The binding of amino acids to the differentiated iron in 3:1 site-differentiated {Fe4S4} clusters

The 3:1 site-differentiated clusters [PPh4](2)[Fe4S4(L) (SEt)] (1, 2) and [NEt4](2)[Fe4S4(L) (SEt)] (3, 4) (L=L-1 or L-2; H3L1=1,4,7-tris(4-sulfanylbenzoyl)-1,4,7-triazacyclononane H3L2=1,5,9-tris(4-sulfanylbenzoyl)-1,5,9-triazacyclododecane) when treated with equimolar amounts of amino acid methyl ester in dimethyl sulfoxide solution form the amino acid substituted site-differentiated clusters [Fe4S4(L) (AA-OMe)](2-) (HAA-OMe=HCys-OMe, L-cysteine methyl ester (5-8); HTyr-OMe, L-tyrosine methyl ester (13-16); HSer-OMe, DL-serine methyl ester (17-20)) or [Fe4S4(L) (HHis-OMe)](-) (HHis-OMe=L-histidine methyl ester (21, 22)). When 0.5 mol equiv. of HCys-OMe were employed in the reaction cysteinato-bridged dimeric {Fe4S4) clusters, [{Fe4S4(L)}(2)(mu-Cys-OMe)](3-) (9-12), were prepared. Clusters 5-22 have been characterised by H-1 NMR, Mossbauer and IR spectroscopies. Some of the clusters reported are coordinated by ligand (L) and amino acid ester, at the fourth, differentiated iron site, in a fashion reminiscent of that for iron-sulfur clusters in some metalloproteins, including the nitrogenase iron-molybdenum cofactor and P-clusters and the distal {Fe4S4} cluster of the nickel-iron hydrogenase of Desulfovibrio gigas.