H. T. TIEN
4512
vice versa. The amount of this isomerization, however, cannot be greater than the amount of olefin formation from these tertiary radicals, i.e., 1.37 (0.9 i 0.47) for the formation of the trans from the cis isomer. We found, however, a G value for the formation of trans1,2-dimethylcyclohexane of 2.2. There is only one other possibility for the isomerization of the cis isomer to the trans isomer and that is by opening and reclosing of the cyclohexane ring. This mechanism also explains the formation of octene-2 by intramolecular disproportionation. Intramolecular disproportionation and combination has been suggested previously by Freeman’2 in the radiolysis of methylcyclohexane. Our results provide experimental proof of such an opening and
reclosing of the cyclohexane ring. Isomerization following ring opening can only take place if a carboncarbon bond between or adjacent to the methyl groups is broken but not if any of the other three carbon-carbon bonds of the cyclohexane ring are broken. Assuming equal probability for breaking of the six ring carbon-carbon bonds, we arrive at an estimated G value for ring opening and reclosing of 3.46, which is about equal to the G value for carbon-hydrogen bond breaking.
Acknowledgment. The author wishes tfo thank Professor Dr. H.J. Born for his generous support of this work and Dr. H. Heusinger for helpful discussions.
Photoelectric Effects in Thin and Bilayer Lipid Membranes in Aqueous Media by H.T. Tien Department of Biophysics, Michigan State University, East Lansing, Michigan 48829
(Received M a y 81, 1068)
When a beam of intense white light is directed onto one side of a lipid membrane constituted from photoactive pigments at a water-oil-water biface, certain photoelectric phenomena are produced. In this paper the experimental findings on the open-circuit photovoltage and the photoconductivity of the membrane are described. Possible explanations for the observed phenomena are discussed. The significance of these observations in relation to photosynthesis and other light-induced processes where membranous structures are believed to be essential is considered.
Introduction The high degree of orderliness and lamellar organization of chloroplasts in nature have been inferred from numerous These studies have led to the suggestion by Bassham and Calvin5 that a crystalline lattice containing chlorophyll molecules and other compounds may be involved in the photosynthetic apparatus. They further suggested that such an organized structure of light-sensitive pigments may possess photoconductive properties resembling those of organic semiconductors. That the photoconductivity may exist in chlorophylls had been suggested earlier by Katz.6 Investigations carried out by Eley,’ Nelson,* and othersQ-” using thin films or compressed disks of chlorophylls and their related compounds have indeed demonstrated the existence of photoinduced electrical effects. In a number of publications, Arnold and coworkers12 have observed semiconductive properties in chloroplast and chromatophore preparations. It should be mentioned that all these experiments were carried out either in vacuo or a d r y state. Although these earlier investigations are significant in their own right, two imT h e Journal of Physical Chemistry
portant questions remain to be answered. (i) Can the photosynthetic pigments be organized in the form of (1) R. Sager and G. E. Palade, E x p . Cell Res., 7, 584 (1954). (2) A. J. Hodge, J. D. McLean, and F. V. Mercer, J. Biophys. Biochem. Cytol., 1, 605 (1955). (3) E.Steinman and F. 8. Sjostrand, E x p . Ce2l Res., 8, 15 (1955). (4) For further references, see (a) E. Rabinowitch, Discussions Faraday Soc., 27, 161 (1959); (b) J. B. Thomas, “Primary Photo-
processes in Biology,” North-Holland Publishing Co., Amsterdam, The Netherlands, 1965,pp 127-135. (5) J. A. Bassham and M. Calvin, “The Path of Carbon in Photosynthesis,” Prentice-Hall, Inc., Englewood Cliffs, N. J., 1957, Chapter 12. (6) E. Katz, “Photosynthesis in Plants,” Iowa State College Press, Ames, Iowa, 1949, p 287. (7) D.D.Eley, Nature, 162, 819 (1948). (8) R. C. Nelson, J. Chem. Phys., 27, 864 (1957). (9) D.R. Kearns, G . Tollin, and M.Calvin, ibid., 32, 1020 (1960). (10) A. Terenin, E. Putzeiko, and I. Akimov, Discussions Faraday SOC.,27, 83 (1959). (11) B. Rosenberg and J. F. Camiscoli, J. Chem. Phys., 35, 982 (1961). (12) (a) W. Arnold and H. K. Sherwood, Proc. Nat. Acad. Sci. U . S., 43, 105 (1967); (b) W.Arnold and R. K. Clayton, ibid., 46, 769 (1960); (c) W. Arnold and H. K. Maclay, Brookhaven S y m p . Biol., 11, l(1959).
4513
PHOTOELECTRIC PHENOMENA IN BIMOLECULAR LIPID MEMBRANES thin membrane, pzeferably of the order of molecular thickness (-100 A) which is assumed to exist in the native state, in an aqueous environment? (ii) I n case such ultrathin membranes could be formed, would they exhibit light-induced electrical phenomena, such as photovoltaic effect and photoconductivity? The present work has been initiated with the aforementioned questions in mind. Recently, techniques have been developed for the formation of ultrathin membranes from a variety of surface-active materials including brain proteolipids, phospholipids, synthetic surfactants, and oxidifed cholesterol. These ultrathin membranes (
