Interaction studies between three ruthenium compounds ([Ru(η5-C5H5)(PPh3)(bopy)] [CF3SO3] (1), [Ru(η5-C5H5)(PPh3)(2- ap)][CF3SO3] (2) and [Ru(η5-C5H5)(PPh3)(isoquinpk)][CF3SO3] (3)) and two variants of albumin, HSA and HSAfaf by fluorescence spectroscopy, showed that the intrinsic fluorescence of Trp-214 resi- due is quenched in the presence of each ruthenium compound.
The most plausible quenching mechanism found for compounds 1 and 3 is the dynamic process, while for compound 2 is a static pro- cess. These results indicate that the ruthenium compounds bind to site I of HSA and that the interaction occurs close to the Trp-214 residue which is located in subdomain IIA, this being confirmed by the experiments carried out with the warfarin. The analysis of the binding equilibrium shows that the number of binding sites between the three compounds and both variants of albumin was about 1.
The thermodynamic parameters indicated that all the studied inter- actions are spontaneous (ΔG b 0), with the hydrophobic interactions being the major forces between these Ru compounds and albumin (ΔH N 0) and (ΔS N 0).
Remarkably, it was observed that small variations on the structures of the (N,O) heteroaromatic ligand, although keeping the same basic structure, greatly influence the binding constant values for the observed interactions.
These interactions, which were found stronger for compounds 1 and 3 than for compound 2 can be reasonably explained by the better double bond conjugation system (thus more extended pla- narity) in cases of 2-benzoylpyridine (bopy) and 1-isoquinolinyl phenyl ketone (isoquinpk) than for 2-acetylpyridine coordinated ligands.
The presence of fatty acids in HSA protein does not hinder the biding of ruthenium compounds; interestingly only in the case of compound 3 this binding is favored in the absence of fatty acids (with HSAfaf). This result might mean that com- pound 3 can also compete with fatty acids for the binding sites on HSA.
The binding constants determined for these three compounds are similar to the constants found for two Ru(II)-arene compounds, namely [(η6- p- cymene)Ru(2- anthracen- 9- ylmethylene - N- ethyl- hydrazinecarbothioamide)Cl]Cl [33] and [(η6-p-cymene)Ru (ATSC) Cl]PF6 [4]. These values might indicate that our complexes could be transported in the blood stream by HSA.