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.