Interactions of tetra-μ-acetato dirhodium(II) with sulfur-containing aminoacids

Reactions of Rh2(CH3COO)4 with thiols and thiolates: a structural study

The structural differences between the aerobic reaction products of Rh2(AcO)4 (1; AcO- = CH3COO-) with thiols and thiolates in non-aqueous media are probed by X-ray absorption spectroscopy. For this study, ethanethiol, dihydrolipoic acid (DHLA; a dithiol) and their sodium thiolate salts were used. Coordination of simple thiols to the axial positions of Rh2(AcO)4 with Rh-SH bonds of 2.5-2.6 Å keeps the RhII-RhII bond intact (2.41 ± 0.02 Å) but leads to a colour change from emerald green to burgundy. Time-dependent density functional theory (TD-DFT) calculations were performed to explain the observed shifts in the electronic (UV-vis) absorption spectra. The corresponding sodium thiolates, however, break up the Rh2(AcO)4 framework in the presence of O2 to form an oligomeric chain of triply S-bridged Rh(III) ions, each with six Rh-S (2.36 ± 0.02 Å) bonds. The RhIII...RhIII distance, 3.18 ± 0.02 Å, in the chain is similar to that previously found for the aerobic reaction product from aqueous solutions of Rh2(AcO)4 and glutathione (H3A), {Na2[Rh2III(HA)4]·7H2O}n, in which each Rh(III) ion is surrounded by about four Rh-S (2.33 ± 0.02 Å) and about two Rh-O (2.08 ± 0.02 Å). The reaction products obtained in this study can be used to predict how dirhodium(II) tetracarboxylates would react with cysteine-rich proteins and peptides, such as metallothioneins.

Methionine Binding to Dirhodium(II) Tetraacetate

The reaction between antitumor active dirhodium(II) tetraacetate and dl-methionine (HMet) was followed in aqueous solution and showed initially mixtures of 1:1 and 1:2 adducts [Rh2(AcO)4(HMet)(H2O)] (AcO- = CH3COO-) and [Rh2(AcO)4(HMet)2] formed at room temperature (RT), as evidenced by UV-vis spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Rh K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy confirmed methionine thioether binding to the axial positions of the Rh2(AcO)4 cage structure. With excess HMet at RT, stepwise displacement of the acetate groups was observed after some time using ESI-MS. Heating the solution to 40° for 24 h accelerated the substitution reaction leading to stable dirhodium(II) species with two acetate ligands displaced by two methionine groups. The crystal structure of the purple [RhII2(AcO)2(d-Met)(l-Met)]·6H2O compound obtained from the solution revealed tridentate coordination of the methionine ligands to the Rh(II) ions, with the thioether S atoms in equatorial positions. A minor amount of a light orange monomeric [RhIII(Met)2](AcO) complex also formed in the solution was isolated by size exclusion chromatography and identified by ESI-MS. Crystals of [RhIII(d-Met)(l-Met)]Cl·3H2O were prepared by reacting RhCl3 and dl-HMet. The crystal structure showed tridentate binding of the methionine ligands to the Rh(III) ion in a trans-S, N, O arrangement.

Glutathione binding to dirhodium tetraacetate: a spectroscopic, mass spectral and computational study of an anti-tumour compound

Glutathione (γ-l-glutamyl-l-cysteinyl-glycine) is a ubiquitous tripeptide found in all plants and animals. Glutathione has key roles as a metallochaperone and as a cellular thiol involved in metabolism. Little is known about how glutathione interacts with organometallic compounds in vivo. Here, we report the reactions of glutathione in vitro with dirhodium(ii) tetraacetate (tetrakis(μ-acetato)dirhodium(ii), Rh₂(OAc)₄), a compound with anti-tumour properties. Electrospray ionization mass spectrometry, UV-Visible absorption and circular dichroism spectroscopic methods were used to determine the stoichiometries and optical properties of the final conjugate. Computational analyses were used to predict the binding modes of glutathione to the Rh₂(OAc)₄, and report on the orbital assignments for the resulting products. We explored the competition by GSH for methionine-bound axial sites on Rh₂(OAc)₄ to investigate the use of weak thioether to protect its cellular-based anti-cancer activity. Our study highlights the important role that axial ligation would play in deactivating or significantly decreasing the efficacy of this bimetallic anti-tumor drug. The computational data explain the stability of the mono-adduct and the appearance of new absorption bands in the UV region including retention of the Rh-Rh single bond. Additionally, these data show that glutathione can effectively disable the potency of these metallo-drugs through orbital overlap of the entire Rh-Rh core as a result of the strong binding. Electronic absorption spectroscopy, mass spectrometry and computational analysis are a powerful combination in understanding possible chemical reactions in vivo and this information can be used to synthetically tune dirhodium complexes for use in the fight against cancer.

