CD of the li‐salt of DNA in ethanol/water mixtures: Evidence for the B‐ to C‐form transition in solution

Abstract

Abstract The conformational change of Li‐DNA in water/ethanol mixtures is followed by the change in the CD spectrum in solutions containing various percentages of ethanol in the range from 0 to 95%. Two main transitions can be distinguished. The first occurs in the range from 0 to 70% and is represented by a large reduction of the intensity of the positive CD band around 275 nm, which is apparently related to a small change in the number of base pairs per turn. Secondly, at higher percentages of ethanol (> 80%) a conformational change is detected, which is expressed as a reduction of the 245‐nm negative CD intensity. According to x‐ray diffraction experiments of fibers of Li‐DNA, the C‐form is attained in 95% ethanol, while at 70% ethanol a B‐like structure is observed. The CD experiments reported here also show that for DNA in solution the dependence on percentage of ethanol is composed of two main transition regions. The C‐form would then be adopted at high (∼ 95%) ethanol percentage. The 0–70% transition, although strongly expressed in optical CD experiments, has to be related to relatively small structural changes within the B‐family of DNA structures, which probably induce an enhanced contribution of n → π* transitions to the CD spectrum.