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 CHCl 3 (−Δ G ° = 4.5−6.3 kcal mol - 1 ). Analysis of the complexation-induced shifts of the carbohydrate OH protons in the 1 H 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-α-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 1 H 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 CHCl 3 suppressed the binding by 1, while addition of polar additives such as water, alcohols, phenols, and ethers assisted the binding by 3 and 4 (−ΔΔ G ° = 0.2−0.4 kcal mol - 1 ) in a low concentration range (0−1.5 mol %).