Photophysical properties of tyrosine and its derivatives with free and blocked functional groups were studied by steady state and time-resolved fluorescence spectroscopy and global analysis in organic solvents, such as methanol, 2-propanol, tetrahydrofuran (THF), and dimethylsulfoxide (DMSO). The mono-exponential fluorescence intensity decays were observed for all tyrosine derivatives in THF and DMSO solutions, whereas in alcohols some derivatives have bi-exponential decays. The rotamer population calculated from 1H nuclear magnetic resonance spectroscopy in DMSO does not correspond to the pre-exponential factors obtained from fluorescence spectroscopy. Moreover in the case of DMSO, the strong interaction of this solvent with the hydroxyl group of the fluorophore's phenol ring causes substantial changes in the fluorescence and nonradiative rate constants of tyrosine derivatives compared with those of tyrosine with a blocked hydroxyl group, Tyr(Me). The steady state and time-resolved fluorescence measurements in pure organic solvents and water–organic solvent mixtures indicate that the fluorescence quenching of the phenol chromophore of tyrosine by an acetyl or amide group or both depends on the polarity of the solvent used as well as the ability of the solvent to form hydrogen bonds with functional groups of tyrosine.
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Vol. 81 • No. 3