Carbohydrates vibrational spectra

Carbohydrates (disaccharides) in aqueous solution are favorable examples for ROA studies furnishing an informative band structure over a wide range of the vibrational spectrum. Their complex and highly coupled normal modes generate strong ROA bands that produce patterns characteristic of the various types of structural units. ROA data can be used to confidently specify the central component of carbohydrate stereochemistry. For disaccharides [92] ROA can provide the nature and conformation of the glycosidic link, and can also probe extended secondary structures of oligosaccharides [93]. 

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. 

It is difficult to assign all of the observed i.r. and Raman vibrations of carbohydrates. The i.r. spectrum is particularly irregular, because it contains combination bands that may overlap with those due to fundamental modes, and interact with one another, leading to distortion of the shapes of the observed bands. Raman spectra show fewer irregularities, because combination bands in them are less important. However, even though the spectra of carbohydrates are complex, advantage can be taken of them by use of such techniques as isotopic substitution, or the model-compound approach. 

The new NIR FT Raman spectroscopy now allows the investigation of food (Keller et al., 1993). Fig. 4.1.20 shows typical Raman spectra of food components carbohydrates, proteins, and lipids. Fig. 4.1-20C shows the Raman spectrum of a banana with the out-of-phase and in-phase vibrations of the C-O-C groups of the carbohydrates at about 1100 and 850 cm. Fig. 4.1-20B, a Raman spectrum of turkey breast shows the amide I and III bands and some bands which can be directly assigned to amino acids. The Raman spectra of lipids allow the determination of the amount of cis and trans disubstituted C=C bonds Fig. 4.1-20A, butter (Keller et al., 1993). NIR Raman spectroscopy has good chances as tool for the investigation of living tissues, especially in medical diagnostics (Keller et al., 1994 Schrader et al., 1995). 

Raman optical activity (RO A) Due to molecular chirality there is a difference in the intensity of Raman scattered right and left circularly polarized light. Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on this difference and the spectrum of intensity differences recorded over a range of wavenumbers reveals information about chiral centers within a sample molecule. It is a useful probe to study biomolecular structures and their behavior in aqueous solution especially those of proteins, nucleic acids, carbohydrates, and viruses. The information obtained is in realistic conditions... 

When making comparisons, it is important that the two substances should have the same physical state. Furthermore, any mono- or oligosaccharide used as a model substance in interpreting the spectrum of the crystalline portion of a polysaccharide should have the same unit cell as the polymer. In this connection, attention should be drawn to the facts that (a) additional complications are introduced into the spectra of crystals, and into those of the crystalline fraction of polymers, by interactions between vibrations in neighboring unit cells, and (b) differences in the degree of order (crystalline, noncrystalline) within polymers also produce complications in the spectra. A detailed discussion of such complexities would be out of place here, as they are probably of primary interest to those engaged in fine-structure examination of polysaccharides rather than to investigators of the chemistry of carbohydrates. 

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. 

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