THE COMPLEX EXCITED-STATE DYNAMICS OF THE EARLY PHOTOCYCLE OF PHYTOCHROME

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 fluore scence decay of the red absorbing form P-r at physiological temperatur es reduces to a single exponential below 150 K with a lifetime tau of (1.3 +/- 0.2) ns at 77 K. This is unambiguously consistent with the dy namics of two excited states which are in thermal equilibrium with eac h other and represent the initially excited P-r and an excited interme diate of the chromophore during the Z-E photoisomerization.