Interaction of metallotexaphyrins with mono- and polysaccharides

Abstract

The interactions of two water-soluble metallotexaphyrins, containing coordinated lutetium(III) and gadolinium(III) cations, with uronic acids (D-galacturonic and D-glucuronic acids), neutral (amylose, galactan) and anionic (pectate, alginate) polysaccharides were studied using UV–VIS titrations. In the case of polyuronides, strong red shifts in the Soret (9–12 nm) and Q-like (4–9 nm) transitions were observed when solutions of the metallotexaphyrin were subject to titration with increasing ligand concentrations. The interaction with neutral polysaccharides leads to significantly smaller bathochromic shifts in these same bands (1–2 nm). No shift of these bands was observed in the case of interaction with uronic acids. If pectate is replaced by partially C-6 methylated pectinates, the extent of the bathochromic shift was seen to decrease as the extent of methylation increased, becoming minimal in the case of methyl pectate (0.5–1 nm). The origin and magnitude of the observed red shifts is rationalised in terms of the conversion of aggregated forms of the metallotexaphyrin into less aggregated (e.g., monomeric) texaphyrin–polysaccharide species. UV–VIS titrations support the conclusion that polyuronides interact with metallotexaphyrins, presumably by acting as polydentate carboxylic ligands for the Lewis acidic lanthanide(III) metallotexaphyrin centres. In the case of the polyuronides, where near complete conversion to monomers is observed, this decrease in aggregation is thought to reflect binding to carboxylate sites. Such binding interactions are not possible in the case of neutral polysaccharides and methyl pectate and the macrocycles remain highly aggregated. Interaction with uronic acids, however, also does not lead to deaggregation.