The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll 663, is chlorophyll alpha esterified with Delta 2,6-phytadienol

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll (BChl) 663, was isolated at high purity by an improved purification proced ure from a crude reaction center complex, and the molecular structure was d etermined by fast atom bombardment mass spectroscopy (FAB-mass), H-1- and C -13-NMR spectrometry, double quantum filtered correlation spectroscopy (DQF -COSY), heteronuclear multiple-quantum coherence (HMQC) and heteronuclear m ultiple-bond correlation (HMBC) spectral measurements. BChl 663 was 2.0 mas s units smaller than plant Chl a. The NMR spectra showed that the macrocycl e was identical to that of Chl a. In the esterifying alcohol, a singlet P7( 1) signal was observed at the high-field side of the singlet P3(1) signal i n BChl 663, while a doublet peak of P7(1) overlapped that of P11(1) in Chl a. A signal of P7-proton, seen in Chl a, was lacking, and the P6-proton app eared as a triplet signal near the triplet P2-proton signal in BChl 663. Th ese results indicate the presence in BChl 663 of a C=C double bond between P6 and P7 in addition to that between P2 and P3. 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 13(2)-epimer of BChl a, BChl a', were found to be present per reaction center, which may constit ute the primary electron donor.