Electron Spin Resonance of Chlorophyll and the Origin of Signal I in Photosynthesis

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A comparison has been made between Signal I, the photo-electron spin resonance signal associated with the primary light conversion act in photosynthesis, and free-radical signals generated in various chlorophyll species in vitro. The esr signals obtained from chlorophyll·monomer, (Chl·L)+., chlorophyll dimer, (Chl2)+., and chlorophyll oligomer, (Chl2)n+., are broader than Signal I, whereas the chlorophyll-water adduct, (Chl·H2O)n+., gives a signal very much narrower than Signal I. The unusually narrow signal from (Chl·H2O)n+. has been ascribed to spin migration, or to unpaired spin delocalization over a large number of chlorophyll molecules. The linewidth of Signal I can be accounted for by a similar delocalization process. A theoretical relationship between the esr linewidth and the number of chlorophyll molecules, N, over which an unpaired spin is delocalized, takes the form ΔHN = 1/√N·ΔHM, where ΔHM is the linewidth of monomer (Chl·L)+.. This relationship for N = 2 accounts well for the linewidths of Signal I in green algae, blue-green algae, and photosynthetic bacteria in both the 1H- and 2H-forms. The linewidth of Signal I (as well as the optical properties of reaction-center chlorophyll) are consistent with unpaired spin delocalization over an entity containing two chlorophyll molecules, (Chl·H2O·Chl)+..

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