The derivation of the oxygen atoms of the 131‐oxo and 3‐acetyl groups of bacteriochlorophyll a from water in Rhodobacter sphaeroides cells adapting from respiratory to photosynthetic conditions: evidence for an anaerobic pathway for the formation of isocyclic ring E

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Abstract
Using mass spectrometry, we have demonstrated 18O‐labelling of both the 131‐oxo and 3‐acetyl groups of newly‐formed bacteriochlorophyll a synthesized by Rhodobacter sphaeroides cells during adaptation from respiratory to photosynthetic conditions in the presence of H2 18O. This derivation of the 131‐oxo group of bacteriochlorophyll a from water provides a stark constrast with that of chlorophylls in higher plants where ring E formation is an aerobic process in which the 131‐oxo group arises from molecular oxygen via an oxygenase activity. The formation of the 3‐acetyl group of bacteriochlorophyll a, however, is consistent with the enzymic hydration of the 3‐vinyl group of a derivative of chlorophyll a.