Coordination Chemistry in Water of a Free and a Lipase-Embedded Cationic NCN-Pincer Platinum Center with Neutral and Ionic Triarylphosphines

Abstract

The coordination chemistry in aqueous media was studied for the platinum center of low-molecular-weight cationic NCN-pincer platinum complexes [RC6H2(CH2NMe2)2-3,5-Pt(H2O)-4]+ (R = −(CH2)3P(═O)(OEt)(OC6H4NO2-4) (1(OH2)), H (2(OH2))) as well as of the platinum center of the NCN-pincer platinum cation embedded in the lipase cutinase (cut-1; molecular weight 20 619.3) with various anionic, neutral, and cationic triarylphosphines. A 31P NMR study of the coordination of triarylphosphines to the cationic NCN-pincer platinum center in low-molecular-weight [2(OH2)][OTf] in both D2O and Tris buffer (Tris = tris(hydroxylmethyl)aminomethane) showed that the phosphine–platinum coordination is strongly affected by Tris buffer molecules. Two crystal structures of a NCN-pincer platinum–phosphine and a NCN-pincer platinum–ethanolamine coordination complex with ethanolamine as a functional model of Tris with hydrogen bridges, provoking a dimeric supramolecular structure, confirmed that the coordination observed in solution occurred in the solid state as well. A 31P NMR and ESI-MS study of the lipase cut-1 showed that the coordination of various triarylphosphines to the enzyme-embedded platinum center is affected by the surrounding protein backbone, discriminating between phosphines on the basis of their size and charge. By using 31P NMR spectroscopy and ESI-MS spectrometry, study of the coordination of triarylphosphines to cut-1 was possible, thereby avoiding the need for the application of laborious biochemical procedures. To the best of our knowledge, this is the first example of a study involving the selective binding of organic ligands to the metal center of a semisynthetic metalloprotein, unequivocally demonstrating that the well-established coordination chemistry for small-molecule complexes can be transferred to biological molecules. This initial study allows future explorations in the field of selective protein targeting and identification, as in protein profiling or screening studies.