Chapter 6 The structure and biosynthesis of bacteriochlorophylls

Bacteriochlorophylls a and b (Structure 4) are the best known Chls produced by photosynthetic bacteria. Specifically, these bacteriochlorophylls (BChl) are the photosynthetic pigments of purple bacteria, and their purple color results from the fact that the parent porphyrin macrocycle is ‘reduced’ not only in ring D [which gives rise to the green color characteristic of Chl a], but also in ring B. Thus, one might regard the BChl a and b as ‘tetrahydroporphyrins\u27. This is certainly true for the case of BChl a, but the reduction state of BChl b might more correctly be described as being at the same ‘dihydro’ level as for the normal plant Chls. Simple migration of the ring B ethylidene double bond into ring B proper would actually yield a macrocycle at the same reduction level as Chl a. However, BChl b also possesses a purple color because the ring B ethylidene moiety still constitutes a structural interference of the n-electron delocalization pathway. BChls a from different sources have been shown to possess variability in their 7-side chain esterifying alcohol; for example, the BChl a from Rhodobacter sphaeroides possesses a phytyl ester (i.e. BChl aphy) while the same pigment from Rlzodospirillum rubrum features geranylgeraniol (i.e. BChl α88). The best known purple BChl, BChl a (3), is found in Rhodospirillaceae. BChl b is the major pigment in Rhodopseudomonas viridis. © 1991, Elsevier B.V. All rights reserved