Allosteric saccharide sensing by a phenylboronic-acids-appended 5,15-bis(triarylethynyl)porphyrin

Porphyrin derivatives bearing two pairs of boronic acid groups (1) and one pair of boronic acid groups (2) were designed and synthesized to construct an allosteric saccharide-sensing system. Compound 1 has a diethynyl porphyrin rotational axis, which is expected to act as a saccharide-binding information transducer. Saccharide binding studies were conducted by UV-vis, fluorescent, and circular dichroism (CD) spectroscopies. In water-methanol 1:1 (v/v) mixed solvent, 1 can bind monosaccharides such as fucose and xylose due to an allosteric mechanism to produce 1:2 host/saccharide complexes. The processes were easily monitored by CD and fluorescent spectral changes. Binding constants K ₁ and K ₂ are evaluated to be 3800 and 3000 M ^-1 for l-fucose and 8900 and 4900 M ^-1 for d-xylose, and these K ₁ values are significantly larger than that for 2 which binds monosaccharides according to conventional 1:1 host-guest stoichiometry. Thus, 1 behaves as a novel saccharide receptor exhibiting the positive homotropic effect.