Optical and photochemical properties of chlorophyll a solubilized in aqueous solutions of surfactants

Abstract

The phenomenon of solubilization of chlorophyll in micellar solutions of surfactants is studied by measuring the dependence of absorption, fluorescence and sensitized photoreduction of methyl orange on the surfactant concentration. Chlorophyll is solubilized well in Triton X-100 and cetyl trimethyl ammonium bromide at concentrations above the critical micelle concentration. Fluorescence and photo-chemical activity rise sharply in this concentration range. Absorption increases gradually with increasing surfactant concentration; it begins to increase below the critical micelle concentration and reaches a maximum above it. This phenomenon is discussed in terms of the “shadow effect” of pigment suspensions. The anionic surfactants investigated solubilize much less chlorophyll and only at high concentrations of surfactant. The relative rates of photoreduction of 2,4-dinitrophenol and chlorophyll in solutions of different surfactants were also measured. They depend on the nature of the oxidant as well as on the surfactant. The photoreduction of viologens in the same system is described. The reaction is reversible. The degree of reduction in the light decreases with decreasing normal potential of the viologen. Finally, the photoreduction of NADP + is described. The reaction requires ferredoxin-NADP + reductase but is almost independent of ferredoxin and plastocyanin. The rate is much lower than with chloroplast fragments. It is concluded that the photochemistry of solubilized chlorophyll is similar to that of chlorophyll dissolved in organic solvent and distinct from that of chloroplasts in which the structure ensures high efficiency and high selectivity for the light reactions of photosynthesis.