Charge effect in protein metalation reactions by diruthenium complexes

The properties of paddlewheel diruthenium compounds significantly depend on the nature of the bridging equatorial ligands. The charge of these complexes is a factor to take into account when studying their interaction with proteins. To compare the reactivity of diruthenium compounds with the model protein hen egg white lysozyme (HEWL), we have prepared the well-known anionic complex [Ru-2(CO3)(4)](3-), two new anionic species, [Ru-2(D-p-FPhF)(CO3)(3)](2-) and [Ru-2(DAniF)(CO3)(3)](2-), and their analogues [Ru2Cl(D-p-FPhF)(O2CCH3)(3)] and [Ru2Cl(DAniF)(O2CCH3)(3)] that generate cationic species in solution (D-p-FPhF- = N,N & PRIME;-bis(4-fluorophenyl)formamidinate and DAniF(-) = N,N & PRIME;-bis(4-methoxyphenyl)formamidinate). The interaction of these compounds with HEWL was investigated by UV-vis absorption spectroscopy, circular dichroism, intrinsic fluorescence and X-ray crystallography. The molecular structures of the adducts differ in the number of metal binding sites, in the binding mode and in the type of fragments that are bound to the protein. The charge of the diruthenium complexes in aqueous solutions strongly influences their protein binding properties. High-negative charge complexes are non-covalently bound. However, the replacement of carbonate ligands changes the negative charge of these complexes and favours covalent binding. These results have great implications for further studies in the tailoring of artificial diruthenium-containing metalloenzymes.