Structures of Chls. (a) Stereochemistry and numbering system in monovinyl-Chl a. Variations of Chl include (b) Chl b (7-formyl, R = phytyl); and (c) Chl c1 (171-dehydro-Pchlide, R1 = methyl; R2 = ethyl or vinyl; R = H). Pchlide is similar to Chl but contains a saturated propionic rather than acrylic acid group on C17. (Structures as in ).