In nucleic acids, most attention has been focused on the IR spectral region 1500-1800cm-1, which contains vibrations of the carbonyl and the double bonds of the purine and pyrimidine rings. The vibrations are highly sensitive to base pairing because the atoms involved participate directly in the formation of hydrogen bonds. Furthermore, each of the four common bases has a distinct IR spectrum in this spectral region (Thomas, 1969). Thus it is possible to examine A-T(U) and G-C pairs separately.
The IR spectra of carbohydrates present several characteristic features from which functional group assignments can be made. There is a strong band at 3500 cm-1 due to the hydrogen bonded O—H groups. The region of 1150-1000cm-1 has several closely spaced vibration bands, probably arising from C—C and C—O vibrations. At lower frequencies, different structural isomers are shown to have different characteristic absorption bands, one of which is the anomeric bands. a Anomers (a-glycosides and glycoses) show a distinct absorption band at approximately 844cm-1 and P anomers at 891cm-1 (Table 7.8). The vibration band observed at around 875 cm-1 is probably due to C—O—C stretching of the pyranose ring. Ring vibrations of pyranoses occur at 917 and 770cm-1 while fura-nose rings exhibit absorption at 924 and 799 cm-1. 2-Keto glycoses show characteristic absorption at 810 and 874 cm-1 characteristic of the ketal group, regardless of whether it is a furanose or pyranose ring.
The IR spectrum offers a means for determining anomeric configuration and is useful in studies of polysaccharides with homologous anomeric linkages, since the anomeric C—H bonds are not influenced by the types (glycose units) and positions of linkages. An estimate of the strength of hydrogen bonds can be made from the separation in wave number (Av) between the IR bands of free versus hydrogen bonded hydroxyl groups. In general, the smaller the length of the hydrogen bond, the stronger the bond itself and the greater the value of the Av. Studies indicate that the crystalline region of cellulose is completely hydrogen bonded and whereas amorphous region is not.
IR dichroism is a useful technique for investigating conformations or oriented poly-saccharide samples. For example, cellulose I possesses a unit cell containing two parallel chains of repeating cellobiose units. Dichroism studies show that the polarization of one of the bands is perpendicular, involved in inter-chain hydrogen bonds and two are parallel to the long axis forming intra-chain hydrogen bonds.
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