Water T2 relaxation in sugar solutions

Abstract

1H spin–spin relaxation times of water were measured with the CPMG sequence in dilute aqueous solutions of glucitol, mannitol, glycerol, glycol, the methyl d-pyranosides of α-glucose, β-glucose, α-galactose, β-galactose, α-xylose, β-xylose, β-arabinose and sucrose, α,α-trehalose, β-maltose, maltotriose and maltoheptaose. The relaxation-time dispersion was measured by varying the CPMG pulse spacing, τ. These data were interpreted by means of the Carver–Richards model in which exchange between water protons and labile solute hydroxyl protons provides a significant contribution to the relaxation. From the dependences on temperature and τ, parameters characteristic of the pool of hydroxyls belonging to a given solute were extracted by nonlinear regression, including: the fraction of exchangeable protons, P, the chemical-shift difference between water protons and hydroxyl protons, δ ω, the intrinsic spin–spin relaxation time, T 2, and the chemical exchange rate, k. These solute-specific parameters are related, respectively, to the concentration, identity, mobility and exchange life-time of the hydroxyl site. At 298 K, values of δ ω, T 2 and k were found to be of the order of 1 ppm, 100 ms and 1000 s −1, respectively. Effects of molecular size, conformation and solute concentration were investigated. The exchange mechanism was characterised by Eyring activation enthalpies and entropies with values in the ranges 50–70 kJ mol −1 and −10 to 60 J K −1 mol −1, respectively.