(3, 2)D 1H, 13C BIRDr,X-HSQC-TOCSY for NMR structure elucidation of mixtures: application to complex carbohydrates

Abstract

Overlap of NMR signals is the major cause of difficulties associated with NMR structure elucidation of molecules contained in complex mixtures. A 2D homonuclear correlation spectroscopy in particular suffers from low dispersion of 1 H chemical shifts; larger dispersion of 13 C chemical shifts is often used to reduce this overlap, while still providing the proton-proton correlation information e.g. in the form of a 2D 1 H, 13 C HSQC-TOCSY experiment. For this methodology to work, 13 C chemical shift must be resolved. In case of 13 C chemical shifts overlap, 1 H chemical shifts can be used to achieve the desired resolution. The proposed (3, 2)D 1 H, 13 C BIRD r,X -HSQC-TOCSY experiment achieves this while preserving singlet character of cross peaks in the F 1 dimension. The required high-resolution in the 13 C dimension is thus retained, while the cross peak overlap occurring in a regular HSQC-TOCSY experiment is eliminated. The method is illustrated on the analysis of a complex carbohydrate mixture obtained by depolymerisation of a fucosylated chondroitin sulfate isolated from the body wall of the sea cucumber Holothuria forskali.

Reduced dimensionality hyphenated NMR experiments for the structure determination of compounds in mixtures

“…3b). As discussed previously, 15 the separation of signals in the U 13C + kU 1H and the U 13C À kU 1H spectra is different. While an accidental overlap can occur in one of the spectra, separation is usually achieved in the other.…”

“…The described approach has so far been applied to produce (3,2)D HSQC-TOCSY experiments. [13][14][15] In the work presented here, we have extended it to a suite of complementary experiments, including (3,2)D HSQC-CLIP-COSY, (3,2)D HSQC-NOESY/ROESY and (3,2)D HSQC-HSQMBC. The new experiments, derived from a set of NMR techniques typically employed in the structure determination of pure, but complicated molecules, were adopted here for use on mixtures.…”

19F-centred NMR analysis of mono-fluorinated compounds

“…Interleaved acquisition of two spectra, differing by 90 in the phase of the last 90 13 C pulse of the pulse sequence, generates inphase and anti-phase F 1 doublets, respectively, allowing spectra to be simplied by spectral editing 58,59 as illustrated in Fig. S5.…”

New 19F NMR methodology reveals structures of molecules in complex mixtures of fluorinated compounds

“…Finally, the outcome of a simultaneous H 1 C n F correlation is illustrated in ⑤ (Fig.4). This intrinsically 3D experiment has been modified using the principles of reduced dimensionality83,84 to produce a (3, 2)D experiment. Here, the13 C chemical shift is coded in the 1 H dimension by the width of the F 1 -doublet.…”