PHOTOREDUCTION OF ZINC PORPHIN BY ASCORBIC ACID*

Abstract— Zinc porphin is photoreduced to zinc chlorin through an intermediate dihydroporphin (PH₂) by ascorbic acid in ethanol containing 1% to 10% (v/v) piperidine. Under the same conditions zinc chlorin is more slowly photoreduced to zinc tetrahydroporphin. The reactions leading to chlorin are photosensitized by the product chlorin and so are autocatalytic in red light. Quantum yields for these reactions range up to 0.05.Other aliphatic amines catalyze these reactions, but at rates peculiar to the amine. The immediate product of reduction of zinc porphin, PH₂, is distinguished by an intense band at 437 nm; it reverts to porphin in the dark in the presence of oxygen or dehydroascorbic acid. Its conversion to chlorin is effected by light absorbed by porphin or chlorin, but not by light absorbed by PH₂ itself. A suggested structure for PH₂, compatible with the observed reactions, has added hydrogens on one bridge carbon and one β-pyrrole carbon. The possibility of an analogy between these reactions and the biochemical conversion of protochlorophyll to chlorophyll is discussed.     

DODECYLPYRIDINIUM ALKANOATES STABILIZE DISPERSED CHLOROPHYLL

In the presence of a surfactant that does not ligate Mg, chlorophyll is adsorbed to polyethylene particles swollen with tetradecane principally as the infrared-absorbing, highly polymeric species Chl 740. Examples of such surfactants are quaternary ammonium salts and soaps. However, when surfactants of opposite charge are present together, in this case dodecylpyridinium iodide and Na butyrate or myristate, chlorophyll may exist entirely in a dispersed form absorbing around 666 nm. Absorption and fluorescence spectral data show that much of the dispersed pigment is still associated, but as dimeric and perhaps short oligomeric species. It is concluded from fluorescence lifetime analysis that most of the observed fluorescence comes from a small population of chlorophyll that is probably monomeric and isolated; fluorescence of more highly associated species decays with a wide range of lifetimes. The capacity for photochemical sensitization of these particles with dispersed chlorophyll is similar to that of particles with ligating surfactants examined earlier. Structures are suggested for chlorophyll association in which Mg is ligated by water hydrogen-bonded to a carboxylate group, while the dodecylpyridinium counterion lies near the chlorophyll ring. The effect of the two surfactants is synergistic. Overall, spectra of dispersed chlorophyll adsorbates resemble most closely those of colloidal chlorophyll suspensions prepared by dilution of solutions in polar organic solvents with water.     

THE CHLOROPHYLL-SENSITIZED REACTION BETWEEN BENZOQUINONE AND ETHANOL*

The electron-transfer reaction between triplet excited chlorophyll and quinones has been extensively studied as a model of the primary reaction in photosystem II. There has also been reported a minor reaction in which the chlorophyll cation radical ostensibly oxidizes the alcohol solvent or even water, leading to a gradual net reduction of quinone, but the exact mechanism and even the existence of this reaction has been uncertain. We have examined the consequences of prolonged irradation of ethyl chlorophyllide and benzoquinone in acidulated ethanol, and find a chlorophyllide-sensitized reaction which is not analogous to the better-known autosensitized reduction of quinones in blue or UV light. In the chlorophyllide-sensitized reaction, benzoquinone is apparently converted to ethoxy-substituted quinones and quinols, and polymeric material. Ethyl chlorophyllide (or chlorophyll) is simultaneously oxidized to more polar products which themselves continue to photosensitize the reaction of quinones. The production of acetaldehyde could not be demonstrated in the sensitized reaction. Chlorophyllide-sensitized reaction of (l-hydroxyethyl)benzoquinone, ethoxybenzoquinone and 2.5-diethoxybenzo-quinone were examined for additional information. A reaction sequence, tentatively proposed to accommodate the known facts, starts with oxidative attack by quinone on an oxidized chlorophyllide radical formed by loss of a hydroxyl proton from alcohol bound as a ligand to Mg²⁺. It is not likely that this reaction is closely related to events at the oxidizing side of photosystem II.     

FLUORESCENCE QUANTUM YIELDS AND LIFETIMES FOR BACTERIOCHLOROPHYLL c

Abstract— Absorption and emission spectra of bacteriochlorophyll c dissolved in a variety of solvents were measured and fluorescence quantum yields determined from the integrated emission spectra. Values for the emission maxima calculated from the positions and bandwidths of the red absorption band using the Stepanov relationship agreed closely with the experimental values. Fluorescence quantum yields varied between 0.10 in methanol and 0.36 in tetrahydrofuran and in dibutylamine. Fluorescence lifetimes were also determined for bacteriochlorophyll c in four of the solvents, and ranged from 2.7 ns in methanol to 7.6 ns in dimethylsulfoxide.     

Parity-violating electron deuteron scattering and the proton's neutral weak axial vector form factor

We report on a new measurement of the parity-violating asymmetry in quasielastic electron scattering from the deuteron at backward angles at Q2=0.038 (GeV/c)2. This quantity provides a determination of the neutral weak axial vector form factor of the nucleon, which can potentially receive large electroweak corrections. The measured asymmetry A=-3.51+/-0.57 (stat)+/-0.58 (syst) ppm is consistent with theoretical predictions. We also report on updated results of the previous experiment at Q2=0.091 (GeV/c)2, which are also consistent with theoretical predictions.