Frequency shifts in the infrared spectrum of human deoxyribonucleic acid

Abstract The infrared spectra of DNA from calf thymus, human normal and human leukaemic leucocytes was recorded in solution of H 2 O and D 2 O. Spectra have been also obtained after the samples in D 2 O have been subjected to the following treatments: heating, acid and enzymatic hydrolysis. The spectrum of apurinic acid has been determined. A detailed study is carried out in the frequency region from 1750 to 1550 cm −1 with improved resolution. The experimental difficulties for recording the spectrum in this spectral range are discussed. In disagreement with previous works it was found that the pair of strong bands at 1645 and 1680 cm −1 in the spectrum of the starting material shifts to 1660 and 1690 cm −1 respectively along with changes in intensity after treatment. Assignments of the bands in the same frequency range are tentatively proposed and the frequency shifts are explained in terms of the intermolecular interaction between purine and pyrimidine units. An infrared criterion for the existence of the double‐stranded helix is postulated as follows: the pair of infrared bands at 1645 and 1680 cm −1 characterizes a regular double‐stranded helix; a strong band at 1660 cm −1 and a weaker one at 1690 cm −1 indicate a deformed or denaturated DNA.