The primary electron acceptor of green sulfur bacteria is chlorophyll a esterified with Δ2,6-phytadienol

Abstract

The primary electron acceptor of green sulfur bacteria, bacterio-chlorophyll (BChl) 663, was isolated at high purity by an improved purification procedure from a crude reaction center complex, and the molecular structure was determined by FAB-mass and NMR spectrometry. The FAB-mass spectra showed that the mass values in the macrocycle of BChl 663 and plant Chl a were the same, but the esterifying alcohol of BChl 663 was 2.0 mass units smaller than phytol. The NMR spectra showed that the macrocycle was identical to that of Chl a and that a C=C double bond between P6 and P7 in addition to that between P2 and P3 of phytol was present in the esterifying alcohol of BChl 663. The structure of BChl 663 was hence concluded to be Chl a esterified with ? 2,6-phytadienol instead of phytol. In addition to BChl 663, two molecules of the 132-epimer of BChl a, BChl a', were found to be present per reaction center, which may constitute the primary electron donor of green sulfur bacteria.