On the Impossibility of an OH-Radical Model for Photosynthetic Water Oxidation / Über die Unmöglichkeit eines OH-Radikalmodells für die photosynthetische Wasseroxidation

Abstract

OH-Radical Model for Photosynthetic Water OxidationIf it is assumed that the oscillating 02 flash yield sequence of algae and chloroplasts predominantly reflects an inherent property of the 02 evolving system of the photosynthetic apparatus it is shown that the OH-radical model (02 production by recombination of four OH radicals) is contradictory to the experimental results because it predicts an aperiodic 02 flash yield sequence.Several experimental facts already known strongly indicate that the OH-radical model is not consistent with the kinetic model for 02 evolution of KOK et al. either. In spite of extensive research in photosynthesis our knowledge of the structure and activities of photosystem II because of its complexity is rather limited. Although considerable progress has been made in recent years the specific mechanism for the photosynthetic oxidation of water to molecular oxygen remains to be elucidated. Several models for this process have been proposed based on a molecular or solid state theory of the primary events of photosynthesis. The kinetic models of KOK et al. *' 2 and of JOLIOT et al. 3 were developed in order to explain the 02 flash yield sequence observed on algae and chloroplasts by these authors. The first model on a specific molecular basis was presented by RENGER 4 . He demonstrated that the 02 flash yield pattern could be deduced quantitatively from several assumptions on the molecular structure of the 02 evolving enzyme system. Although it is not known whether there are "free" oxygen intermediates in photosynthetic water oxidation it is assumed in this model as well as in that of JOLIOT et al. that the water cleavage leads to the production of oxygen atoms which combine to form 02 .According to the solid state theory of photosynthetic 02 evolution the decomposition of the water molecule is thought to be caused by its interaction with randomly diffusing molecular excitons of the chlorophyll aggregates at the surface of the thylakoid membrane 5 ' 6 . But no explanation for the 02Requests for reprints should be sent to Dr. G. VIERKE, Institut für physikalische Biochemie der Universität Frankfurt, D-6000 Frankfurt, Sandhofstr. 2. flash yield pattern in terms of the solid state theory has been offered as yet.Several authors 5 ' 7-10 presented the theory that the photooxidation of water could be brought about by the oxidation of OH G -ions to OH radicals or equivalents of these 11-15 the recombination of which w r ould lead to H20 and 02 .In this paper it is shown that the OH-radical model of photosynthetic water oxidation cannot provide an explanation for the 02 flash yield oscillations as determined by JOLIOT et al. 3 and KOK et al. 2 . Some Operational and Structural Principles of PS IIOnly the most important facts concerning the 02 evolution mechanism will be reported in the following short summary. Details which are not necessary for the subsequent proof have been omitted.Since 02 evolution is a four quantum process 16 ' 17 (quantum requirement of PS II) the oxidation of water to m...