Investigation of exchange couplings in [Fe3S4](+) clusters by electron spin-lattice relaxation

We have studied four proteins containing oxidized 3Fe clusters ([Fe3S4](+), S=1/2, composed of three, antiferromagnetically coupled high-spin ferric i ons) by continuous wave (CW) and pulsed EPR techniques: Azotobacter vinelan dii ferredoxin I, Desulfovibrio gigas ferredoxin II, and the 3Fe forms of P yrococcus furiosus ferredoxin and aconitase. The 35 GHz (Q-band) CW EPR sig nals are simulated to yield experimental g tensors, which either had not be en reported, or had been reported only at X-band microwave frequency. Pulse d X- and Q-band EPR techniques are used to determine electron spin-lattice (T-1, longitudinal) relaxation times at several positions on the samples' E PR envelope over the temperature range 2-4.2 K. The T-1 values vary sharply across the EPR envelope, a reflection of the fact that the envelope result s from a distribution in cluster properties, as seen earlier as a distribut ion in g(3) values and in Fe-57 hyperfine interactions, as detected by elec tron nuclear double resonance spectroscopy. The temperature dependence of 1 /T-1 is analyzed in terms of the Orbach mechanism, with relaxation dominate d by resonant two-phonon transitions to a doublet excited state at similar to 20 cm(-1) above the doublet ground state for all four of these 3Fe prote ins. The experimental EPR data are combined with previously reported Fe-57 hyperfine data to determine electronic spin exchange-coupling within the cl usters, following the model of Kent et al. Their model defines the coupling parameters as follows: J(13) = J, J(12) = J(1+epsilon'), J(23) = J(1+epsil on), where J(ij) is the isotropic exchange coupling between ferric ions i a nd j, and epsilon and epsilon' are measures of coupling inequivalence. We h ave extended their theory to include the effects of epsilon' not equal 0 an d thus derived an exact expression for the energy of the doublet excited st ate for any epsilon, epsilon'. This excited state energy corresponds roughl y to epsilon J and is in the range 5-10 cm(-1) for each of these four 3Fe p roteins. This magnitude of the product epsilon J, determined by our time-do main relaxation studies in the temperature range 2-4 K, is the same as that obtained from three other distinct types of study: CW EPR studies of spin relaxation in the range 5.5-50 K, NMR studies in the range 293-303 K, and s tatic susceptibility measurements in the range 1.8-200 K. We suggest that a n apparent disagreement as to the individual values of J and epsilon be res olved in favor of the values obtained by susceptibility and NMR (J greater than or similar to 200 cm(-1) and epsilon greater than or similar to 0.02 c m(-1)), as opposed to a smaller J and larger epsilon as suggested in CW EPR studies. However, we note that this resolution casts doubt on the accepted theoretical model for describing the distribution in magnetic properties o f 3Fe clusters.