A MECHANOCHEMICAL STUDY OF MGDNA FIBERS IN ETHANOL-WATER SOLUTIONS

Highly oriented calf-thymus MgDNA fibers, prepared by a wet spinning m ethod, were studied with a simple mechanochemical set-up. The relative fiber length, L/L(0), was measured with the fibers submerged in ethan ol-water solutions. In one type of experiment L/L(0), was measured as a function of ethanol concentration at room temperature. No substantia l decrease in L/L(0), with increasing ethanol concentration was observ ed, indicating that MgDNA fibers stay in the B form even when the wate r activity is very low. For low ethanol concentrations the fiber struc ture is stable and does not dissolve even at very high water activitie s. In a second type of experiment, the heat-induced helix-coil transit ion was manifested by a marked contraction of the fibers. The transiti on temperature decreases linearly with increasing ethanol concentratio n between 52 and 68% ethanol. At higher ethanol concentrations the hel ix-coil transition temperature increases due to strong aggregation wit hin the DNA fibers, and above 77% ethanol the fibers do not contract a t all, not even at the upper temperature limit of the experiments, sim ilar to 80 degrees C. This behavior is discussed with reference to dri ed DNA and the P form of DNA. The helix-coil transition temperature of the MgDNA fibers in 70% ethanol does not show any dependence on the M gCl2 concentration, ii is shown that the Poisson-Boltzmann cylindrical cell model can account qualitatively for this lack of salt dependence .