New Di- and Triorganotin(IV) Tyrosylalaninates as Models for Metal—Protein Interactions: Synthesis, Structural Characterization, and Potentiometric Studies of Tyrosylalanine, Glycyltyrosine, and Glycylisoleucine with Di- and Trimethyltin(IV) Moieties in Aqueous Medium
Di- and triorganotin(IV) derivatives of tyrosylalanine (H2Tyr-Ala) with general formula R2Sn(Tyr-Ala) (where R？=？Me, n-Bu, n-Oct, and Ph) and R3Sn(HTyr-Ala) (where R？=？Me and Ph) have been synthesized and structurally characterized in the solid state as well as in solution on the basis of various spectroscopic techniques, namely. FT-IR, multinuclear (1H, 13C and 119Sn) NMR and 119Sn M？ssbauer. These investigations suggest that tyrosylalanine in R2Sn(Tyr-Ala) acts as dianionic tridentate ligand coordinating through carboxylate oxygen [–C(O)O？], amino (–NH2), and (CO) N ？ p e p t i d e nitrogen, while in the case of R3Sn(HTyr-Ala), the ligand acts as monoanionic bidentate coordinating through –C(O)O？ and –NH2, and the polyhedron around tin in R2Sn(Tyr-Ala) and R3Sn(HTyr-Ala) is a distorted trigonal-bipyramidal. Equilibrium (pH-metric) studies of the interaction of Me2Sn(IV)2+ and Me3Sn(IV)+ with dipeptides namely, tyrosylalanine (H2Tyr-Ala), glycyltyrosine (H2Gly-Tyr), and glycylisoleucine (H2Gly-Ile), in aqueous solution (I？=？0.1？M KNO3, 298？K) have also been carried out. The concentration distribution of the various complex species in solution has been evaluated as a function of pH. It has been found that in these dipeptides, [–C(O)O？, N？, NH2] coordinated complexes are dominant in the neutral pH range with a trigonal-bipyramidal structure. The complex species formed are water soluble in the pH range 2.7–10.5. In all of the studied systems, no polymeric species have been detected in the experimental pH range. Beyond pH 8.0, significant amounts of hydroxo species, namely. Me3Sn(OH) and Me2Sn(OH)2, are formed. 1. Introduction Organotin compounds have emerged as potential future pharmaceuticals as antitumor agents among nonplatinum chemotherapeutic metallopharmaceuticals in the last two decades [1–5]. Though tin-based drugs usually are less active than the corresponding platinum antitumor drugs, however, they have the advantage of lower toxicity [6]. In order to get a better insight in how the organotin species behave inside the biological systems, it is necessary to study their coordination behavior with biomolecules that can occur in the biological medium. Therefore, considerable efforts have been made to understand the binding mode of organotin compounds with biologically relevant ligands such as amino acids [1, 2, 7–13] and dipeptides [1, 2, 12–24], which constitute a very important class of biomolecules. Further, diorganotin(IV) derivatives of dipeptides [12, 14, 18] have been found to exhibit potent anti-inflammatory activity; thus such compounds may
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