbonyl group can contact the phenol ring, and (3) in the tyrosine zwitterion, either rotamer interconversion is fast and an average lifetime is seen or rotamer interconversion is slow and the individual fluorescence decay constants are similar.
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