Self-regulation phenomenon of electron-conformational transitions in biological electron transfer under nonequilibrium conditions

We present a theoretical description and experimental evidence for a nonlin ear effect of repetitive stepwise alteration of photoexcitation intensity o n quasi-stationary-state optical absorption changes of an electron trans fe r biomolecular system. This effect was studied for photosynthetic reaction centers, and analyzed with a model that takes into account correlative beha vior of charges and slow structural motions of the macromolecule. Theoretic ally, photosynthetic reaction centers can be described with a conformationa l potential, the shape of which depends on the intensity of the photoexcita tion. This is in agreement with previous studies on hysteretic behavior of reaction center absorbance, causing bifurcational appearance of a "light" c onformational state that coexists with a "dark" conformational state within a particular range of exciting light intensity. The essential assumption t hat allows for observation of the effects reported here is the nonequilibri um character of macromolecule transitions between dark and light conformati onal states. Experimentally observed behavior of absorbance changes, caused by a sequence of photoexcitation intensity alternations, indicates unambig uously that the reaction center functions as a highly nonequilibrium, nonli near dynamic system with a pronounced capability for regulation of the phot oelectron Bur by slow structural motions. This system could be considered a s a molecular motor for pumping the charge separated state. [S1063-651X(99) 08703-6].