A conformational study of methyl β-cellobioside-d8 by 13C nuclear magnetic resonance spectroscopy: dihedral angle dependence of 3JC–H in 13C—O—C—1H arrays

Abstract

Methyl β-cellobioside (2) has been deuterated at positions C-2, -3, -3′, -4, -6, and -6′ by 1H–2H exchange with deuterium oxide in the presence of Raney nickel catalyst. The resulting simplification in the 13C nmr spectrum of this isotopically-labelled compound has facilitated the measurement of interresidue 13C–1H coupling, as a means of determining the orientation of the glycosidic linkage in 2. Three-bond coupling across the linkage between 13C-4′ and H-1 is found to be 4.3 Hz and that between 13C-4′ and H-1, 4.2 Hz. By reference to other 13C–1H coupling data, which exhibit a Karplus type of relationship between the magnitude of 3JC—H and dihedral angle in 13C—O—C—1H arrays of bonded atoms, it appears that torsion angles [Formula: see text] and ψ of the disaccharide in aqueous solution are in the region of 25–30°, i.e., close to the angles of crystalline 2. Corresponding data for the hepta-O-acetyl derivative indicate that ψ is smaller by ∼10° and [Formula: see text] is unchanged. An analysis of the 1H-coupled 13C spectrum of cyclohexaamylose-2,3,6,6′-d4 is presented in relation to these findings. Computer simulations of the observed spectra have been used to determine coupling constants and to assess the importance of second-order effects on the measurement of JC—H.