Chemically Substituted Polynucleotides

The substitution of phosphate O by other atoms or groups is another field which has received attention. These substitutions, which may render the P atom chiral, include O, S, Se, CHj, NRj. BH3 and OR groups (10.95). Sulphur has also been introduced into the main polynucleotide chain [68] and there has also been very recent evidence that As can be substituted for P in the DNA of a certain variety of bacteria (see Chapter 9.9). 

An oligomer having N chiral centres gives rise potentially to 2 isomers. The stereoselective synthesis of such isomers has not yet been completely developed. 

Varieties of phosphonate DNA have been made (10.96c) and the P atom itself has also been replaced to give oligonucleotide analogues devoid of phosphate bonds (10.96a,b). 

Locked nucleic acids (LNA) (10.96d,e), are types of polynucleotide (RNA) chain in which eithCT the 2 or the 3 O atom is linked to the 4 C atom by a methylene bridge, thns restricting the conformation of the sugar ring. 

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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. 

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