The photophysics and photochemistry of the plant photosensor pigment phytochrome

Abstract

The similarity of the photophysical (e. g., fluorescence lifetimes) and photochemical (e. g., transients kinetic behaviour) properties determined for differently-sized phytochromes (small: 60 kD, large: ca. 114-118 kD, and native: 124 kD) indicates that the first steps of the photochromic transformation between the two thermally stable forms of the pigment, P(r) and P(f r), 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. A set of parallel reactions from P(r) to P(f r) can best fit the kinetic data available. In solution, the radiationless processes of the fully conjugated stretched tetrapyrroles (serving as chromophore models) mainly occur around the central C-10 methine group, while at an early stage during the P(r)→ P(f r) photoconversion a Z/E isomerization takes place at the C-15 methine group of the chromophore. The protein therefore appears to impair the twisting around C-10, creating the very efficient energy wasting step responsible for the more than 80% heat loss after light absorption by P(r) and by P(f r). © 1984 IUPAC