Photophysical Properties of Tyrosine and Its Simple Derivatives Studied by Time-Resolved Fluorescence Spectroscopy, Global Analysis, and Theoretical Calculations

Abstract

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 1H NMR spectroscopy, do not correspond to the pre-exponential factors obtained from fluorescence spectroscopy. The calculated energy barriers of rotations about the Cα−Cβ 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.