In this study, we
have used [1H, 15N] NMR
spectroscopy to investigate the interactions of the trinuclear platinum
anticancer drug triplatin (1) (1,0,1/t,t,t or BBR3464) with site-specific sulfated and carboxylated disaccharides.
Specifically, the disaccharides GlcNS(6S)-GlcA (I) and GlcNS(6S)-IdoA(2S) (II) are useful models of longer-chain glycosaminoglycans (GAGs) such
as heparan sulfate (HS). For both the reactions of 15N-1 with I and II, equilibrium conditions
were achieved more slowly (65 h) compared to the reaction with the
monosaccharide GlcNS(6S) (9 h). The data suggest both carboxylate
and sulfate binding of disaccharide I to the Pt with
the sulfato species accounting for <1% of the total species at
equilibrium. The rate constant for sulfate displacement of the aqua
ligand (k
L2) is 4 times higher than the
analogous rate constant for carboxylate displacement (k
L1). There are marked differences in the equilibrium concentrations
of the chlorido, aqua, and carboxy-bound species for reactions with
the two disaccharides, notably a significantly higher concentration
of carboxylate-bound species for II, where sulfate-bound
species were barely detectable. The trend mirrors that reported for
the corresponding dinuclear platinum complex 1,1/t,t, where the rate constant for sulfate displacement of the aqua ligand
was 3 times higher than that for acetate. Also similar to what we
observed for the reactions of 1,1/t,t with the simple
anions, aquation of the sulfato group is rapid, and the rate constant k
–L2 is 3 orders of magnitude higher than
that for displacement of the carboxylate (k
–L1). Molecular dynamics calculations suggest that extra hydrogen-bonding
interactions with the more sulfated disaccharide II may
prevent or diminish sulfate binding of the triplatin moiety. The overall
results suggest that Pt–O donor interactions
should be considered in any full description of platinum complex cellular
chemistry.