The reaction center and associated cytochromes of Thiocapsa pfennigii: Their thermodynamic and spectroscopic properties, and their possible location within the photosynthetic membrane

Abstract

The majority of photosynthetic bacteria possess bacteriochlorophyll a, but two species, Rhodopseudomonas viridis and Thiocapsa pfennigii, possess bacteriochlorophyll b, which absorbs at significantly longer wavelengths. The reaction center of Rps. viridis has been extensively studied; this paper presents studies on the reaction center of T. pfennigii, with the following results. 1. (i) The “primary donor” bacteriochlorophyll dimer of T. pfennigii has a midpoint redox potential of +490 mV at pH 7, and the oxidized form has a gaussian EPR signal centered close to g = 2.0025 with a peak-to-peak width of 13 G. The zero field splitting characteristics of the light-induced triplet state of the bacteriochlorophyll dimer, seen when the “primary acceptor” is reduced before illumination, are D = 158 · 10 −4 cm −1, E = 39 · 10 −4 cm −1. These EPR properties are very similar to those of the bacteriochlorophyll dimer of Rps. viridis, and are quite distinct from those of bacteriochlorophyll a-containing species. 2. (ii) The intermediary carrier, I, which is probably a bacteriopheophytin b molecule in close association with the “primary acceptor”, can be trapped in its reduced form by the illumination of appropriately poised samples at 200 K. The EPR signals associated with I −· are very similar to those of Chromatium vinosum, with the majority of the spins being observed in the form of a broad signal centered close to g = 2.003, and split by 68 G; the magnitude of the splitting of this signal is only half that seen in Rps. viridis. We have been unable to measure the E m of the I/I −· couple in T. pfennigii. 3. (iii) The “primary acceptor” of T. pfennigii is very similar to that of the majority of other purple bacterial species, being characterized by an EPR signal at g = 1.82 and g = 1.62. The E m of the redox couple is pH-dependent below pH 6.5, and the E m of the unprotonated couple, which is believed to function in electron flow, is −130 mV. 4. (iv) Four cytochrome hemes can apparently donate electrons to the photooxidized bacteriochlorophyll dimer free radical cation, two of cytochrome c-555 ( E m7 = 340 mV) and two of cytochrome c-550 ( E m7 = 0). If all are reduced prior to illumination, the latter are preferentially oxidized. 5. (v) The carotenoid bandshift of T. pfennigii can be interpreted as indicating that the bacteriochlorophyll dimer is located near the middle of the membrane dielectric, with the high potential cytochrome c-555 closer to the inside, and quinone · iron closer to the outside of the chromatophore membrane. The low potential cytochrome c-550 seems to be nearer the outside of the membrane than the bacteriochlorophyll dimer.