Ethanol-induced structural transitions of DNA on mica

The effect of ethanol on the structure of DNA confined to mica in the prese nce of Mg2+ was examined by varying the ethanol concentration and imaging t he DNA by atomic force microscopy. Contour length measurements of the DNA s how a transition from all-B-form at 0% ethanol to all-A-form at >25% ethano l. At intermediate ethanol concentrations, contour lengths suggest that ind ividual molecules of air-dried DNA are trapped with mixed compositions of A -form and B-form. The relative composition depends on the ethanol concentra tion. Fitting the length distributions at intermediate ethanol concentratio ns to a simple binomial model results in an upper bound estimate for the A- form and B-form domains of similar to 54 bp in the individual molecules. In addition to length changes, the apparent persistence length of DNA decreas es with increasing ethanol concentration. At high concentrations of ethanol (>20%), DNA formed several higher order structures, including flower shape d condensates and toroids.