O

In the IR spectrum the P-O modes are considerably stronger than those for the P-O-R linkage: 1230 cm'1 for P02 asymmetric stretch, 1080 cm-1 for P02-symmetric stretch, and 980 cm"1 for POf~.

Sugar Moiety

The most characteristic group of bands occurs in a broad distribution in the range of 1000 to 1100 cm-1 and arises from C-O-C and C-C(OH)-C modes. The deoxy derivatives exhibit a sharp band at 975 cm-1, which is absent in the ridonucleosides. There is some evidence that DNAs and RNAs may be distinguished in this manner.

DNA and RNA

As expected, the spectra of DNA and RNA show features in common with their simpler analogs. In the 1500 to 1750 cm-1 region there are broad, poorly resolved bands corresponding to the complex ring modes. The strong phosphate (asymmetric) mode is in the 1240 cm-1 region, and the broad and complex band is in the 1000 to 1100 cm ~1 region for the sugar vibrations. The strong band in the 3400 cm"1 region, which contains the NH stretching mode, is influenced by the presence of water. One of the major difficulties of studying even solid-state nucleotides is that water almost inevitably crystallizes with the nucleotides.

Dichroic Ratios

The measurement of dichroic ratios of biological systems to determine their structure requires polarized light and oriented structures. Polarized IR light may be produced by reflection from or transmission through films of selenium 4 fim thick, set at the polarizing angle to the incident light. This technique has been applied to the analysis of nucleic acid structure (DNA). The position of the N-H and O-H frequencies indicates that these groups participate in hydrogen bonds. The dichroism of the N-H, O-H, and C = O groups indicates that the hydrogen bonds are preferentially oriented perpendicular to the molecular axis, and so they probably represent intermolecular regions of attraction. The P-O-C groups also have a component perpendicular to the molecular axis, but the absence of any dichroism in the P = O absorption band at 1250 cm ~1 implies that they are oriented at angles of 55° to the molecular axis, which is in good agreement with the structural model of DNA discussed previously.

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