J. W. COLEMAN AND E. RABINOWITCH
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centrations of reactant it reacts further with the of this type of dyestuff. Further work is now rereactant, an apparently negligible amount then quired to determine: (a) whether this holds also undergoing dismutation, thus for other ionic forms of the intermediate (ic., when photoreduction is carried out in solutions of B + Fez++--B- + Fe3+ widely different pH value), (b) whether it is true 2B- +--B + B2also for other dyestuffs such as thionine., (c) to compared with what degree other oxidizing or reducing agents can compete with the dismutation reaction by reacting Cz042-+ Fe3+ +-Cz04- Fez+ with the semiquinone and (d) to determine the GO4Fe3++- 2C02 Fez+ effect of polymeric substances upon the dismutation Conclusions reaction (Le., in systems previously investigated by The results obtained in the present experiments Wotherspoon and OsterSby methods involving conare only of a preliminary nature, they cover only tinuous irradiation at normal light intensities). a very limited range of conditions and require more The author thanks Dr. E. J. Bowen, F. R. S., for precise investigation. They do however show that much helpful advice and discussion, This paper is the high rate of the dismutation reaction of the published by permission of the Admiralty. semiquinone intermediate can be the dominating (8) N. Wotlierspoon and G . Oster, J . Am. Chem. Soc., 79, 3992 factor governing the course of the photoreduction (1957).
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EVIDENCE OF PHOTOREDUCTION OF CHLOROPHYLL I N VIVO? BY J. W. COLEMAN AND E. RABINOWITCH Photosynthesis Laboratory, Departmen.t of Botany, University of Illinois, Urbana, Ill. Received July 11, 1968
It has been suggested repeatedly that chlorophyll plays in photosynthesis the role of a “photoenzyme”-more specifically, a “photo-oxydoreductase”-mediating, in the light-excited state, the transfer of hydrogen atoms (or electrons) from a reductant with a low reduction potential to a n oxidant with a low oxidation potential, thus storing a part of its excitation energy as chemical energy of the products. From this point of view, it would be very important to find evidence of reversible formation of reduced (or oxidized) chlorophyll during photosynthesis. Observations of changes in the absorption spectrum of photosynthesizing organisms in light provide one possible approach to this problem. Sensitive measurements of the “difference spectrum” (difference between the absorption spectra of dark and illuminated cells), by a method similar to that first used by Rabinowitch’ for the study of photodissociation of halogen molecules, were first made by Duysens2-6 later by Witt,?-lo ( i )Work supported by a grant from the Office of Naval Research. (1) E. Rabinowitch and H. L. Lehman, Trans. Faraday Soc., 167, 689 (1935). (2) L. N. M. Duysens, Thesis, Univ. of Utrecht, 1952. (3) L. N. A?. Duysens, Nature, 173, 692 (1954). (4) L. N. M. Duysens, Science, 120, 353 (1954). (5) L. N. M. Duysens, ibid., 121, 210 (1955). (6) L. N.M. Duysens, W. J. Huiskarnp, J. J. Vos and J. M. van der Hart, Bzochzm et Bzophys. Acto, 19, 188 (1956); see also “Research in Photosynthesis,” Interscience Pub., New York, N. Y., 1957,pp. 164173. (7) H. T. Witt, Naturwiss., 42,72 (1955); 2. physik. Chem., 4, 120 (1955), (8) H. T. Witt, Z.Elektrochem., 69, 10,981 (1955). (9) H.T. Witt, zbid.. 60, 1148 (1956). (IO) H. T. Wi‘itf, ”Research in Photosynthesis,” Ed. by Hans Graffron, et al., Interscience Publishers, New York, N. Y., 1957, pp. 75-84. (loa) H. T. Witt and R. Moraw, 2. physik. Chem., 12, 343 (1957); 13, 113 (1957). (lob) H. T. Witt, R. Moraw and A. Miller, ibid., 13, 113 (1957).
LundegHrdh,ll Strehler et aZ.,12 Chancel3 and Kok.14-16 With the exception of Kok, these authors covered only the spectral region
