THE SIZE OF THE PHOTOSYNTHETIC UNIT IN PURPLE BACTERIA

Pigment analysis was performed by means of normal phase HPLC on a numb er of bacteriochlorophyll a and b containing species of purple bacteri a that contain a core antenna only. At least 99% of the bacteriochloro phyll in Rhodobacter sphaeroides R26, Rhodopseudomonas viridis and Thi ocapsa pfennigii was esterified with phytol (BChl ap and BChl bp, resp ectively). Rhodospirillum rubrum contained only BChl a esterified with geranyl-geraniol (BChl a(GG)). Rhodospirillum sodomense and Rhodopseu domonas marina contained, in addition to BChl ap, small amounts of BCh l a(GG), and presumably also of BChl a esterified with dihydro and tet rahydro geranyl-geraniol (Delta 2,10,14-phytatrienol and probably Delt a 2,14-phytadienol). In all species bacteriopheophytin (BPhe) esterifi ed with phytol was present. The BChl/BPhe ratio indicated that in thes e species a constant number of 25 +/- 3 antenna BChls is present per r eaction centre. This number supports a model in which the core antenna consists of 12 alpha-beta heterodimers surrounding the reaction centr e. Determination of the in vivo extinction coefficient of BChl in the core-reaction centre complex yielded a value of ca. 140 mM(-1) cm(-1) for BChl a containing species and of 130 mM(-1) cm(-1) for Rhodopseudo monas viridis.