Determination of Stereochemistry of Bacteriochlorophyll gF and 81-Hydroxy-chlorophyll aF from Heliobacterium modesticaldum¶

The reaction center complex of heliobacteria contains three kinds of chlorophyll pigments, bacteriochlorophyll gF (BChl gF), its 132-epimer BChl gF′ and 81-hydroxy-chlorophyll aF (81-OH-Chl aF). Because the full stereochemistry of these naturally occurring chlorophyllous pigments has remained unknown, we determined the stereochemistry of both BChl gF and 81-OH-Chl aF extracted from Heliobacterium modesticaldum. The configurations of the specific functional groups at ring-B as well as those at ring-D and -E were investigated by use of nuclear Overhauser effect correlations in their 1H-NMR spectra and circular dichroism spectra, as well as by modified Mosher's method in their chemical modification: (1) E-configuration was confirmed for the 8-ethylidene group at ring-B in BChl gF, (2) R-configuration was identified for the 1-hydroxyethyl group at ring-B in 81-OH-Chl aF and (3) 132-(R)-, 17-(S)- and 18-(S)-configurations at ring-D and -E in both BChl gF and 81-OH-Chl aF were confirmed. These stereochemistries enabled us to discuss their biosynthesis and to propose possible routes for preparation of ethylidene and 1-hydroxyethyl groups at the 8-position.