Glycans as Ligands in Bioinorganic Chemistry. Probing the Interaction of a Trinuclear Platinum Anticancer Complex with Defined Monosaccharide Fragments of Heparan Sulfate.

We report herein a detailed NMR study of the aquation and subsequent covalent binding of the trinuclear clinical agent [{ trans-PtCl(15NH3)2}2{μ- trans-Pt(15NH3)2(15NH2(CH2)615NH2)2}]4+ (1, 1,0,1/ t, t, t or Triplatin) with three d-glucosamine residues containing varied O-sulfate and N-sulfate or N-acetyl substitutions, which represent monosaccharide fragments present within the repeating disaccharide sequences of cell surface heparan sulfate (HS). The monosaccharides GlcNS(6S), GlcNS, and GlcNAc(6S) were synthesized in good yield from a common 4,6-diol α-methyl glucopyranoside intermediate. The reactions of 15N-1 with sodium sulfate, GlcNS(6S), GlcNS, and GlcNAc(6S) were followed by 2D [1H,15N] heteronuclear single quantum coherence (HSQC) NMR spectroscopy using conditions (298 K, pH ≈5.4) similar to those previously used for other anionic systems, allowing for a direct comparison. The equilibrium constants (p K1) for the aquation of 1 in the presence of GlcNS(6S) and GlcNS were slightly higher compared to that of the aquation in a sulfate solution, while a comparable p K1 value was observed in the presence of GlcNAc(6S). A comparison of the rate constants for sulfate displacement of the aqua ligand showed preferential binding to 2- N-sulfate compared to 6- O-sulfate but a more rapid liberation. For disulfated GlcNS(6S), equilibrium conditions were achieved rapidly (9 h) and strongly favored the dichloro form, with <2% sulfato species observed. The value of kL1 was up to 15-fold lower than that for binding to sulfate, whereas the rate constant for the reverse ligation ( k-L1) was comparable. Equilibrium conditions were achieved much more slowly (∼ 100 h) for the reactions of 1 with GlcNS and GlcNAc(6S), attributed to covalent binding also to the N-donor of the sulfamate (GlcNS) group and the O-donor of the N-acetyl [GlcNAc(6S)] group. The rate constants ( kL2) were 20-40-fold lower than that for binding to the 2- N- or 6- O-sulfate, but the binding was less reversible, so that their equilibrium concentrations (5-8%) were comparable to the 2- N- or 6- O-sulfate-bound species. The results emphasize the relevance of glycans in bioinorganic chemistry and underpin a fundamental molecular description of the HS-Pt interactions that alter the profile of platinum agents from cytotoxic to metastatic in a systematic manner.