Adduct formation between ternary Pt(II)–amino acid–aromatic diimine complexes and flavin mononucleotide and its effect on redox properties

Adduct formation of ternary Pt(II) complexes composed of an amino acid and an aromatic diimine, [Pt(A)(DA)] (A = glycinate (Gly), alaninate (Ala), valinate, or arginine (Arg); DA = 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen)), with flavin mononucleotide (FMN) and anthraquinone-2-sulfonate (AQS) were investigated by spectroscopic, X-ray diffraction, and electrochemical methods. The Pt(II) complexes formed 1:1 [Pt(A)(DA)]–FMN adducts by stacking with the aromatic moiety of FMN, and the stability constants, log K, for the systems with [Pt(A)(phen)] (A = Gly, Ala, and Arg) and [Pt(Arg)(bpy)] were determined to be 2.83(8)–3.42(6) from 1H NMR spectra at 25 °C in D2O (I = var.). The structure of the adduct [Pt(Ala)(phen)](AQS) (1) was determined by X-ray analysis to involve a π–π stacking interaction between coordinated phen and AQS with the distance of 3.400(7) Å and a hydrogen bond between the sulfonate moiety of AQS and the amino group of coordinated Ala. Cyclic voltammetry of the 1:1 [Pt(A)(DA)]–FMN systems in a phosphate buffer (pH 7.0) showed that the potentials, E1/2, for the two-electron redox process of FMN shifted to higher values by 18–31 mV as compared with the value for free FMN.