Anion radicals of bacteriochlorophyll a and bacteriopheophytin a. Electron spin resonance and electron nuclear double resonance studies

Chemical, electrochemical, and photochemical reductions of bacteriopheophytin a (BPh) and deuterated BPh yielded stable anion radicals which were characterized by electron spin resonance and electron-nuclear double resonance techniques. Radicals of the model bacteriochlorin compound, 5,10,15,20-tetraphenyl-2,3,12,13-tetrahydroporphyrin, selectively deuterated, were prepared to guide the interpretation of the BPh data and the experimental results compared with self-consistent field-molecular orbital calculations. ESR and ENDOR results for bacteriochlorophyll a (BChl) and deuterated BChl are also assigned with the aid of a model, zinc tetraphenyltetrahydroporphyrin, and molecular orbital calculations. These data thus help define the electronic profile of the BPh and BChl anions, the radical species presently postulated to result, within picoseconds, from the primary charge separation of bacterial photosynthesis.