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Bacteriochlorophyll a. R is phytyl or geranylgeranyl.

Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct photosynthesis, but do not produce oxygen. They use wavelengths of light not absorbed by plants or Cyanobacteria. Different groups contain different types of bacteriochlorophyll:

Pigment Bacterial group in vivo infrared absorption maximum (nm)
Bacteriochlorophyll a Purple bacteria, Heliobacteria, Green Sulfur Bacteria, Chloroflexi, Chloracidobacterium thermophilum[1] 805, 830-890
Bacteriochlorophyll b Purple bacteria 835-850, 1020-1040
Bacteriochlorophyll c Green sulfur bacteria, Chloroflexi, C. thermophilum 745-755
Bacteriochlorophyll cs Chloroflexi 740
Bacteriochlorophyll d Green sulfur bacteria 705-740
Bacteriochlorophyll e Green sulfur bacteria 719-726
Bacteriochlorophyll f Green sulfur bacteria (currently found only through mutation; natural may exist)[2] 700-710
Bacteriochlorophyll g Heliobacteria 670, 788

Bacteriochlorophylls a, b, and g are bacteriochlorins, meaning their molecules have a bacteriochlorin macrocycle ring with two reduced pyrrole rings (B and D). Bacteriochlorophylls c, d, e, and f are chlorins, meaning their molecules have a chlorin macrocycle ring with one reduced pyrrole ring (D).

Chemical structures comparing porphin, chlorin, bacteriochlorin, and isobacteriochlorin. Note relocation of C=C double bond between the two bacteriochlorin isomers. There are two π electrons (symbolized by π e) for every double bond in the macrocycle.