Photoreactions of bacteriopheophytins and bacteriochlorophylls in reaction centers of Rhodopseudomonas sphaeroides and Chloroflexus aurantiacus

Abstract

A comparison of spectral properties of reaction centers from Chloroflexus aurantiacus and Rhodopseudomonas sphaeroides (R-26) is reported. Treatment of reaction centers from Rps. sphaeroides with NaBH 4 leads to a decrease of the dipole strength of the 800-nm band by factor of approx. 1.75-1.95 and to the formation of new bacteriopheophytin, BPh-715, which is almost completely removed during the purification of reaction centers. The modification of the reaction centers does not change the quantum yield of P photooxidation and the spectrum of BPh-545 (H 1) photoreduction which includes the changing of the 800-nm band. This implies the preservation of the photoactive chain P-B 1-H 1-Q A (where B 1 is the bacteriochlorophyll (BChl)-800 molecule situated between P and H 1) and the modification of the second BChl-800 (B 2). The preparation of modified reaction centers is a mixture of at least three types of reaction centers with different contents of B 2 and of the second BPh (H 2). Some of the reaction centers (5-25%) contain the original B 2 and H 2 molecules (type I). In the CD spectrum of modified reaction centers a decrease of the 800-nm band and the appearance of a positive band at 765 nm is observed. This spectrum is similar to the CD spectrum of Chloroflexus reaction centers containing 3 BPh's and 3 BChl's. This implies that in some (approx. 40%) of the modified Rps. sphaeroides reaction centers (type II) B 2 has been replaced by BPh a which interacts with H 2. Probably some of the modified reaction centers (approx. 40%) have lost both B 2 and H 2 (type III). The modification of reaction centers leads to a considerable decrease of the CD bands at 800 (+) nm and 810 (−) nm and to a decrease of the absorbance changes near 800 nm in the difference absorption spectrum of the oxidation of P. The data are interpreted in terms of the interaction between P and B 1 molecules which gives two transitions at 790-800 and 810 nm with different orientations in modified Rps. sphaeroides as well as in Chloroflexus reaction centers. Similar transitions are observed for the interaction between P and B 2. The spectral analysis shows the existence of two chains P-B 1-H 1, and P-B 2-H 2 in which the distances between the centers of molecules are 1.3 nm or less.