Gold-induced spin-state changes in haem proteins

Abstract During the course of our work on anti-arthritic gold drugs, we discovered that Et 3 PAuCl converted ferric cytochrome into a novel, green high-spin form at pH 7, ambient temperature [1]. The reaction was readily reversed of gold by gel filtration or addition of competing gold-binding ligands. Such a facile spin-state conversion of cytochrome c is unusual [2], and we have now extended our studies to other haem proteins with the aim of identifying the critical gold-bonding sites. The axial ligands for iron in the proteins studied are shown in Fig. 1. The ferric cytochrome b 5 ( Erwinia chrysanthememi ) was predominantly low-spin at pH 7 [3]. Titration with Et 3 PAuCl gave rise to an increase in absorption at 610 nm similar to that observed previously with cyt c [1], 24 equivalents of gold giving complete conversion to a green high-spin form. The reaction was reversed when gold was removed (gel filtration), but, in contrast to cyt c, too rapid for any kinetic observations by conventional means. From this we concluded that gold attack on either histidine or the haem group itself is involved. Although Et 3 PAuCl rapidly denatured ferric methaemoglobin, metmyglobin was fully converted into a green high-spin product by about 30 equivalents of Au at similar haem concentrations (0.07 m M ). However, in contrast to cytochrome c , Et 3 PAuCl also reacted with the ferrous proteins. With MbO 2 , a two-phase reaction was observed leading to the same high-spin green product as from metMb. HbO 2 behaved similarly but reacted more rapidly [4]. These gold-induced autoxidations appeared to involve release of O − 2 , and that of myoglobin, but not haemoglobin, was reversed by gold removal and Na 2 S 2 O 4 addition. Direct evidence for structural perturbations around the haem group as a result of gold binding were obtained by a comparison of the paramagnetically-shifted resonances of high-spin ferric myoglobin before and after gold treatment. Since similar reactions occurred with haemin chloride itself, direct attack of gold on the haem group as well as histidines may be responsible for the observed spin-state changes. Studies on other gold compounds suggest that AuP bond remains intact and that N displaces Cl − . A direct attack by gold on pyrrole nitrogens might be enhanced with a bifunctional gold compound, and indeed we find that only a 12-fold molar excess of [AuCl] 2 [depe], where depe is (1,2-diethylphosphino)ethane, is required for full spin-state conversion of ferric cyt c.