Formulation of anomerization and protonation in D-glucosamine, based on ¹H NMR

The major anomer of non-protonated neutral D-glucosamine GlcN⁰ is the β-form, while the α-anomer is dominant for protonated cationic glucosamine GlcNH⁺. The present work confirmed correlation between the anomerization and the protonation by simultaneous determination of signal intensity and chemical shift in pD-variation ¹H NMR, and formulated the equilibrium constants between subspecies α-GlcN⁰, β-GlcN⁰, α-GlcND⁺, and β-GlcND⁺ to interpret the correlation. The individual anomerization constants, K_an = [βGlcN]/[αGlcN] and K_anD = [βGlcND⁺]/[αGlcND⁺], are linked to each other through the relation K_Dα∙K_anD = K_Dβ∙K_an with the deuteration constants K_Dα and K_Dβ of the anomers. The anomer populations are stimulated by OD⁻ and D⁺ ions in the dose−response form. The acidic deuteron in α-GlcND⁺ is populated mostly at the nitrogen atom, whereas the population in β-GlcND⁺ is comparable at nitrogen and anomeric oxygen; this difference is consistent with the basicity of the nitrogen and the anomerization process of glucosamine.