The iron-quinone acceptor complex in Rhodospirillum rubrum chromatophores studied by EPR

Abstract

A new EPR signal is reported in Rhodospirillum rubrum chromatophores. The signal is attributed to Q − BFe 2+, the semiquinone-iron complex of the secondary quinone electron acceptor, on the basis of the following observations. (1) It is induced by a single laser flash given a room temperature and is stable. (2) It is present after odd-numbered flashes and absent after even-numbered flashes when a series of flashes is given. (3) When it is already present, low-temperature illumination results in the disappearance of the signal due to formation of the Q − AFe 2+Q − B state. (4) Its formation is inhibited by the presence of orthophenanthroline at normal values of pH. The Q − BFe 2+ signal has two main features, one at g = 1.93 and the other at g = 1.82. The two features have different microwave power and temperature dependences, with the g = 1.82 signal being more difficult to saturate and requiring lower temperatures to be observable. Raising the pH leads to an increase in the g = 1.82 feature, while the g = 1.93 signal decreases in amplitude. It is suggested that the two parts of the signal may represent two EPR forms due to structural heterogeneity. The low-field feature of the Q − BFe 2+ signal shifts to lower field as the pH is raised and a p K for this change seems to occur at pH 9.4. The Q − AFe 2+ signal at g = 1.88 also shifts as the pH is increased; however, the shift is less marked than that seen for Q − BFe 2+, the shift is to higher field and the range over which it occurs is wider and depends upon the temperature of Q − AFe 2+ formation. This effect may be due to a p K on a protein group being shifted to higher pH by the presence of Q − A. ortho-Phenanthroline broadens and shifts the Q − AFe 2+ signal. The inhibition of electron transfer between Q − A and Q B by ortho-phenanthroline becomes less effective at high pH. The new Q − BFe 2+ signal is unlike other semiquinone-iron signals reported in the literature in bacteria; however, it is remarkably similar to the Q − BFe 2+ signal reported in Photosystem II.