Aqueous Solutions of Nucleosides, Nucleotides, Polynucleotides, and DNA

Infrared spectra of heavy-water solutions have been used to determine the tautomeric forms of nucleosides, nucleotides, and polynucleotides and to study the 

In order to make quantitative measurements. Miles (1958f)) has determined integrated infrared absorption intensities for some nucleosides, nucleotides, and polynucleotides in DjO solution (Fig. 12.8 and Table 12.2) by application of Ramsay s method I (Ramsay, 1952). When there is a well-resolved band, the application of Ramsay s method encounters no difficulty, but when overlapping bands occur, as in uridine and its derivatives, some uncertainty exists in determining the halfband width of a particular band. (The half-band width is a factor in Ramsay s equation and is defined as, Av, 2 = the width of the band in cm at half-maximum intensity.) The equation as used by Miles to obtain A, the true integrated absorption intensity 

Marked changes occurred in the infrared spectra when polyuridylic acid and polyadenylic acid were mixed, and the changes were considered to result from hydrogen-bonding interaction between them. These quantitative measurements supported the conclusion that infrared spectra of such mixtures provide experimental evidence that the uracil units are present in the keto form and that the adenine units are probably present in the amino form, as in the Watson-Crick DNA model. Earlier it had been demonstrated (Miles, 1956 1958o) by means of infrared spectra of appropriate model compounds that the band at 1692 cm is characteristic of the keto form of uridine and would be absent in the enol form. It had also been shown that the 1630 cm band is probably indicative of the amino form of adenosine. If interaction of the polymers involved the enol form of uridine, there would be no band at 1695 cm in the mixture. 

The aqueous method has been used for the determination of tautomeric structures (Miles, 1961 Miles et al., 1963) the demonstration of helix strandedness [i.e., the stoichiometry of interaction of polynucleotides to form two- or three-stranded helices (Miles and Frazier, 1964a)] the demonstration of a strand disproportionation reaction (Miles and Frazier, 1964b), of a strand displacement reaction (Sigler et al., 1962), and the quantitative analysis of different helical structures (Miles and Frazier, 1964a,b) in complex mixtures. It has been important in most of these applications that there are comparatively large differences of the spectra of the components from one another and there are marked and characteristic changes that the spectra undergo upon interaction to form helical secondary structures. 

Miles (1960) has shown spectra of the three-stranded helices formed between polyadenylic acid (poly A) and polyuridylic acid (poly U) and between poly U and tetra A (tetraadenylic acid, pApApApA). For the related spectra, the assignment of bands, and the experimental methods used, some of Miles s previous papers (1956 1958a,b 1959) should be consulted. The infrared spectra of the double helix formed 

---