5, 10, 15, 20-Tetrakis[3- and 4-(dihydroxyboryl)phenyllporphinatoiron(III)s (1a and 1b, respectively) were synthesized and the saccharide-binding ability of their μ-oxo diniers was investigated. The saccharide-binding process with boronic acids can be conveniently monitored by CD spectroscopy. The μ-oxo dimer of 1b with 4-(hydroxyboryl) groups can bind glucose and galactose among monosaccharides with extremely high selectivity and sensitivity (association constants 104-105 dm3 mol-1), whereas that of la with 3-(hydroxyboryl) groups shows only a weak affinity with these monosaccharides. A similar trend was also observed fordisaccharides. The origin of the CD activity in 2b can be ascribed to the formation of 1 :1 μ-oxo dimer/saccharide complexes, in which two porphyrin rings are chirally bridged by one saccharide molecule. The CD sign is well-correlated with the absolute configuration of these saccharides. Thus, the present paper demonstrates that the μ-oxo dimer of 1b, which is formed in a self-assembled manner in aqueous alkaline solution, acts as an excellent pair of artificial "sugar tweezers".
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