The complex excited state dynamics of the early photocycle of photochrome

The primary relaxation rate of excited phytochrome is measured by time-resolved laser fluorescence techniques at temperatures between 77 and 300 K in a buffer glycerol mixture. The known multiexponential fluorescence decay of the red absorbing form Pr at physiological temperatures reduces to a single exponential below 150 K with a lifetime τ of (1.3 ± 0.2) ns at 77 K. This is unambiguously consistent with the dynamics of two excited states which are in thermal equilibrium with each other and represent the initially excited Pr and an excited intermediate of the chromophore during the ZE photoisomerization.