Determination of Sugar Structures in Solution from Residual Dipolar Coupling Constants:  Methodology and Application to Methyl β-d-Xylopyranoside

TN Pham, SL Hinchley, DWH Rankin… - Journal of the …, 2004 - ACS Publications
TN Pham, SL Hinchley, DWH Rankin, T Liptaj, D Uhrín
Journal of the American Chemical Society, 2004ACS Publications
We have developed methodology for the determination of solution structures of small
molecules from residual dipolar coupling constants measured in dilute liquid crystals. The
power of the new technique is demonstrated by the determination of the structure of methyl β-
d-xylopyranoside (I) in solution. An oriented sample of I was prepared using a mixture of
C12E5 and hexanol in D2O. Thirty residual dipolar coupling constants, ranging from− 6.44
to 4.99 Hz, were measured using intensity-based J-modulated NMR techniques. These …
We have developed methodology for the determination of solution structures of small molecules from residual dipolar coupling constants measured in dilute liquid crystals. The power of the new technique is demonstrated by the determination of the structure of methyl β-d-xylopyranoside (I) in solution. An oriented sample of I was prepared using a mixture of C12E5 and hexanol in D2O. Thirty residual dipolar coupling constants, ranging from −6.44 to 4.99 Hz, were measured using intensity-based J-modulated NMR techniques. These include 15 DHH, 4 1DCH, and 11 nDCH coupling constants. The accuracy of the dipolar coupling constants is estimated to be <±0.02 Hz. New constant-time HMBC NMR experiments were developed for the measurement of nDCH coupling constants, the use of which was crucial for the successful structure determination of I, as they allowed us to increase the number of fitted parameters. The structure of I was refined using a model in which the directly bonded interatom distances were fixed at their ab initio values, while 16 geometrical and 5 order parameters were optimized. These included 2 CCC and 6 CCH angles, and 2 CCCC and 6 CCCH dihedral angles. Vibrationally averaged dipolar coupling constants were used during the refinement. The refined solution structure of I is very similar to that obtained by ab initio calculations, with 11 bond and dihedral angles differing by 0.8° or less and the remaining 5 parameters differing by up to 3.3°. Comparison with the neutron diffraction structure showed larger differences attributable to crystal packing effects. Reducing the degree of order by using dilute liquid crystalline media in combination with precise measurement of small residual dipolar coupling constants, as shown here, is a way of overcoming the limitation of strongly orienting liquid crystals associated with the complexity of 1H NMR spectra for molecules with more than 12 protons.
ACS Publications
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