The thermally stimulated depolarization current (TSDC) and d.c. conductivity methods have been used in the search for a glass transition in solid NaDNA and in the studies of its origin. The existence and properties of a dipolar TSDC relaxation peak with a maximum at temperature, T,,, , in the range of (160 + 250) K (depending on the water content in the samples) and an exponentially increasing d.c. conductivity above T, indicate the existence of a glass transition effect in solid DNA. The use of the same methods in the samples of pure ice and frozen aqueous solutions of NaCl (models of the ion-water shell in DNA) confirms that the glass transition origin is in melting of hydrogen bonds of adsorbed (bound) water molecules on the surface of the biopolymer.
The thermally stimulated depolarization current (TSDC) measurements in frozen aqueous solutions, gels and solid layers of NaDNA show typically up to three dipolar overlapping peaks in the low-temperature range of 80-150 K. Up to four discrete relaxation peaks have been observed at higher temperatures above 150 K. The low-temperature TSDC peaks are due to the dipolar relaxations of free and loosely bound water which crystallizes. Part of bound water especially in the first hydration shell of DNA molecule is at low temperatures in the form of glass. The transition of this glass from solidlike behavior to liquidlike behavior observed mainly in gels and solid samples is associated with a previously founded TSDC relaxation peak. The peak is at its maximum at 165-250 K depending on the sample humidity. Existence of this relaxation in the samples with water contents in a broad range confirms, that the slowly relaxing shell (minimally 5-7 water molecules/nucleotide) closely associated with DNA double helix retains its characteristics. Also another peak of the high-temperature band at 180-205 K which was observed in the samples at hydration 2-1800 g H2O/g dry NaDNA is due to a relaxation in the sample volume. At the highest temperatures relax the space charges trapped at the electrodes.
Abstract. The d.c. conductivity and daelectric properties of solid low-humidity NaDNA layers have been examined over frequency and temperature ranges up to 105 Hz and 80-330 K respectively. The results presented are basically consistent with a model in which the majority charge carriers are protons (H30 +, OH-) moving on the surface of a NaDNA chain in the neighbourhood of the phosphate groups. The increasing hydration also increases the mobility of counterions (Na +) and their contribution to a d. c. conductivity. The Arrhenius d. c. conductivity seems to be limited by electrochemical processes on the electrodes. Low-frequency dispersion is also caused by this effect. The local and long-range motion of charge carriers is limited below temperatures of a dipolar thermally stimulated depolarization current (TSDC) peak observed in the range 165-255 K. The amplitude and position of the peak depend strongly on the water content in the sample.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.