We have observed in-situ the hybridization (double helix formation) and denaturation (separation of double helix at elevated temperatures) of DNA in aqueous solution using infrared absorption spectroscopy (IRAS) in the multiple internal reflection (MIR) geometry. We demonstrated that conformational changes of DNA strands due to hybridization and denaturation are reflected in the infrared absorption spectra of the bases of DNA. Comparing with the results of ab-initio cluster calculation, we found that hybridization produces the specific C=O stretching vibration modes in the hydrogen-bonded bases, and also that the C=O stretching vibration modes of the bases of a single strand may be greatly influenced by its surrounding water molecules that interact with the bases.
