Study of calcium ion binding to D-ribose in aqueous solutions using hydroxy-proton resonance shifts

Abstract

Resolved hydroxy-proton resonance features for various forms of D-ribose in dilute aqueous solutions shift markedly on the addition of calcium chloride. Down-field shifts are assigned to OH groups which bind to Ca²⁺ ions. These shifts level off in the 1.0–1.5 mol dm^–3 region when complexing is complete. Resonances for OH groups which are not involved in complex formation shift linearly to high-field, the shift being greater than that expected for the sum of general Ca²⁺ and Cl^– ionic effects. This is because the Ca²⁺ ions are removed by complex formation, the up-field shift being largely due to the effect of chloride ions. When chloride is replaced by perchlorate, this up-field shift is greatly enhanced. Approximate equilibrium constants for complex formation have been derived, and the influence of conformational changes is discussed.