Evidence for photoreduction of monomeric bacteriochlorophyll a as an electron acceptor in the reaction center of the green photosynthetic bacterium Prosthecochloris aestuarii

Photooxidation and triplet formation of the primary electron donor of the green photosynthetic bacterium Prosthecochloris aestuarii observed with ESR spectroscopy

“…These values for k, and kv differ considerably from those measured in purple bacteria [ 19-211. Fig.3 shows the kinetics of the absorbance change at 425 nm, induced by a short flash in the RCPP complex at 5 IS, which is ascribed to the triplet of P-840 [6]. Semilogarithmic analysis yielded a monophasic decay within the limits of error, with a halftime of -280 /.is, which is in good agreement with the average halftime (290 f. 30 ps) measured with ESR.…”

“…As was reported [6], only in -50% of the reaction centers in the PP complex a stable charge separation is produced at <120 K. In the other half, electron transport at the acceptor side appears to be inhibited, resulting in the formation of the BChl triplet of P-840, which decayed with a halftime of 165 /.JS at 120 K [6]. The ESRspectrum ofthe BChla triplet, together with the P-840' signal around g = 2, obtained upon illumination of the PPcomplex at 5 K is shown in fig.2.…”

“…In [ 131 a partial reversal of P-840 oxidation in complex I of C. limicola was observed; however, lowering the redox potential to 590 mV did not eliminate the irreversible signal [ 131. The decay of the P-840' signal at 5 K after a short flash had a halftime of 13 ms, probably due to the back reaction between P-840' and a reduced acceptor, as observed at 77 K [6]. Flash-induced absorbance changes at 5 K, measured at 606 nm yielded the same halftime.…”

“…This is also indicated by the difference in position of the maximal bleaching of the Qy-bands (830--840 nm in green bacteria [2,5,6] and X70-890 nm in purple bacteria).…”

“…At room temperature both complexes show photooxidation of P-840, the primary electron donor, and of 1 cytochrome c-553/reaction center [3]. Upon flash excitation the formation of the triplet state of P-840 was observed with optical difference spectroscopy in both complexes [6]. At <120 K the amount of P-840 triplet formed in a saturating flash was 0.4405 in the PP complex and close to 1 /reaction center in the RCPP complex.…”