Picosecond fluorescence decay of lens protein γ-II crystallin

Abstract

The fluorescence decay of tryptophan residues in the bovine lens protein γ-II crystallin has been measured in aqueous buffer solutions. Results were obtained as a function of emission wavelength, temperature, dissolved oxygen, and denaturing solvent. The protein displayed complex fluorescence decay which fit a biexponential model with a long component (ns) and a short component (few hundred ps). Measured fluorescence quantum yields data for γ-II crystallin allowed calculation of radiative and non-radiative rate constants. The radiative rate constant was consistent with that observed in other indole derivatives, while the nonradiative rate constant was quite large and accounted for the short lifetime in γ-II. The temperature dependence of the non-radiative decay in γ-II crystallin yielded a small activation energy of only 1–2 kcal/mol, compared to 4 kcal/mol for the reference compound NATA whose barrier is known to derive from the rotamer model.