Reactions of Antitumor Active Dirhodium(II) Tetraacetate Rh2(CH3COO)4 with Cysteine and Its Derivatives

We have combined results from several spectroscopic techniques to investigate the aerobic reactions of Rh₂(AcO)₄ (AcO^- = CH₃COO^-) with l-cysteine (H₂Cys) and its derivatives d-penicillamine (3,3'-dimethylcysteine, H₂Pen), with steric hindrance at the thiol group, and N-acetyl-l-cysteine (H₂NAC), with its amino group blocked. Previous investigations have shown that antitumor active dirhodium(II) carboxylates may irreversibly inhibit enzymes containing a thiol group at or near their active sites. Also, cysteine, the only thiol-containing proteinogenic amino acid, interacts in vivo with this class of antitumor compounds, but structural information on the products of such reactions is lacking. In the present study, the reactions of Rh₂(AcO)₄ and H₂L were carried out in aqueous solutions at the pH of mixing (acidic) and at physiological pH, using the different mole ratios 1:2, 1:4, and 1:6, which resulted in the same products in increasing yields. Electrospray ionization mass spectrometry (ESI-MS) indicates formation of dimeric [Rh^III ₂Pen₄]^2- or oligomeric {Rh^III ₂L₄} _n (L = Cys, NAC) complexes with bridging thiolate groups. Analyses of Rh K edge extended X-ray absorption fine structure (EXAFS) data reveal 3-4 Rh-S and 2-3 Rh-(N/O) bonds around six-coordinated Rh(III) ions at mean distances of 2.33 ± 0.02 and 2.09 ± 0.02 Å, respectively. In the N-acetyl-l-cysteine compound, the Rh^III···Rh^III distance 3.10 ± 0.02 Å obtained from the EXAFS spectrum supports trithiolate bridges between the Rh(III) ions, as was also found when using glutathione as ligand. In the cysteine and penicillamine complexes, double thiolate bridges join the Rh(III) ions, with the nonbridging Cys^2- and Pen^2- ligands in tridentate chelating (S,N,O) mode, which is consistent with the Δδ_C = 7.3-8.4 ppm shift of the COO^- signal in their carbon-13 cross polarization magic angle spinning (CPMAS) NMR spectra. For the penicillamine complex, the 2475.6 eV peak in its S K edge X-ray absorption near edge structure (XANES) spectrum shows partial oxidation, probably caused by peroxide generated from reduction of dissolved O₂, of thiolato to sulfenato (S=O) groups, which were also identified by ESI-MS for all three {Rh^III ₂L₄} _n compounds.

A circular dichroism and fluorescence quenching study of the interactions between rhodium(II) complexes and human serum albumin

Various divalent rhodium complexes Rh2(L)4 (L = acetate, propionate, butyrate, trifluoroacetate and trifluoroacetamidate) have been found to bind to non-defatted human serum albumin (HSA) at molar ratios about 8:1. The circular dichroism measurements showed that the more liposoluble carboxylates, butyrate and trifluoroacetate, caused the major alterations of the secondary structure of HSA. Stern-Volmer constants for the fluorescence quenching of the buried Trp214 residue by these complexes were also higher for the lipophilic metal compounds. In the case of the rhodium carboxylates it was observed that their denaturating and quenching properties could be explained in terms of their liposolubilities: the higher their lipophilic characters, the higher their abilities to penetrate inside the protein framework leading to structural alterations, and the closer they could get to the Trp residue causing fluorescence quenching. The liposoluble amidate complex, Rh2 (tfc)4, presented an intermediate quenching and did not cause structural alterations in the protein, presumably not penetrating inside the peptidic backbone. This study shows that it is possible to design new antitumor metal complexes which bind, to a large extent, to a transport protein causing little structural damage.

Interaction of tetra-mu-acetatodirhodium(II) with human serum albumin

The interaction of Rh2(OAc)4 with human serum albumin (HSA) has been studied by absorption difference spectroscopy, CD spectroscopy, and quantitative precipitating HSA-antibody test. Our results demonstrate that this rhodium complex reacts easily with HSA at several ratios of reagents. The Rh atoms are coordinated to protein molecules via the imidazole rings of His residues. The structural studies have shown the conformational change of HSA modified by rhodium. Rhodium binding lowers the helicity of the native protein between 8 to 18% depending upon the molar ratios (from 1:1 to 10:1). Denaturation measurements of free HSA and HSA in the presence of dirhodium(II) acetate complex with 8-M urea followed by CD spectroscopy, suggest that rhodium affects the secondary protein structure and might stabilize HSA against denaturing agents. 8-M urea caused the unfolding of the native HSA secondary structure by about 40% and the structure of Rh(OAc)4-HSA by about 10%. The modification of native HSA by rhodium causes its decreased ability to precipitate with HSA antibodies. The decrease of antigenic properties can be connected with the unfolding of the antigen structure, which brings about perturbation of complementarity of the antigen-antibody reactive sites.