Low-frequency four-wave mixing spectroscopy of biomolecules in aqueous solutions

Four-wave mixing (FWM) spectroscopy is used to detect the rotational resonances of H₂O and H₂O₂ molecules in DNA and denatured DNA aqueous solutions in the range ±10 cm^-1 with a spectral resolution of 3 GHz. It is found that the resonance contribution of the rotational transitions of these molecules increases significantly in solutions rather than in distilled water. This fact is interpreted as a manifestation of specific properties of a hydration layer at DNA—water and denatured DNA—water interfaces. Analysis of the FWM spectra shows that the concentration of H₂O₂ molecules in the hydration layer of the DNA solution increases by a factor of 3 after denaturation. The FWM spectra of aqueous solutions of α-chymotrypsin protein are obtained in the range ±7cm^-1 at the protein concentrations between 0 and 20 mg cm^-3. It is found that the hypersound velocity in the protein aqueous solution, measured by the shift of Brillouin components in the scattering spectrum, obeys a cubic dependence on the protein concentration and reaches a value of about 3000 m s^-1 at 20 mg cm^-3.