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Through the use of an Escherichia coli strain deficient in menaquinone biosynthesis, purified nitrate reductase A (NarGHI)-enriched inner membrane vesicles were titrated and monitored by electron paramagnetic resonance (EPR) spectroscopy, revealing the formation of protein-bound ubisemiquinone (USQ) species. Two-dimensional ESEEM (HYSCORE) experiments on these radicals revealed the same magnetic interaction with an (14)N nucleus as found for menasemiquinone stabilized at the Q(D) site of E. coli NarGHI and assigned to His66 N(delta), a distal heme axial ligand. Moreover, this signature was lost in the NarGHI(H66Y) mutant, which is known to be unable to react with quinols. These findings demonstrate that NarGHI-bound USQ can be formed and detected by EPR. They also provide the first direct experimental evidence for similar binding of natural menasemiquinones and ubisemiquinones within the same protein Q site of NarGHI.

Q -1 “metallic” range (01, = 1.8(2) cm s , the diffusion constant Dl, exhibits a temperature activated behaviour in the low temperature range, at variance with the prediction invoking the model of an intrinsic semiconductor. A low-temperature divergence of the homogenous ESR line width, indicative of one-dimensional fluctuations, was also observed. LOW-DIMENSIONAL ORGANIC conductors have attracted much interest since the discovery of superconductivity in several different crystalline charge transfer (CT) complexes of the Bechgaard-type [l-3]. Up to date no system using pure hydrocarbons (HC) as donor molecules were found to exhibit superconductivity. Instead they are characterized by broad metal-toinsulater (MI) transitions in the temperature range of 120-200 K. In all systems studied so far the low temperature ground state was non-magnetic, thus rendering the HC charge transfer salts ideal candidates for magnetic resonance investigations. However, conventional cw ESR probing susceptibility and line width is of limited value for unravelling the microscopic behaviour of the charge transport. A few systems, exhibiting extremely narrow ESR lines in the range of IO-30mG, have been investigated by pulsed ESR. Especially the CT-complex (fluoranthenyl)sX was utilized as a model compound [4]. In the high-temperature metallic phase the extreme onedimensionality was proven from the analysis of the Hahn-echo decay function with [S] and without [6] the application of an external field gradient. The application of the well-established experimental method to other HC-CT complexes was impeded by electron-spin dephasing times T,, in the sub-MS range. We therefore developed a pulsed ESR apparatus with optimized time resolution, allowing the detection of the echo signal within at least 200ns referenced to the beginning of the first microwave pulse. With such a time resolution not only the spin diffusion constant in the metallic regime can be measured but also the relaxation rates in the low-temperature insulating phase can be investigated. In particular, we were interested, if remnants of SDW fluctuations could be detected via T,, in the low-temperature regime, were susceptibility measurements showed a T-O.” behaviour, indicative of a random exchange Heisenberg antiferromagnetic chain (REHAC) [7].

The electron-spin-echo decay function was determined for the conducting phase of [(C16H10)2]+X- in the temperature range 183-300 K. The decay function exhibited an exp[-(γt)3/2] dependence, characteristic of one-dimensional spin diffusion. The ratio 5×106≤D∥/D⊥≤5×108 for the inner-stack-to-out-of-stack diffusion rates was determined from the characteristic time t0, at which the electron-spin-echo decay function changed to the "conventional" exp(-2γ′τ)) form. The deduced value 2×1013≤D∥≤2×1016 rad/s is consistent with the bulk dc conductivity and with recently determined nuclear spin-lattice relaxation rates. The overall assumption of highly mobile electronic spins was confirmed by an Overhauser-type experiment.