essential that in vitro esr observations be interpreted in terms of the chlorophyll species actually present. Chlorophyll is able to act both as electron donor and electron acceptor in chargetransfer complexes. The central Mg atom of chlorophyll is coordinatively unsaturated when it has the coordination number 4, and at least one of the Mg axial positions must always be occupied by an electron donor group. In the absence of other nucleophiles, the ketone C-O function in Ring V of one chlorophyll molecule serves as donor to the Mg atom of another, forming chlorophyll dimers, (Chl2), or oligomers, (ChlW)3. Extraneous nucleophiles (bases) can compete for the coordination site at Mg, with disruption of the chlorophyllchlorophyll interactions, to form chlorophyll-ligand adducts, (Chl-L). The nature of the nucleophile determines whether the chlorophyll-nucleophile adduct is monomeric or polymeric. Bifunctional ligands, such as dioxane, pyrazine, 1, diazobicyclo(2.2.2.)octane, and, in particular, water can cross-link chlorophyll molecules or chlorophyll dimers by coordination to Mg to form large (chlorophyll-nucleophile) micelles (to be published). The chlorophyll species present in a particular experiment are very sensitive to temperature, adventitious nucleophiles such as water, and solvent, factors not always taken into account in previous work. EXPERIMENTAL METHODSChlorophyll samples were dried, prior to solution preparation, by codistillation with CCL (36). The solvents used for in vitro measurements were first dried over Linde 3A molecular sieve and degassed under reduced pressure. Solutions were prepared and oxidant was added in a nitrogen-filled dry box or on the vacuum line.Irradiations were performed with a 150-W Varian Eimac lamp. Infrared components were removed by 2 cm of water and 2 dichroic infrared-rejecting filters. All irradiations were by red light with a Corning 2404 sharp cut-off filter. All in vitro measurements were made in 4-mm quartz esr tubes at -170'C; in vivo measurements were made at room temperature on concentrated slurries of cells held in a Varian water cell (V4548).625

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