Assignment of the Sugar and Polyol Carbon Atoms

Most naturally occurring nucleosides and nucleotides contain a ribofuranose moiety, From detailed 13C NMR studies of carbohydrates [88, 89, 696] it is known that primary CH2OH groups resonate at higher field than the secondary ring CHOH groups, and thus 

C-5 of the ribofuranose residue can easily be assigned. Another rule is found for the anomeric carbons C-l of the ribofuranose residue which always absorb at lowest field [696]. C-4 can be assigned by noting the phosphorus-carbon long-range coupling in the proton broadband-decoupled 13C NMR spectra of nucleotides [750] as demonstrated in Fig. 5.9. 

The ribitol residue of the riboflavin moiety was assigned on the following basis (the same numbering of the C-atom as in Fig. 5.9 is used here)  

Several nucleosides containing other sugar residues, such as arabinopyranose, arabino-furanose, mannopyranose, talopyranose, xylopyranose or xylofuranose, have been investigated by 13C NMR [676, 684]. The assignments of the signals of the carbohydrate moieties were mainly performed by comparison with the 13C resonances of the parent sugars. 

A further aid in assigning vicinal cis hydroxy groups in carbohydrate residues of nucleosides and nucleotides makes use of boric acid as a complex shift reagent at various pH values [686]. In the presence of boric acid and at pH 7 all complexing 

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