Reaction of dirhodium(II) tetraacetate with S-methyl-L-cysteine

Abstract

The reaction of antitumor active dirhodium(II) tetraacetate, [Rh₂(AcO)₄], with S-methyl-L-cysteine (HSMC) was studied at the pH of mixing (=4.8) in aqueous media at various temperatures under aerobic conditions. The results from UV–vis spectroscopy and electrospray ionization mass spectrometry (ESI–MS) showed that HSMC initially coordinates via its sulfur atom to the axial positions of the paddlewheel framework of the dirhodium(II) complex, and was confirmed by the crystal structure of [Rh₂(AcO)₄(HSMC)₂]. After some time (48 h at 25 °C), or at elevated temperature (40 °C), Rh-SMC chelate formation causes breakdown of the paddlewheel structure, generating the mononuclear Rh(III) complexes [Rh(SMC)₂]⁺, [Rh(AcO)(SMC)₂] and [Rh(SMC)₃], as indicated by ESI–MS. These aerobic reaction products of [Rh₂(AcO)₄] with HSMC have been compared with those of the two proteinogenic sulfur-containing amino acids methionine and cysteine. Comparison shows that the (S,N)-chelate ring size influences the stability of the [Rh₂(AcO)₄] paddlewheel cage structure and its Rh^II–Rh^II bond, when an amino acid with a thioether group coordinates to dirhodium(II) tetraacetate.

Keywords:

• Dirhodium(II) tetraacetate
• S-methyl-L-cysteine
• ESI–MS
• UV–vis spectroscopy
• crystal structure

Acknowledgments

We express our appreciation to Mr. Wade White at the instrumentation facility at the Department of Chemistry, University of Calgary, for his assistance with the ESI–MS measurements. V.J.B acknowledges the NSERC Undergraduate Student Research Awards (USRA). A.E.G appreciates the University of Calgary Eyes High, Faculty of Graduate Studies and Faculty of Science Dean’s Doctoral Scholarships. M.W. visit from the China University of Petroleum to the University of Calgary was supported through China Scholarship Council.

Disclosure statement

The authors declare no competing financial interest.

Additional information

Funding

This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) funding reference number RGPIN 2016-04546, Canadian Foundation for Innovation (CFI) and the Province of Alberta (Department of Innovation and Science).