Ubisemiquinone in membranes from Escherichia coli.

The e.s.r. t spectra of respiratory membranes from mitochondria show an asymmetric signal with the major component in the region of g = 1.9 that has been attributed to 'non-haem iron' (Sands & Beinert, 1960; see also Beinert & Palmer, 1965). A symmetrical signal due to an organic radical (g 2.00) has also been found in respiratory membranes (Sands & Beinert, 1960), but has not been definitely assigned owing to the lack of hyperfine structure in protein-bound radicals (see Beinert & Palmer, 1965). Two possible sources of the latter signal have been suggested, namely the semiquinones of flavin and ubiquinone (see Beinert & Palmer, 1965). As part of a study of the electron-transport system of Escherichia coli K12, the e.s.r. spectra of membrane suspensions were recorded. Membranes were isolated by ammonium sulphate fractionation (Cox, Snoswell & Gibson, 1968) from E. coli K12 strainAN 62 (Cox, Young, McCann & Gibson, 1969), which possessed a normal respiratory system. The e.s.r. spectrum of these membranes is depicted in Fig. 1(a). The spectrum consists of a symmetrical signal with g = 2.003i 0.001 and with a peak-to-peak width of 12G. The g value, symmetry, peak-to-peak width and the saturation at relatively low power levels are consistent with an organic free radical of the semiquinone type being the source of the signal (Beinert & Palmer, 1965). The maximum value of the signal corresponded to about 2% of the total ubiquinone present and was reached at different times depending on themembrane preparation used. The signal was always relatively intense initially, but was removed by continued incubation in the e.s.r. tube at 00C for 24h, to be replaced by an asymmetric signal with two peaks at g values of 2.02 and 1.92 (Fig. lb). These two peaks are part of the same signal, as they appeared simultaneously at a constant intensity ratio and, in addition, they