The effect of ethanol on the structure of DNA confined to mica in the presence of Mg2+ was examined by varying the ethanol concentration and imaging the DNA by atomic force microscopy. Contour length measurements of the DNA show a transition from all-B-form at 0% ethanol to all-A-form at > 25% ethanol. At intermediate ethanol concentrations, contour lengths suggest that individual molecules of air-dried DNA are trapped with mixed compositions of A-form and B-form. The relative composition depends on the ethanol concentration. Fitting the length distributions at intermediate ethanol concentrations to a simple binomial model results in an upper bound estimate for the A-form and B-form domains of ~ 54 bp in the individual molecules. In addition to length changes, the apparent persistence length of DNA decreases with increasing ethanol concentration. At high concentrations of ethanol (> 20%), DNA formed several higher order structures, including flower shaped condensates and toroids.
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