Tyrosyl Fluorescence Decays and Rotational Dynamics in Tyrosine Monomers and in Dipeptides

Abstract

Picosecond time-correlated single-photon counting was used to measure fluorescence lifetimes and fluorescence anisotropy decays of tyrosine and the tyrosine–alanine and tyrosine–leucine dipeptides. After excitation of tyrosine at 287 nm two emitting species were observed, one at 303 nm with a lifetime of 3.3 ns and another at 340 nm with a lifetime of 360 ps. The rotational correlation time of tyrosine at 303 nm is 38 ps in water at pH 7 and depends linearly on viscosity with a slope of 44 ps/cP, consistent with Stokes–Einstein–Debye theory. We calculated a value of 45 ns for the radiative lifetime of tyrosine, yielding a fluorescence quantum yield of 0.07. The dipeptides Tyr–Ala and Tyr–Leu exhibit two- or three-exponential decays. The amplitudes of the decay components for three-exponential fits correlate closely with the populations of rotamers in these peptides as determined by NMR. The quenching of dipeptide fluorescence is shown to depend on the solvent polarity, strongly supporting the hypothesis that tyrosyl fluorescence in peptides is quenched by charge transfer. The rotational correlation times of tyrosine, Tyr–Ala, and Tyr–Leu increase linearly with the van der Waals volumes. However, rotational relaxation is somewhat faster than expected from Stokes–Einstein–Debye theory with “stick” boundary conditions.