Submillimeter-wave Fourier transform spectroscopy of biological macromolecules

Abstract

In this article we report experimental results on Fourier-transform infrared spectroscopy of deoxyribonucleic acid (DNA) macromolecules and related biological materials in the submillimeter range (i.e., ∼10–500 cm−1). Film samples made from commercial DNA fibers, polyadenylic acid potassium salt, and cellular agents such as the spore form of Bacillus subtillis have been prepared and measured. A broad series of measurements carried out in the low frequency region (10–50 cm−1) with a higher resolution of 0.2 cm−1 revealed fine features—multiple dielectric resonances in the submillimeter-wave spectra obtained from DNA samples. These long-wave absorption features are shown to be intrinsic properties of biological materials determined by phonon modes. The emphasis is on reproducibility of experimental spectra and on receiving reliable results. The effects of differences in sample preparation, including sample geometry, orientation, and aging are studied and separated from the phonon effects that determine the fine structure of transmission spectra. A direct comparison of spectra between different DNA samples reveals a large number of modes and a reasonable level of sequence-specific uniqueness. A theoretical study of two double helical DNA fragments has applied a normal mode analysis to predict spectra in the far infrared. Most of the modes determined by long-distance interactions are at frequencies below 220 cm−1, with the density higher than one mode per cm−1, which is approximately what was observed experimentally.