Mechanochemical study of conformational transitions and melting of Li‐, Na‐, K‐, and CsDNA fibers in ethanol–water solutions

Abstract

AbstractHighly oriented fibers of Li‐, Na‐, K‐, and CsDNA were prepared with a previously developed wet spinning method. The procedure gave a large number of equivalent fiber bundle samples (reference length, L0, typically = 12–15 cm) for systematic measurements of the fiber length L in ethanol–water solutions, using a simple mechanochemical set up. The decrease in relative length L/L0 with increasing ethanol concentration at room temperature gave evidence for the B‐A transition centered at 76% (v/v) ethanol for NaDNA fibers and at 80 and 84% ethanol for K‐ and CsDNA fibers. A smaller decrease in L/L0 of LiDNA fibers was attributed to the B‐C transition centered at 80% ethanol. In a second type of experiment with DNA fibers in ethanol–water solutions, the heat‐induced helix–coil transition, or melting, revealed itself in a marked contraction of the DNA fibers. The melting temperature Tm, decreased linearly with increasing ethanol concentration for fibers in the B‐DNA ethanol concentration region. In the B‐A transition region, Na‐ and KDNA fibers showed a local maximum in Tm. On further increase of the ethanol concentration, the A‐DXA region followed with an even steeper linear decrease in Tm. The dependence on the identity of the counterion is discussed with reference to the model for groove binding of cations in B‐DNA developed by Skuratovskii and co‐workers and to the results from Raman studies of the interhelical bonds in A‐DNA performed by Lindsay and co‐workers. An attempt to apply the theory of Chogovadze and Frank‐Kamenetskii on DNA melting in the B‐A transition region to the curves failed. However, for Na‐ and KDNA the Tm dependence in and around the A‐B transition region could be expressed as a weighted mean value of Tm of A‐ and B‐DNA. On further increase of the ethanol concentration, above 84% ethanol for LiDNA and above about 90% ethanol for Na‐, K‐, and CsDNA, a drastic change occurred. Tm increased and a few percentages higher ethanol concentrations were found to stabilize the DNA fibers so that they did not melt at all, not even at the upper temperature limit of the experiments (∼ 80°C). This is interpreted as being due to the strong aggregation induced by these high ethanol concentrations and to the formation of P‐DNA. Many features of the results are compatible with the counterion–water affinity model. In another series of measurements, Tm of DNA fibers in 75% ethanol was measured at various salt concentrations. No salt effect was observed (with the exception of LiDNA at low salt concentrations). This result is supported by calculations within the Poisson–Boltzmann cylindrical cell model. © 1994 John Wiley & Sons, Inc.