BINARY OSCILLATIONS IN THE KOK MODEL OF OXYGEN EVOLUTION IN OXYGENIC PHOTOSYNTHESIS

The flash-induced kinetics of various characteristics of Photosystem I I (PS II) in the thylakoids of oxygenic plants are modulated by a peri od of two, due to the function of a two-electron gate in the electron acceptor side, and by a period of four, due to the changes in the stat e of the oxygen-evolving complex. In the absence of inhibitors of PS I I, the assignment of measured signal to the oxygen-evolving complex or to quinone acceptor side has frequently been done on the basis of the periodicity of its flash-induced oscillations, i.e. four or two. Howe ver, in some circumstances, the period four oscillatory processes of t he donor side of PS II can generate period two oscillations. It is sho wn here that in the Kok model of oxygen evolution (equal misses and eq ual double hits), the sum of the concentrations of the S-0 and S-2 sta tes (as well as the sum of concentrations of S-1 and S-3 states) oscil lates with period of two: S-0 + S-2-->S-1 + S-3-->S-0 + S-2-->S-1 + S- 3. Moreover, in the generalized Kok model (with specific miss factors and double hits for each S-state) there always exist such epsilon(0), epsilon(1), epsilon(2), epsilon(3) that the sum epsilon(0)[S-0] + epsi lon(1) [S-1] + epsilon(2)[S-2] + epsilon(3)[S-3] oscillates with perio d of two as a function of flash number. Any other coefficients which a re linearly connected with these coefficients, <(epsilon)over cap> = c (1) epsilon(i) + c(2) (i = 0,1,2,3), also generate binary oscillations of this sum. Therefore, the decomposition of the dash-induced oscilla tions of some measured parameters into binary oscillations, depending only on the acceptor side of PS II, and quaternary oscillations, depen ding only on the donor side of PS II, becomes practically impossible w hen measured with techniques (such as fluorescence of chlorophyll a, d elayed fluorescence, electrochromic shift, transmembrane electrical po tential, changes of pH and others) that could not spectrally distingui sh the donor and acceptor sides. This property of the Kok cycle puts l imits on the simultaneous analysis of the donor and acceptor sides of the RC of PS II in vivo and suggests that binary oscillations are no l onger a certain indicator of the origin of a signal in the acceptor si de of PS II.