MECHANOCHEMICAL STUDY OF CONFORMATIONAL TRANSITIONS AND MELTING OF LI-, NA-, K-, AND CSDNA FIBERS IN ETHANOL-WATER SOLUTIONS

Highly 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, L(0), typ ically = 12-15 cm) for systematic measurements of the fiber length L i n ethanol-water solutions, using a simple mechanochemical set up. The decrease in relative length L/L(0) with increasing ethanol concentrati on at room temperature gave evidence for the B-A transition centered a t 76% (v/v) ethanol for NaDNA fibers and at 80 and 84% ethanol for K- and CsDNA fibers. A smaller decrease in L/L(0) of LiDNA fibers was att ributed to the B-C transition centered at 80% ethanol. In a second typ e of experiment with DNA fibers in ethanol-water solutions, the heat-i nduced helix-coil transition, or melting, revealed itself in a marked contraction of the DNA fibers. The melting temperature T-m, decreased linearly with increasing ethanol concentration for fibers in the B-DNA ethanol concentration region. In the B-A transition region, Na- and K DNA fibers showed a local maximum in T-m. On further increase of the e thanol concentration, the A-DNA region followed with an even steeper l inear decrease in T-m. The dependence on the identity of the counterio n is discussed with reference to the model for groove binding of catio ns in B-DNA developed by Skuratovskii and co-workers and to the result s from Raman studies of the interhelical bonds in A-DNA performed by L indsay and co-workers. An attempt to apply the theory of Chogovadze an d Frank-Kamenetskii on DNA melting in the B-A transition region to the curves failed. However, for Na- and KDNA the T-m dependence in and ar ound the A-B transition region could be expressed as a weighted mean v alue of T-m of A- and B-DNA. On further increase of the ethanol concen tration, above 84% ethanol for LiDNA and above about 90% ethanol for N a-, K-, and CsDNA, a drastic change occurred. T-m 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 tem perature limit of the experiments (similar to 80 degrees C). This is i nterpreted as being due to the strong aggregation induced by these hig h 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, T-m of DNA fibers in 75% ethanol was measured at various salt concentrations. No salt effect was observ ed (with the exception of LiDNA at low salt concentrations). This resu lt is supported by calculations within the Poisson-Boltzmann cylindric al cell model. (C) 1994 John Wiley & Sons, Inc.