L

2200 2400 2600 2800 3000 A (A)

Figure 7. Absorption UV spectra of nucleoside that constitutes DNA at neutral pH in the dry state.

sorption spectra for nucleic acids would have a maximum at 260 fim. The optical densities shown in the figure were calculated from data obtained from thinner and more dilute solutions on the assumption that the Beer-Lambert law holds. At the longer ultraviolet wavelengths, nucleic acids will have about 20 times the optical density of an equal mass concentration of protein. Most of the amino acids are nonaromatic and contribute practically nothing to the protein absorption spectrum at wavelengths greater than 210 /im. The high and characteristic absorption by nucleic acids makes it relatively simple to detect them by ultraviolet spectroscopy and to localize them in cells by ultraviolet microscopes.

In Table 4, the wavelengths of maximum absorbency and the intensity (e) for various bases, nucleosides, and nucleotides are summarized, which are tentative values compiled by the National Academy of Science.

Infrared Absorption Spectra

Infrared (IR) absorption methods are used mainly for two different purposes: one is as a "fingerprint" to identify a particular compound and to detect it in the mixture, the other is for structural studies. The logical approach to a spectroscopic study of the nucleic acids is to examine separately the base, sugar, and phosphate moieties; their combination in nucleosides and nucleotides; and their comparison with nucleic acids. The approximate vibrational frequency ranges for the components of functional groups related to nucleic acids are shown in Table 5.

Bases and Nucleosides

The fundamental modes associated with base residues occur throughout the IR spectrum and thus overlap bands

Table 4. The Maximum Absorbance Intensity (e) for Various Nucleic Acids

Ratio of observance

The maximum absorbance between two wavelengths

Table 4. The Maximum Absorbance Intensity (e) for Various Nucleic Acids

Ratio of observance

The maximum absorbance between two wavelengths

Nucleic acid

Intensity (e x 103)

Wavelength im ft)

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