Bacteriopheophytin, the magesium-free base of bacteriochlorophyll, undergoes reversible one-electron reduction in organic solvents to yield an anionic free radical with characteristic optical and electron spin resonance spectra. The which was pumped away to codistill water, and the pigment was dried under reduced pressure (10-3 Pa) for several hours. RESULTSThe anion radical of bacteriopheophytin BPh exhibits a reversible, one electron, halfwave reduction potential E1/2 of -0.58 against NHE in CH2Cl2.

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Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl) a and its cation radicals. Oxidation potentials for Chl -Chl+' as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl+' ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P480 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment. Photosynthesis in algae and green plants functions via two chlorophyll (Chl)-mediated systems that cooperatively oxidize water (photosystem II, PS II) and reduce carbon dioxide (PS I) (1-14). The primary chemical products generated by the light harvested by the antenna network and funnelled to the reaction centers are believed to be oxidized Chls, P-700+ in PS I and P-680+-in PS II1(1-14), and reduced Chl or pheophytin (11,(15)(16)(17). The optical changes associated with the oxidations of P-700 and P-680 (18-24), particularly an enhanced absorption at -820 nm, and the electron spin resonance (ESR) linewidths and g-values (10, [24][25][26][27][28][29][30][31][32] are characteristic of Chl ir cation radicals (33). ESR (10,(25)(26), ENDOR (electron-nuclear double resonance) (34-35), optical, and circular dichroism (36) results have further established the dimeric or "special pair" nature of P-700+ . ESR data show (25) that the 7 G (1 G = 1o-4 tesla) peak-to-peak linewidth (AH) of P-700+ is narrower by V-than that of Chl+-(AH = 9 G) in dichloromethane and dichloromethane/methanol solutions, as expected if the unpaired electron of the radical is equally shared between two Chl molecules. Additional support for delocalization of spin over two Chls in P-700+-derives from ENDOR spectra of oxidized chloroplasts (34,35) in which several proton hyperfine splittings are found to be approximately half those of Chl+-in vitro (37,38). The nature of P-680+' in PS II is considerably more ambiguous: linewidths of ESR signals attributed to P-680+-range from 7 to 9 G (27-32), resulting in proposals of monomer (28), dimer (29-31), and even trimer (32) configurations. Even more puzzling are the large differences in oxidation potentials of P-700 and P-680. Titrations of P-700 yield a midpoint potential (39-41) ranging between +0.4 and +0.5 V against the normal hydrogen electrode, whereas the minimu...

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