Interactions of Chlorophyll Monolayers with Electron Donors and Acceptors

Abstract

Photoreactions and complexation reactions between monolayers of chloroplyll a, at a water-air interface, and various electron donors and acceptor in a Wilhelmy film balance. When phenazine methosulfate (PMS), ascorbate, benzyl viologen (BV), or dehydroascorbic acid are present in the aqueous subphase, the area/molecule increases with substrate concentration in accordance with a Langmuir- type adsorption isotherm. The increase in area is attributed to a reorientation of chlorophyll upon complex formation. The surface potentials and areas/molecule (at zero pressure) are 350 mV and 179 Å2 for PMS + ascorbate in the subphase, 300 mV and 175 Å2 for PMS, 310 mV and 146 Å2 for BV, 310 mV and 156 Å2 for ascorbate, 230 mV and 146 Å2 for dehydroascorbic acid. The equilibrium constants are 170 × 103 ᴍ-1 for PMS + ascorbate, 64 × 103 ᴍ-1 for PMS, 41 × 103 ᴍ-1 for BV, and 3.9 × 103 ᴍ-1 for ascorbate. Formation of a complex decreases the slope of the isotherm, thereby giving the film a more liquid nature. Chlorophyll does not complex with oxidized PMS. Illumination of the chlorophyll · BV complex in a nitrogen atmosphere results in an oxidation of chlorophyll; the photoproduct (probably Chl⊕ · BV⊖) has a larger area/molecule, of 165 Å2, and a lower surface potential, of 280 mV. Illumination of the chlorophyll · ascorbate complex or the chlorophyll · PMS · ascorbate complex results in a reduction of chlorophyll. The photoproduct of the former reaction (probably Chl⊖ · -oxidized ascorbate) has a larger area/molecule, of 169 Å2, and a lower surface potential, of 290 mV. The photoproduct of the latter reaction (probably Chl⊖ · PMS · oxidized ascorbate) has a larger area/molecule, of 211 Å2, and a larger surface potential, of 380 mV. No evidence is observed for a photoreaction with the Chl · PMS complex or the Chl · dehydroascorbic acid complex. A value of 0.03 is determined as a lower limit for the quantum yield of these reactions. In all cases the absorption spectrum of the chlorophyll recovered from surface after irradiation shows no significant differences from the control.