Quantum Chemical and Spectroscopic Study on Hydrogen Bonds in Hairpin DNA

Broadband far-infrared absorption spectra were measured for pellets containing guanine or cytosine and an aqueous solution containing hairpin DNA with the two bases. When the solution was dried at 80 °C, the absorption increased around 540 cm⁻¹. Quantum chemical calculations were conducted by assuming models simulating one, two, and three molecules of each base and another model representing the hydrogen bonding between the two bases. By comparing the spectra acquired experimentally and numerically, it has become clear that almost all absorption peaks appearing in a range from 16.7 to 650 cm⁻¹ are due to skeletal and intermolecular vibrations. Two vibrational levels at 527 cm⁻¹ in guanine and 524 cm⁻¹ in cytosine decrease if hydrogen bonds are formed. Assuming that the two modes are causal vibrations of the 540-cm⁻¹ absorption, the spectroscopic experiment catches that hydrogen bonds between guanine and cytosine in DNA are dissociated around 80 °C.