Picosecond fluorescence decay of lens protein gamma-II crystallin

The fluorescence decay of tryptophan residues in the bovine lens protein gamma-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 gamma-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 non-radiative rate constant was quite large and accounted for the short lifetime in gamma-II. The temperature dependence of the non-radiative decay in gamma-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.