Composition and optical properties of reaction centre core complexes from the green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum

Photosynthetically active reaction centre core (RCC) complexes were isolate d from two species of green sulfur bacteria, Prosthecochloris (Ptc.) aestua rii strain 2K and Chlorobium (Chl.) tepidum, using the same isolation proce dure. Both complexes contained the main reaction centre protein PscA and th e iron-sulfur protein PscB, but were devoid of Fenna-Matthews-Olson (FMO) p rotein. The Chl. tepidum RCC preparation contained in addition PscC (cytoch rome c). In order to allow accurate determination of the pigment content of the RCC complexes, the extinction coefficients of bacteriochlorophyll (BCh l) a in several solvents were redetermined with high precision. They varied between 54.8 mM(-1) cm(-1) for methanol and 97.0 mM(-1) cm(-1) for diethyl ether in the Q(Y) maximum. Both preparations appeared to contain 16 BChls a of which two are probably the 13(2)-epimers, 4 chlorophylls (Chls) a 670 a nd 2 carotenoids per RCC. The latter were of at least two different types. Quinones were virtually absent. The absorption spectra were similar for the two species, but not identical. Eight bands were present at 6 K in the BCh l a Q(Y) region, with positions varying from 777 to 837 nm. The linear dich roism spectra showed that the orientation of the BChl a Q(Y) transitions is roughly parallel to the membrane plane; most nearly parallel were transiti ons at 800 and 806 nm. For both species, the circular dichroism spectra wer e dominated by a strong band at 807-809 nm, indicating strong interactions between at least some of the BChls. The absorption, CD and LD spectra of th e four Chls a 670 were virtually identical for both RCC complexes, indicati ng that their binding sites are highly conserved and that they are an essen tial part of the RCC complexes, possibly as components of the electron tran sfer chain. Low temperature absorption spectroscopy indicated that typical FMO-RCC complexes of Ptc. aestuarii and Chl. tepidum contain two FMO trimer s per reaction centre.