Dynamics of deoxyribonucleic acid solutions as studied by dielectric relaxation spectroscopy and dynamic mechanical spectroscopy

An investigation was carried out by dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS) on the dynamics of aqueous solutions of deoxyribonucleic acid (DNA). Novel information is generated and presented through our use of wide temperature and frequency range in DRS and DMS measurements. Two relaxation processes were detected at temperatures below 273 K. The higher frequency, Debye-like process has an activation energy of 27 kJ/mol and is assigned to the bound water around DNA molecules. The lower frequency process is of the Cole-Cole type and has an activation energy of 55 kJ/mol, the same as that of pure ice. In the higher temperature range, encompassing the physiological condition, conductivity dominates the dielectric response. A pronounced peak in the dielectric modulus spectrum is observed, and its molecular origin is found to lie in the migration of counterions along the DNA surface. Using Manning's model, it was calculated that the subunit length over which counterions fluctuate increases from 70 nm at a concentration of 2 mg/mL to 153 nm at a concentration of 0.125 mg/mL. The results of DMS measurements of aqueous solutions of calf thymus DNA reveal a G'/G" crossover point whose frequency (omega(C)) scales with concentration as omega(C) similar to C-DNA(-2.4). The implication is that the DNA molecules behave as semiflexible polymers in aqueous solution. The temperature dependence of omega(C) indicates that the breakup of the DNA base pairs and the chain melting begin at a temperature as low as 50 degrees C.