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Only few studies have attempted to characterize biological materials by THz spectroscopy. Most of these used either solid samples or biological tissues. In this work, we present results of THz spectroscopic characterization of dilute solutions of DNA samples. Water and heavy water (D2O) have strong absorbance that overlap significantly with important absorption bands of biomolecules in conventional FTIR spectroscopy. Cumbersome spectral subtractions and highly concentrated samples are therefore required to partially overcome problems of water interference in FTIR spectra of biomolecules. Although liquid water absorbs and contributes to
background in the THz spectral range of interest, the level of water absorption in the low THz range is at least 2.5 orders of magnitude less than in the far IR. Here, we demonstrate that reproducible spectra of dilute solutions of DNA in the frequency range 10-24 cm-1 can be obtained. We show that dilute aqueous samples of DNA produce THz spectra with signals that do not overlap with those of water. This is a significant achievement towards the goal of developing THz resonance spectroscopy as a useful tool for the biological sciences because all biological functions of DNA and proteins take place in aqueous environments. A simple technique for sample preparation and characterization is described. Samples containing as little as 100 ng of DNA in 10 μl of water (0.01 mg/ml or 0.001%) have been prepared and measured. The signal/noise ratios of THz spectra of these samples are sufficient to detect reproducible resonances at several characteristic frequencies. The effect of orientation on different substrates and the mechanism of sensitivity enhancement in these samples is discussed. It should be possible to extend these methods to also study proteins in dilute solutions. Advantages of using dilute samples include small quantities of biological material required, the absence of interference from interactions between neighboring molecules, and the absence of problems with light-scattering that are often encountered with short wave-length optical techniques.
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