Temperature-dependent conformational analysis of gentiobiose octa-acetate in solution. Proton and carbon nuclear magnetic relaxation study

Abstract

The carbon spin–lattice relaxation rates of a 0.15 mol dm^–3 solution of gentiobiose octa-acetate have been measured in the temperature range 22–70 °C. R₁ is shown to be purely dipolar and dominated by an isotropic rotational tumbling motion of the molecule. From these results the activation energy of molecular reorientation has been determined. The NOESY spectrum of gentiobiose octa-acetate measured at 70 °C has been used to detect dipolar connectivities between glycosidic protons. For a quantitative determination of interproton cross-relaxation contributions selective and bi-selective proton spin–lattice relaxation experiments have been used. In this way and using the correlation time previously determined from carbon R₁ measurements, selected proton–proton distances have been calculated. The conformation of gentiobiose octa-acetate in solution as determined from n.m.r. relaxation experiments has then been compared with results obtained from proton vicinal coupling constant analysis.