Cryogenic electron acceptors

Attempts to identify the primary electron acceptor that takes an electron from P-870 focused first on nonheme iron. Illumination of chromatophores at cryogenic temperatures gave rise to a broad EPR signal at g = 1.8, in addition to the sharp signal at g = 2.0025 characteristic of P [18-21]. However, the photooxidation of P-870 still occurred in preparations which were depleted of Fe, and in these the reduced electron acceptor gave a sharp EPR signal consistent with an organic se-miquinone [22]. Purified reaction centers from Rhodobacter sphaeroides were... 

Fig. 4. (A) EPR spectra of TSF lla particles poised at -450 mV and after 90-s illumination at 295 or 220 K and measured at two different microwave powers. (B) shows effect of microwave power (P) on the amplitude of the photoinduced narrow (singlet) and doublet EPR signals at 7 K, Figure source Klimov, Dolan and Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 98,99 and Klimov, Dolan, Shaw and Ke (1980) Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-lron complex in photosystem-ll reaction centers. Proc Nat Acad Sci, USA. 77 7228...

VV Klimov, E Dolan and B Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 97-100... 

The membrane-bound iron-sulfiir centers were discovered by Dick Malkin and Alan Bearden in 1971 in spinach chloroplasts using EPR spectroscopy. Since the EPR spectrum was found to resemble that of the iron-sulfur protein ferredoxin and since the soluble ferredoxin had already been removed from the chloroplast sample used in the measurement, the substance represented by the newly found EPR spectrum was initially called membrane-bound ferredoxin. And since the iron-sulfur center was also found to be photo-reducible at cryogenic temperature, it was therefore suggested that it was the primary electron acceptor of photosystem I. 

Electron flow beyond the pheophytin has also been demonstrated in the presence of the artificial electron acceptor, silicomolybdate [SiMo] (6,8). More recently, we have reported that in the presence of SiMo, P680 and the radical of the monomeric chlorophyll can be detected at cryogenic temperatures by esr (9). In a small proportion of centres, electron transfer from the tyrosine radical to P680" also occurred (9). Here, we report upon the reconstitution of the reaction centre complex with the exogenous quinones, DPQ and DBMIB. 

---