IDENTIFICATION OF EPR SIGNALS FROM THE STATES QA.-QB.- AND QB.- IN PHOTOSYSTEM-II FROM PHORMIDIUM-LAMINOSUM

We have studied the EPR signal in PS II from Phormidium laminosum with a g-value of 1.66, which we assign to an interaction between the semiquinones of Q(a) and Q(b) and the non-heme iron. 77 K illumination of samples from dark-poised redox titrations show the rise of the signal has a midpoint potential (E(m)) of about +60 mV, and it is lost with an E(m) of about -10 mV. Under the same conditions, the rise of the g = 1.9 signal from Q(a).--Fe2+ in the dark was found to be about +10-mV. The g = 1.66 signal can also be formed with a high yield by first illuminating dark-adapted PS II particles at 293 K, followed by a short period of darkness at 273 K and subsequent illumination at 77 K. We have measured the effect on signal yield of varying the period of darkness following 293 K illumination. Over 60% of the maximum signal size is seen after 1 min darkness, and increases further over 2 h. In these samples a signal attributed to Q(b).--Fe2+ is seen prior to 77 K illumination. Confirmation of the presence of Q(b).- was obtained by reductant-linked oxidation of the non-haem iron using phenyl-para-benzoquinone (PPBQ). Samples treated with the Q(b)-analogue tribromotoluquinone (TBTQ) give a modified EPR signal. We propose (i) that Q(b) is preserved in PS II preparations from P. laminosum; (ii) that Q(b)-semiquinone can be readily formed and trapped by freezing; and (iii) the g = 1.66 signal arises from a coupling between the primary and secondary plastosemiquinones and the non-haem iron.