ABSTRACT
264Dehydration, melting, and thermal degradation of sodium salt of calf thymus DNA in dry state were investigated. For investigation of kinetics of reactions taking place during the nonisothermal heating, thermal gravimetric analysis (TGA) data recorded at different heating rates were analyzed using various model-free isoconversional methods. Differential scanning calorimetry (DSC) analysis was performed both in open and hermetically sealed cells. Melting temperature of the DNA was higher in the sealed cells (110°C) than in the open cells (80°C). The residues formed after the DNA was repeatedly heated and cooled in sealed DSC cells were analyzed by transmittance Fourier transform infrared (FTIR) spectroscopy. These residues were also analyzed by scanning electron microscope (SEM) for the morphological changes. The original fibrous form of the DNA turned to a black residue during heating to 175°C in the hermetically sealed cells with upper pressure limit of 0.3 MPa. Microbubbles indicating the intumescence characteristic of the DNA were observed on the surface of the residues. In the transmission FTIR spectrum of the residue obtained by heating the DNA to 600°C under nitrogen flow, bands characteristics for inorganic phosphates were detected. The DNA was also analyzed in situ by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy during heating/cooling under vacuum. The results were discussed based on the effects of heating rate and reaction atmosphere on the intumescence behavior of the DNA.
