Anomeric carbons, chemical shifts

The presence of two different probes, namely, Gal C-l and Thr C7, giving rise to signals situated in two different, uncrowded regions of the spectrum, with chemical-shift differences (AdC-l up to 8.35 p.p.m., and Ad Thr C7, up to 6.7 p.p.m.), allows the facile detection of any racemiza-tion occurring at either, or both, of the asymmetric carbon atoms of the threonine. It would be difficult to distinguish a-D-Gal — L-Thr (27) from its P analog (28), or / -D-Gal — D-alloThr (58) from / -D-Gal — L-alloThr (56), on the basis of the anomeric-carbon chemical-shifts only. However, they can be differentiated on the basis of Thr Cr chemical-shift data. It should be noted that neither optical rotation nor -n.m.r. spectroscopy could have elucidated this point.85... 

However, measurement of anomeric carbon chemical shifts provided the answer due to significant chemical... 

The carbon chemical shift (i582.9 ppm) for the anomeric carbon Cl was indicated from the HMQC experiment, which is consistent with the N-conjugation... 

Proton and carbon chemical shift data for several rhamnobioses and their acetylated derivatives have been used to compute the effect of substitution on the anomeric carbon atoms and glycosidically linked carbon atoms, xanthorhamnin. 

These observations have several implications for studies of glycoproteins (i) the resonances of anomeric carbon atoms that are involved in N-glycosylic linkages will be rather difficult to identify, because of their proximity to nonanomeric-carbon resonances and (ii) due to the proximity of the chemical shifts of C-l and C-5 (they are only 0.6 p.p.m. apart for / -d-G1cNAc — Asn), it may be difficult, but not impossible, to ascertain whether N-glycosylic linkages exist in the glycoprotein (see later). 

Chemical shift (p.p.m.) of model compounds in H20 relative to internal 1,4-dioxane (67.86 p.p.m.). Chemical shifts for these compounds are given at pH 5.5-7.5. Estimated precision for the chemical shifts is 0.05 p.p.m.h See Refs. 82 and 83. See Ref. 20.d These assignments may have to be interchanged. See Ref. 21 numbers in the brackets below the given chemical shift values refer to those published in Ref. 86. f See Ref. 19. The chemical shift for the -anomeric carbon atom was found to be 100.6 p.p.m. and was determined from an anomeric mixture of this compound. The existence of the a-Man — Ser unit was confirmed by the l]CH value (169 Hz) obtained for this compound. See Ref. 84. 

C-N.m.r. Chemical-shift Data for the Anomeric-Carbon Atoms and C, and Coupling Constants ( /ch) Measured for Selected Gland /S-D-Glycosyl-L-threonine Model Compounds19- 1,82-84... 

Table XVIII contains the 13C-n.m.r.-spectral data for the model compounds a,/ -Araf — Hyp and a,/ -Ara/— Hyp. The assignments of Qfi and Cs of Hyp were readily obtained19 by comparison with 13C data for L-Hyp in the free amino acid form and for L-Hyp residues of peptides.109 The chemical shift of C 5 was, as expected, 7-8 p.p.m. downfield from the values reported for nonglycosylated L-Hyp. The chemical shifts of C and Cy appeared in the same region as carbohydrate resonances and were therefore identified through their chemical shift-pH dependence.19 The chemical shift of the / -anomeric carbon atom in compound 60 is rather up-field, in agreement with a syn orientation for the C-1 - O-1 and...

As may be seen from the comparisons of the chemical-shift data for compounds 59 and 60 with those for compounds 61 and 62, two carbon atoms, C-l and C-4, could be monitored in order to distinguish between the two tautomeric and anomeric forms of arabinose. There is an 6-p.p.m. difference between the C-4 signal of arabinose in the pyra-nose and the furanose form. Determination of the chemical shift of the anomeric-carbon atom is, however, not that straightforward, as there is an overlap between the C-l chemical-shifts of arabino-furanosyl and -pyranosyl derivatives (compare the data for compounds 60,61, and 62). The 13C chemical-shift data for C-l, in conjunction with the 13C chemical-shift data for C-4, allow the detection and assignment of arabinose in either tautomeric and either anomeric form. 

The data conclusively showed that the C-l chemical-shift of these model compounds shifts 20 p.p.m. upheld ( 80 p.p.m.) relative to the oxygen derivatives this means that assignment of C-l of this type of linkage would be difficult. Moreover, the use of the integrated, anomeric spectral-region in order to obtain quantitative information may present some problems, because of the fact that some of the anomeric-carbon atoms may not now resonate in this region. 

Chemical shifts are expressed in p.p.m. downfield of the I3C resonance of tetra-methylsilane., J Except for the general anomeric-earbon and primary (CH2OH) carbon assignments, these are tentative. c Measured in water. Data from Ref. 165. Data from Ref. 167. Data from Ref. 168. 

The C-l chemical-shifts of furanosides are generally at lower field than those of their anomeric counterparts in the pyranose series.13 Sometimes, an immediate identification may be made when very low-field signals of 8C 107 or more are present, for example, for /3-galacto-furanoside14-17 and a-arabinofuranoside.18 Generally, characteristic signals of furanoside-ring carbon atoms are present at low field (8C 80 -85) also. 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

NMR is predictive of reactivity within a series Within a set of common donors (i.e., galactose) in which the C2 position is constant, the H chemical shift of the anomeric carbon appears to be a good predictor of relative reactivity. Little correlation is found when the C2 position is varied, or between different donor pyranoses. The extent to which this correlation will prove useful remains in question. We envision that its primary applicability may be in trouble shooting a failed synthesis when a complex donor does not behave as predicted. Checking the H NMR of the advanced intermediate may quickly reveal that the RRV value for the pyranose of interest is not as expected, and hence the reactivity is not as expected. 

Fig. 1 also may be used to note some general characteristics of C spectra of carbohydrate polymers ( -11) Chemical shifts of anomeric carbons (C-l), in the region of 100-110 p.p.m., are typically well separated from other signals. As compared with C-l of the related monosaccharides (12-15)j the anomeric carbon is strongly deshielded (commonly by 7-10 p.p.m.) through glycoside formation (9)> i.e., by the change from 0-H to an 0-C bond. 

A parameter that has been useful for defining the anomeric stereochemistry of glycopyranosides is the chemical shift of the anomeric carbon atom, C-l. Similarly, the 13C NMR shifts of C-l in methyl septanosides also diagnostic for their anomeric configuration. In general, the Sc.i values for a-septanosides are slightly upfield (99-104 ppm in completely deprotected septanosides) relative to (S-septanosides (104-111 ppm) (Table I). This distribution of chemical shifts mirrors the trends for pyranosides namely, the C-l chemical shifts for a-pyranosides lie upfield from those of p-pyranosides. This observation suggests that the anomeric effect is operative in... 

---