¹H NMR studies of molecular interaction of D-glucosamine and N-acetyl-D-glucosamine with capsaicin in aqueous and non-aqueous media

Complex formation of D-glucosamine (Gl) and N-acetyl-D-glucosamine (AGl) with capsaicin (Cp) were studied by ¹H NMR titrations in H₂O-d₂ and DMSO-d₆; capsaicin is the major bioactive component of chili peppers. Every titration curve has been interpreted by formulating a suitable model for the reaction equilibrium, to elucidate intermolecular interactions. In DMSO, glucosamine cations associate with each other to yield linear aggregates, and undergo pseudo-1:1-complexation with capsaicin, the formation constant being ca. 30 M⁻¹. N-Acetylglucosamine, without self-association, forms a 2:1-complex AGl₂Cp with the stability of ca. 70 M⁻². These complexations are achieved by intermolecular hydrogen bonds. In D₂O, glucosamine undergoes reversible protonation equilibrium between Gl⁰ and GlH⁺ with the logarithmic protonation constants log K_D = 8.63 for α-glucosamine and 8.20 for β-isomer. Both anomeric isomers of deprotonated glucosamine form Gl⁰Cp-type complexes of capsaicin, in a competitive manner, with a formation constant of 1040 M⁻¹ for the α-glucosamine complex and 830 M⁻¹ for the β-complex; the anomeric carbons result in the difference in thermodynamic stability. The reactant molecules are closed up by the solvent-exclusion effect and/or the van der Waals interaction; the resulting pair is stabilized by intermolecular hydrogen bonding within a local water-free space between the component molecules. By contrast, neither protonated glucosamine (GlH⁺) nor N-acetylglucosamine yields a capsaicin complex with the definite stoichiometry. The monosaccharides recognize capsaicin under only a controlled condition; the same phenomena are predicted for biological systems and nanocarriers based on polysaccharides such as chitosan.