Interactions of Chlorophyll Monolayers with Electron Donors and Acceptors

Photoreactions and complexation reactions between monolayers of chloroplyll a, at a water-air interface, and various electron donors and acceptor in a W il h e 1 m y 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 L a n g m u i r 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 Ä2 for PMS + ascorbate in the subphase, 300 mV and 175 A 2 for PMS, 310 mV and 146 A2 for BV, 310 mV and 156 Ä2 for ascorbate, 230 mV and 146 Ä2 for dehydroascorbic acid. The equilibrium constants are 170X 10 3 M~ 1 for PMS + ascorbate, 64 x 103 M _ 1 for PMS, 41 x 103 M 1 for BV, and 3.9 x 103 M _ 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 C h l ® B V e ) has a larger area/molecule, of 165 A2 , 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 Chi®-oxidized ascorbate) has a larger area/molecule, of 169 A2, and a lower surface potential, of 290 mV. The photoproduct of the latter reaction (probably Chi°-PMS-oxidized ascorbate) has a larger area/molecule, of 211 A2, and a larger surface potential, of 380 mV. No evidence is observed for a photoreaction with the Chi • PMS complex or the Chi • 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.