The primary events in the photoactivation of yellow protein

The first steps in the photochemistry of the photoactive yellow protein are investigated with light pulses of 200 fs duration. Transient absorption kinetics are measured in the spectral region from 430 to 550 nm at room temperature upon excitation at 400 and 464 nm. The first intermediate is an excited state which develops in 0.7 ps and has a lifetime of 3.6 ps. Stimulated emission appears red-shifted and reduced in intensity in 0.7 ps, after which it decays with the 3.6 ps lifetime. The spectrum and quantum yield of the product state are consistent with the already known red-shifted photocycle intermediate pR.