Interaction of metallotexaphyrins with mono- and polysaccharides

The interactions of two water-soluble metallotexaphyrins, containing coordi nated lutetium(III) and gadolinium(III) cations, with uronic acids (D-galac turonic and D-glucuronic acids), neutral (amylose, galactan) and anionic (p ectate, alginate) polysaccharides were studied using UV-VIS titrations. In the case of polyuronides, strong red shifts in the Soret (9-12 nm) and Q-li ke (4-9 nm) transitions were observed when solutions of the metallotexaphyr in were subject to titration with increasing ligand concentrations. The int eraction with neutral polysaccharides leads to significantly smaller bathoc hromic shifts in these same bands (1-2 nm). No shift of these bands was obs erved in the case of interaction with uronic acids. If pectate is replaced by partially C-6 methylated pectinates, the extent of the bathochromic shif t was seen to decrease as the extent of methylation increased, becoming min imal 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 aggre gated forms of the metallotexaphyrin into less aggregated (e.g., monomeric) texaphyrin-polysaccharide species. UV-VIS titrations support the conclusio n that polyuronides interact with metallotexaphyrins, presumably by acting as polydentate carboxylic ligands for the Lewis acidic lanthanide(III) meta llotexaphyrin centres. In the case of the polyuronides, where near complete conversion to monomers is observed, this decrease in aggregation is though t to reflect binding to carboxylate sites. Such binding interactions are no t possible in the case of neutral polysaccharides and methyl pectate and th e macrocycles remain highly aggregated. Interaction with uronic acids, howe ver, also does not lead to deaggregation.