Carbohydrates infrared spectra

To assist in the interpretation of carbohydrate infrared spectra, one approach is to make use of model compounds which are of a simple nature but have similarities with the structure of the carbohydrate being examined. For example, tetrahydropyran is such a model compound since it contains the pyranose ring which is found in sugars. A knowledge of its spectrum can assist in the study of, and interpretation of, carbohydrate spectra. 

The intense absorption of water over most of the infrared spectrum restricts the regions where aqueous solutions of carbohydrates can be usefully studied. Absorbance subtraction makes it possible to eliminate water absorbance and magnify the remaining spectral features to the limit of the signal-to-noise ratio. Many other data-processing techniques, such as the ratio method,4 the least-squares refinement,5 and factor analysis,6 should be of benefit in the study of carbohydrate mixtures. 

Glycogen shows the characteristic infrared absorption spectrum of starch-type polysaccharides.—The infrared spectrum of glycogen, in the frequency range 730-960 cm., has three absorption peaks, at 928 3, 838 3, and 760 2 cm.- the absorption peak at 838 cm.- is displayed by all carbohydrates containing a-D-glucopyranose units, whilst the peaks... 

The Fourier Trairsform Infrared (FTIR) spectrum obtained from non-adapted tomato cell walls is very similar to that from the onion parenchyma cell wall (both contain cellulose, xyloglucan and pectin) although there is more protein in the tomato walls (amide stretches at 1550 and 1650 cm-i) (Fig 4). In DCB-adapted tomato cell walls, the spectrum more closely resembles that of either purified pectins or of a commercial polygalacturonic acid sample from Sigma with peaks in common at 1140, 1095, 1070, 1015 and 950 cm-t in the carbohydrate region of the spectrum as well as the free acid stretches at 1600 and 1414 cm-i and an ester peak at 1725 cm-k An ester band at 1740 cm-i is evident in both onion parenchyma and non-adapted tomato cell wall samples. It is possible that this shift in the ester peak simply reflects the different local molecular environment of this bond, but it is also possible that a different ester is made in the DCB-adapted cell walls, as phenolic esters absorb around 1720 cm-i whilst carboxylic esters absorb at 1740 cm-k The... 

Infrared and Raman spectroscopy are in current use fo r elucidating the molecular structures of nucleic acids. The application of infrared spectroscopy to studies of the structure of nucleic acids has been reviewed,135 as well as of Raman spectroscopy.136 It was noted that the assignments are generally based on isotopic substitution, or on comparison of the spectrum of simple molecules that are considered to form a part of the polynucleotide chain to that of the nucleic acid. The vibrational spectra are generally believed to be a good complementary technique in the study of chemical reactions, as in the study76 of carbohydrate complexation with boric acid. In this study, the i.r. data demonstrated that only ribose forms a solid complex with undissociated H3B03, and that the complexes are polymeric. 

In Fig. 12.2 ATR infrared spectra of starch (a complex carbohydrate) (A) soy protein (B), canola oil (mostly triglycerides) (C), and a 1 1 1 mathematical mixture of the three (D) are presented. The regions containing diagnostic bands appropriate for infrared imaging are identified with dashed boxes. Note how easily all three components may be identified in the mixture spectrum (D). 

This Section Ls restricted to a description of some of the work of Ander-gon, 8-a> who has ably applied the quantitative analysis of vapors by infrared spectroscopy to analytical problems in carbohydrate chemistry, principally to the Zeisel alkoxyl determination. In this particular application, the usual Zeisel apparatus was used, and the volatile iodide liberated was carried by a flow of nitrogen into a cold trap where it was collected quantitatively Anhydrone (magnesium perchlorate) was used for removing water vapor which would otherwise interfere in the spectrum. The contents of the trap were allowed to vaporize into an evacuated gas-cell, and air was then admitted through the trap to sweep all the vapor into the gas-cell. Double-beam compensation of atmospheric water vapor and carbon dioxide was not upset by this procedure, which also served the purpose of increasing the sensitivity of the infrared method by the well known pressure-broadening effect. The complete spectrum of the vapor... 

Crystalline samples sometimes produce spectra with distorted band shapes, an effect known as the Christiansen effect [see Potts (1963) and Table 1.4]. Also, polymorphic forms of the same substance frequently show differences in infrared spectra. An example is N-benzoyl-2,3,4,6-tetra-0-benzoyl- -D-glucosylamine, a compound that exists in a form with melting point 113-115°C which, when heated to 117-120°C and allowed to crystallize from the melt, gives a form with melting point 184°C having a somewhat different spectrum in Nujol (Tipson, 1968). Also, different crystal habits (same melting point) of a compound may display partially differing spectra, especially if examined as mulls, in which little pressure is applied. Shifts of up to 20 cm" for certain bands have been observed (Barker et al., 1956) for crystalline and amorphous forms of some carbohydrates. In all such instances, however, spectra of samples of each of the forms, recorded after dissolution in the same solvent, or as a molten substance, are identical. 

Eremin et al. (1965) have precipitated carbohydrate material with ethanol after alkaline hydrolysis of cultures of Whitmore s bacillus Pseudomonas pseudomailer), Pasteurella pestis, and Vibrio comma, and have subjected these polyoses to infrared spectroscopy. All spectra had strong absorption at 1660 and 1550 cm the former was related to double-bond vibrations and the latter was associated with stretching vibrations of C—N. The latter absorption was almost completely absent in the spectrum of a complex from V. comma. Absorption at 970 cm (the C=C double bond in the trans position) and traces of absorption at 790 cm characteristic of the 1 — 3 bond were always present. A polysaccharide from the cell wall of P. pestis had a wide band at 1170-1000 cm the low intensity bands at 1190 and 1160cm indicated the presence of P—O—Me and P—O—Et groups. The spectrum of a complex from Whitmore s bacillus differed from the others by the presence of a band at 1735 cm due to esters of fatty acids. 

Near-infrared (NIR) spectroscopy is widely used in the food industry as a fast routine analytical method for the quantitative measurement of water, proteins, fats and carbohydrates [13]. Although the near-infrared bands are less useful for qualitative analysis of foods because of their broad overlapped appearance, these bands are suitable for quantitative analysis when using chemometric techniques. Figure 8.12 illustrates the appearance of the major food components in the near-infrared, showing the spectrum of a sample of dehydrated tomato soup. 

Examples from other groups include gas-phase thermal dissociation experiments, implemented with blackbody infrared radiative dissociation (BIRD) and FT-ICR MS on a series of protein-carbohydrate complexes, and the detection of fusion peptide-phospholipid noncovalent interactions using nano-ESI FTICR-MS. An interesting example of protein-DNA interaction smdied by ESI-MS is the trp repressor-DNA operator complex. Escherichia coli trp apo-repressor (TrpR), a homodimer, is a DNA-binding protein that binds to two molecules of co-repressor L-tryptophan to form a holorepressor complex at higher salt concentrations. The mass spectrum of noncovalent... 

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