t NH

Base residue

Note: Modes: v, stretching; <5, in-plane bending; t, out-of-plane bending.

Note: Modes: v, stretching; <5, in-plane bending; t, out-of-plane bending.

from the other entities in nucleic acids. Between 1500 and 1750 cm-1 several strong bands occur that are free from interference by phosphate and sugar groups and are related to the structure of the purine and pyrimidine rings in nucleosides and nucleotides, and nucleic acids. These bands also arise from strongly coupled vibration of the C = 0 (1725 cm"1 stretch), C = N (1600 cm'1 stretch), and C = C types with in-plane NH (1680 cm-1 bend) deformation.

On the basis of protonation studies of the base-sugar, the following are concluded (16):

1. The acidic form of adenosine has a proton attached to one of the ring nitrogens, not to — NH2.

2. Guanosine exists in acidic, neutral, and alkali forms, with two possible neutral forms. Each form has a different IR spectrum. The alkali form is deproton-ated in position 1, and the acidic form in position 7.

3. Deprotonation of inosine also causes ^-electron de-localization with concomitant decrease in C=0 stretching frequency from 1675 cm-1 to 1595 cm-1.

Phosphate Group

Linking the sugars in the nucleotide backbone, the phosphate diester linkage can exist in two different states, pro-tonated and charged:

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