Molecular recognition of carbohydrates by zinc porphyrins: Lewis acid Lewis base combinations as a dominant factor for their selectivity

A systematic study of the binding of carbohydrates by functionalized zinc porphyrins indicated that [5 15-bis(8-quinolyl)porphyrinato] zinc(II) (1) showed marked affinity for octyl glucoside and mannoside in CHCl3 (-Delta G degrees = 4.5-6.3 kcal-mol(-1)). Analysis of the complexation-induced shifts of the carbohydrate OH protons in the H-1 NMR revealed that receptor 1 bound the 4-OH group of mannoside and glucoside by coordination to the zinc and the 6-OH and 3-OH groups by hydrogen bonding to the quinolyl nitrogen atoms. These NMR results and comparison of binding affinity with reference receptors and ligands indicated that receptor 1 recognized the trans,trans-1,2-dihydroxy-3-(hydroxymethyl) moiety of carbohydrates by the combination of Lewis acid (zinc) and Lewis bases (quinolyl nitrogens). Poor affinities of 1 to octyl galactosides and octyl 2-O-methyl-alpha-mannoside were ascribed to neighboring group effects, where a neighboring group in ligands not directly involved in the receptor-ligand interactions had considerable influence on the affinity through destabilizing the hydrogen-bonding-network(s) in the receptor-ligand complex. The circular dichroism induced in the porphyrin Soret band by complexation with the carbohydrates displayed characteristic patterns, which parallel the patterns of the complexation-induced shifts in the H-1 NMR. The CD patterns sensitively reflected the receptor-ligand interaction modes, particularly Ligand orientation and fluctuation in the complex. variable-temperature CD revealed that glucoside was fluctuating on 1 while mannoside was rigidly fixed on 1 at room temperature. Addition of alcohols to CHCl3 suppressed the binding by 1, while addition of polar additives such as water, alcohols, phenols, and ethers assisted the binding by 3 and 4 (-Delta Delta G degrees = 0.2-0.4 kcal mol(-1)) in a low concentration range (0-1.5 mol %).