SPIN-DENSITIES AND SPIN COUPLING IN IRON-SULFUR CLUSTERS - A NEW ANALYSIS OF HYPERFINE COUPLING-CONSTANTS

We present a new analysis of Fe-57 isotropic hyperfine coupling consta nts in iron-sulfur clusters containing one, two, three, or four iron a toms. Instead of relying on a unique set of site values for ferric and ferrous ions which depend on the degree of covalency of the iron atom with the surrounding atoms and which contain a variable spin-orbit co ntribution, we propose the use of semi-empirical free ion constants (a ) over bar(Fe3+) and (a) over bar(Fe-2.5+) whose values are semiempiri cally found to be about -31 MHz and -32 MHz respectively (we found, in addition, -38 MHz for the pure core-polarization constant (a) over ba r(c)(Fe2+), in excellent agreement with theoretical calculations). The se are transferrable from one system to another, and can be combined w ith estimated covalency factors to define ''site values.'' These value s allow us to derive a set of spin projection coefficients for a varie ty of iron-sulfur clusters (two to four irons, and some mixed-metal co mplexes) in different oxidation states, These can be compared to those deduced from proposed spin-coupling schemes. For the [Fe4S4](3+) clus ter, which forms the active site of the high potential iron protein (H iPIP), we conclude that the best simple spin state, within a pairwise model in which two ions dimers combine to make a tetramer, is \(S-mv,S -,S-ferric,S-t] = \7/2,3,1/2], and not \9/2,4,1/2) as is often assumed . The stabilization of this spin state is rationalized in terms of spi n frustration. For Fe-4 ferredoxins, we find spin projection coefficie nts intermediate between those of the \7/2,3,1/2 and \5/2,2,1/2] state s, whereas the aconitase cluster (both with and without substrate) has coefficients intermediate between those of the states \9/2,4,1/2] and \7/2,3,1/2). In aconitase, the change upon release of substrate appea rs to be accompanied by,a relocalization of the mixed-valence pair. We also report analyses for mixed-metal complexes of the form [MFe(3)S(4 )](n+), with (M,n) = (V,2), (Co,2), (Ni,1), (Zn,1), and (Mo,3), in an attempt to gain some insight into a wide variety of polynuclear spin-c oupling schemes.