Spectra of Polysaccharides

The torsion angles of glycosidic bonds of maltose oligomers and cyclohexaamylose, and their O-acetyl derivatives, have been deter- 

Detection of the C-6 signal of the nonreducing end-group of amylopectin was possible for the 4,6-diphenylboronate derivative, which is associated with nonreducing end-groups and whose C-4 and C-6 signals are displaced99 upfield and downfield, respectively (see Fig. 7). 

[Temperature 70° chemical shifts (8C) based on external tetramethylsilane.] 

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) 

The diagnostic nature of the 8C 70-75 spectral region, in terms of degree and type of dextran branching, was demonstrated on investigation of dextran fractions from NRCC strains of L. mesenteroides B-742, 

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Fig. 16.— 13C-N.m.r. Spectra of Polysaccharides (in DsO) Containing (1— 3)- and (1—>4)-Linked /3-D-Mannopyranosyl Residues, Obtained from [2H]-Labelled D-Glu-coses. (Temperature, 70° chemical shifts expressed as 8C, relative to external tetra-methylsilane the number marked with a superscript asterisk refers to the position of labelling in each D-glucose derivative.)...

As illustrated on this example, interpretation of NOESY spectra of polysaccharides might be complicated by the presence of spin-diffusion and higher order elfects. ID NOESY-NOESY experiment can provide some assistance in identification and assignment of spin-diffusion signals and thus prolong networks of spins experiencing NOE contacts beyond immediate neighbours normally detected in NOESY experiments. 

Polysaccharides. NIR spectra of polysaccharides contain features due to OH, CHp, and CH. OH groups produce broad bands near 1.4 and 1.0 um, the first and second overtones of the 0-H stretching fundamental. Sharp narrow 0-H features may occur if OH groups are weakly hydrogen bonded, as, for example, in the sapropel spectrum in Figure 3 In addition, two or more broad bands occur between 1.9 and 2.3 Jim due to combinations of 0-H stretch and C-O-H bending modes (42, 44> 56,... 

Coates, M. L., and Wilkins, C. L. (1987). Laser-desorption Fourier transform mass spectra of polysaccharides. Anal. Chem. 59 197-200. 

Fig. 2.—Absorption Spectra of Polysaccharide-Iodine Complexes. (I) Mytiltis edulis glycogen, (II) rabbit liver glycogen, (III) waxy-maize starch (amylopectin). [Solutions contained 0.01% of polysaccharide and 0.02% of iodine in 0.2% of potassium iodide, and were read against an iodine—potassium iodide reference solution.]...

Nevertheless, even at 360 MHz, assignment of the H n.m.r. spectra of higher oligosaccharides, and polysaccharides, is still a formidable problem. For example, Perlin and coworkers, with a 220-MHz instrument, made a study on heparin that led only to the assignment of the anomeric protons again, it is interesting that, on those occasions when it is possible to esterify the substance fully, prior to study, the H n.m.r. spectra show a substantial increase in dispersion.However, it seems reasonable to conclude that, at least for the present, detailed interpretations of the H n.m.r. spectra of polysaccharides are unlikely to be achieved, even were the expensive equipment needed for such measurements to become more widely available. 

Figure 13. NMR spectra of polysaccharide preparations from various strains...

The literature indicates that the proton NMR spectra of polysaccharides display severe overlap of signals in the chemical shift region of 3.5-5.5 ppm and it is difficult to assign them using ID H NMR spectrum alone [90,127]. This difficulty can be overcome by the fact that polysaccharides display a wide range of chemical shifts when compared to their H shifts. Carbon chemical shifts of polysaccharides appear in the range of 60-110 ppm. 

Further applications of a semieittpirical theory of optical activity have been reported. Calculated sodium-D line molar rotations for methyl pyranosides were consistently low, but the general dependence on structure was reproduced.The dominant origin of sodium-D rotation has been correlated with a vacuum-u.v. band near 150 nm, observed in c.d. spectra of polysaccharide films. 

Figure 7.37 Typical mechanical spectra of polysaccharide gels ...

The other spectra of polysaccharides, such as starch, pectin, and agar show a general resemblance to that of liquid HjO, having a broad maximum in the region of 200 cm and then increasing rapidly with increasing wavenumber [185],... 

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