THz-spectroscopy of biological molecules

The terahertz frequency absorption spectra of DNA molecules reflect low-frequency internal helical vibrations involving rigidly bound subgroups that are connected by the weakest bonds, including the hydrogen bonds of the DNA base pairs, and/or non-bonded interactions. Although numerous difficulties make the direct identification of terahertz phonon modes in biological materials very challenging, our research has shown that such measurements are both possible and fruitful. Spectra of different DNA samples reveal a large number of modes and a reasonable level of sequence-specific uniqueness. In an attempt to show that the long wavelength absorption features are intrinsic properties of biological materials determined by phonon modes, a normal mode analysis has been used to predict the absorption spectra of polynucleotide RNA Poly[G]-Poly[C]. Direct comparison demonstrated a correlation between calculated and experimentally observed spectra of the RNA polymers, thus confirming that the fundamental physical nature of the observed resonance structure is caused by the internal vibration modes in the macromolecules. In this work we demonstrate results from Fourier-Transform Infrared ( FTIR) spectroscopy of DNA macromolecules and related biological materials in the terahertz frequency range. Careful attention was paid to the possibility of interference or etalon effects in the samples, and phenomena were clearly differentiated from the actual phonon modes. In addition, we studied the dependence of transmission spectra of aligned DNA and polynucleotide film samples on molecule orientation relative to the electromagnetic field, showing the expected change in mode strength as a function of sample orientation. Further, the absorption characteristics were extracted from the transmission data using the interference spectroscopy technique, and a strong anisotropy of terahertz characteristics was demonstrated.