Chiral discrimination of fructo-oligosaccharides toward amino acid derivatives by induced-fitting chiral recognition†

Abstract

Among linear permethylated fructo-oligosaccharides containing various monosaccharide moieties at the reducing terminal, MeSorFru₃3333, MeGlc⁶6666Fru₃3333, and MeFruNys showed high degrees of enantioselectivity toward binding some amino acid 2-propyl ester salts. In particular MeFruNys indicated remarkable chiral discrimination toward [ValOPrⁱ]⁺ (I_R/I_S-Dn = 0.14 corresponding to −ΔΔG_enan = 1.2 kcal mol⁻¹, S-selectivity) and [PheOPrⁱ ]⁺ (I_R/I_S-Dn = 0.18 corresponding to −ΔΔG_enan = 1.0 kcal mol⁻¹, S-selectivity). The results of FAB mass, UV–visible spectrometries, thermodynamic parameters, and NMR induced shifts provided evidence for the chiral discrimination of MeFruNys toward amino acid ester salts in the “induced-fitting chiral recognition system”: the conformation of MeFruNys drastically changes from a linear to a pseudo-ring structure with a given cation such as a chiral organic ammonium ion or an alkali metallic ion. This is the first example of chiral discrimination of linear oligosaccharide derivatives toward amino acid derivatives based on the induced-fitting chiral recognition mechanism.