The photophysical properties of tyrosine and its derivatives with free and blocked functional groups in water were studied by steady-state and time-resolved fluorescence spectroscopy and global analysis. Tyrosine fluorescence intensity decays in water at pH ) 5.5 in the short-wavelength region (290-320 nm) are monoexponential, whereas, at longer wavelengths, they are biexponential. The monoexponential fluorescence intensity decay of O-methyl tyrosine across the fluorescence band is observed. The fluorescence lifetimes of Tyr calculated using a global analysis are equal to 3.37 ( 0.04 ns at the short-wavelength region and 0.98 ( 0.12 ns at the longer-wavelength region. This observation, together with the decay-associated spectra, indicate that the short-lifetime component can be attributed to tyrosine with phenol hydroxyl groups hydrogen-bonded with water molecules. The rotamer populations calculated from potentials of mean forces, as well as those obtained from 1 H NMR spectroscopy, do not correspond to the pre-exponential factors obtained from fluorescence spectroscopy. The calculated energy barriers of rotations about the C R -C β bond indicate that the interconversion rate constant for tyrosine and N-acetyl-tyrosinamide are much greater than the fluorescence rate constant. Monoexponential fluorescence intensity decay of tyrosine derivatives in acetonitrile solution is observed for all derivatives studied and, contrary to the aqueous solution, the amide group does not quench the fluorescence. Thus, specific conformation(s) stabilized by the hydrogen-bond network seem to be responsible for the heterogeneous fluorescence intensity decay of tyrosine derivatives in aqueous solution.

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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