Binding of histidine and human serum albumin to dirhodium(II) tetraacetate

Reactions of the anticancer active dirhodium tetraacetate (1), Rh₂(AcO)₄ (AcO^- = CH₃COO^-), with the amino acid histidine (HHis) and human serum albumin (HSA) were monitored over time and different metal: ligand ratios using UV-vis spectroscopy and/or electro-spray ionization mass spectrometry. Initially, histidine formed 1:1 and 1:2 adducts in aqueous solutions. The crystal structure of Rh₂(AcO)₄(L-HHis)₂·2H₂O (2) confirmed the axial coordination of histidine imidazole groups (average Rh-N_axial 2.23 Å). These adducts, however, were found to be unstable in solution over time (24 h). Heating Rh₂(AcO)₄ -histidine solutions to 40 °C (near body temperature) or 95 °C accelerated the formation of Rh^II₂(AcO)₂(His)₂ and Rh^III(His)₂(AcO) complexes. The corresponding pH change from neutral to mildly acid (pH 4-5) indicates deprotonation of histidine NH₃⁺ groups due to coordination to Rh ions, which simultaneously bind to histidine COO^- groups, as evidenced by ¹³C NMR spectroscopy. In the case of HSA with 16 histidine and one cysteine residues, UV-vis spectroscopy indicates that mono- and di-histidine HSA adducts with Rh₂(AcO)₄ are formed. X-ray absorption spectroscopy showed almost the same Rh-Rh distance (2.41 ± 0.01 Å) for the Rh₂(AcO)₄ units as in 2, and a contribution from an axial thiol coordination (Rh-S_axial 2.62 ± 0.05 Å). The Rh₂(AcO)₄ - HSA complex was found to decompose partially (~15%) over 24 h at ambient temperature. The partial decomposition of Rh₂(AcO)₄ both through coordination to histidine or to human serum albumin, the most abundant protein in blood plasma, is a factor to consider for its efficacy as a potential anticancer agent.