Calorimetric investigation on liquid crystals of deoxyribonucleic acid in concentrated solutions

Abstract The phase states of DNA in concentrated solutions have been studied by simultaneous measurement using differential thermal analysis (DTA) and a visible optical method (HeNe gas laser, λ o = 633 nm) equipped with a computer, and using a polarization microscope equipped with a hot stage, and IR spectra. From the polarization microscope results, DNA below a concentration of 2.00 wt% showed an isotropic phase; in the concentration range 3.00–5.00 wt%, DNA showed an anisotropic phase; and in concentrations above 6.00 wt%, DNA showed an anisotropic phase with a distinct birefringence from the measurement of the refractive index at 298 K, i.e. at concentrations above 6.00 wt% DNA forms a liquid crystal phase state. In order to obtain information about the mechanism of formation of the liquid crystal of DNA, IR spectra for solutions containing 10.0 wt% of DNA and Mg 2+ ion have been measured at room temperature. DNA does not form liquid crystals in the presence of Mg 2+ ion. Therefore, an electrostatic interaction between the PO − 4 group in the main chain of DNA and the Mg 2+ ion plays an important role in blocking the formation of the liquid crystal. Thus, it is suggested that DNA in concentrated solutions forms liquid crystals by a molecular orientation arrangement based on the repulsive force between the negative PO − 4 groups in the main chain of DNA. The change in the enthalpy of formation of the liquid crystal of DNA has been estimated to be about − 2.8 kJ mol −1 of nucleotide, by considerating the change in the intensity of transmitted laser light, using the DTA apparatus equipped with a laser.