THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF THE PLANT PHOTOSENSOR PIGMENT PHYTOCHROME

The similarity of the photophysical and photochemical properties determined for differently-sized phytochromes indicates that the first steps of the photochromic transformation between the two thermally stable forms of the pigment, P(r) and P(fr), are reactions of the conjugated tetrapyrrole chromophore interacting with protein regions still present in small P(r). I(700), the primary photoproduct of P(r), with which it is in a photochromic equilibrium, decays biexponentially within microseconds. These and further dark steps of the transformation, taking place within milliseconds and even seconds, might be proton transfers, conformational changes of the chromophore and, in the last steps leading to P(fr), of the protein.