NONLINEAR LIGHT-INDUCED PROPERTIES OF PHOTOSYNTHETIC REACTION CENTERSUNDER LOW-INTENSITY IRRADIATION

Absorbance changes (Delta A) were studied in reaction centers (RCs) fr om the photosynthetic bacteria Rhodobacter sphaeroides and Chloroflexu s aurantiacus under extremely low Light intensity irradiation. The dep endence of Delta A as a function of the intensity of the actinic light was linear for Rb. sphaeroides wild type (Q(B)-free) and o-phenanthro line-inhibited Rb. sphaeroides RCs (strain R-26). Significant nonlinea r Delta A changes as a function of the intensity of photoactivation we re observed in the RCs from C. aurantiacus and for Q(B)-containing RCs from Rb. sphaeroides. The investigations of Delta A changes upon the history of the photoactivation revealed pronounced hysteresis loop-typ e dependencies for these RCs under a wide range of illumination intens ities. It is proposed that the two branches of the hysteresis loop cor respond to different electron-conformational (structural) states of th e protein induced by the light variation, which substantially influenc es the rate constants of certain electron transfer reactions. These ex perimental data can be explained in terms of nonequilibrium self-organ ization effects arising due to self-consistent switching effects in th e flexible cofactor-protein conformation and controlled by the actinic light. It is concluded that RCs exist in at least two conformational states, i.e. a light-adapted state and a dark-adapted state. These sta tes differ strongly in the rates of the electron transfer to Q(B) and the subsequent recombination rate.