In Vitro Photoreactions of Chlorophyll and Photosynthetic Energy Conversion: Chlorophyll-Quinone-Ethanol at Low Temperature as an Analog of Photosystems I and II

Abstract

Illumination at low temperatures of ethanol solutions of p-benzoquinone, alone or in the presence of chlorophyll or pheophytin, leads to the reversible formation of radical complexes. Based on g-value determinations and deuterium-substitution effects, the radicals are identified as a semiquinone and an ethoxy radical. In the porphyrin-sensitized system, ternary complexes are produced. Chlorophyll also forms a cation radical upon illumination, whereas pheophytin does not. The generation of this cation radical is independent of the presence of quinone, and proceeds via the chlorophyll singlet state, as opposed to quinone and ethoxy radical production, which involve triplet states. The significance of these reactions to photosynthetic energy conversion is discussed.Our previous studies of chlorophyll-sensitized quinone radical production in solution (1-3) have provided indirect evidence, mainly kinetic in nature, for the following primary photoprocess:(solvent-Chl-quinone) + hp --(solvent-ChlT-quinone) (solvent-ChlI-quinone) solvent0X+ Chl + quinonered This mechanism proposes that absorption of a photon within a ternary complex results, via the chlorophyll triplet state, in the formation of an oxidized solvent radical and a reduced quinone radical (semiquinone). The purpose of this communication is to report on attempts to obtain further support for this mechanism by the use of low temperatures to stabilize the solvent radical, and deuterated solvent and quinone to permit individual identification of ESR signals due to radicals derived from these components. As will be shown, evidence has been obtained that demonstrates that illumination of these solutions does indeed lead to the formation of the above-mentioned radicals, in addition to the chlorophyll cation radical, which is produced in a separate parallel reaction. A more complete account of these experiments, which will be published separately (manuscript in preparation), includes a detailed interpretation of the ESR spectra in terms of the nature of the intermediates that are formed. METHODSAll of the experimental techniques involved in sample preparation, chlorophyll purification, and ESR observation have been described (2, 4). g-Value determinations were made with diphenylpicryihydrazyl as a standard. Fully-deuterated ethanol was obtained from Merck, Inc. The fully-deuterated benzoquinone was prepared by the method of Charney and Becker (5). RESULTSUpon irradiation of degassed solutions of either benzoquinone (with blue light: 330-500 nm, or white light) or pheophytin plus benzoquinone (with red light: Corning CS 2-73 filter) in ethanol in an ESR spectrometer at 1300K, the spectrum shown in Fig. 1 is reversibly produced. Note the poorly-resolved, but still evident, hyperfine structure that is present, particularly on the low-field side of the spectrum. Under similar experimental conditions, replacement of ethanol by its fully-deuterated analog resulted in a considerable improvement in spectral resolution (Fig. 1). A possible cause of such enhanced re...

tert-Butoxycarbonylaminoacyl-4-(oxymethyl)phenylacetamidomethyl-resin, a more acid-resistant support for solid-phase peptide synthesis

“…The materials and methods for solid-phase synthesis were similar to those described earlier. Boc-Valyloxymethyl-Resin (2). Chloromethyl-polyfstyrene-co-1% divinylbenzene) (0.24 mmol of Cl/g, 200-400 mesh, Lab Systems; 10.0 g) was stirred for 16 h at 50 °C with a solution of the cesium salt of Boc-Val12 (3.15 mmol) in DMF (60 ml).…”

Photooxidation of Chlorophyll b by Quinones Studied by Chemically-Induced Dynamic Nuclear Polarization

“…Using the g value assigned by Harbour and Tollin (17) for the ethoxy radical, g = 2.0049, and assuming that the methoxy radical will have a similar g value, we find that the Ag between the methoxy and the semiquinone radicals is too small as compared to the hyperfine interactions to account for the observed polarization. Simulated spectra that are based on those values look totally different from those observed experimentally.…”

“…It was impossible to fit the spectra to the radical pair proposed by Tollin et al (13,14,17), namely, CH,30 Q-.…”

“…As a way out of this dilemna, Tollin (13,14,17) suggested that a reaction occurs involving a ternary complex of the form:…”

“…The photosensitized oxidation of chlorophyll by different oxidizing agents is being investigated in several laboratories (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17), in order to provide guidelines for understanding the primary steps in photosynthesis. Among the various oxidants, the reaction of chlorophyll with various quinones is of special interest.…”