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 g F (BChl g F ), its 13 2 ‐epimer BChl g ′ F and 8 1 ‐hydroxy‐chlorophyll a F (8 1 ‐OH‐Chl a F ). Because the full stereochemistry of these naturally occurring chlorophyllous pigments has remained unknown, we determined the stereochemistry of both BChl g F and 8 1 ‐OH‐Chl a F 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 1 H‐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 g F , ( 2 ) R ‐configuration was identified for the 1‐hydroxyethyl group at ring‐B in 8 1 ‐OH‐Chl a F and ( 3 ) 13 2 ‐( R )‐, 17‐( S )‐ and 18‐( S )‐configurations at ring‐D and ‐E in both BChl g F and 8 1 ‐OH‐Chl a F were confirmed. These stereo‐chemistries enabled us to discuss their biosynthesis and to propose possible routes for preparation of ethylidene and 1‐hydroxyethyl groups at the 8‐position.