SynopsisThe 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 difiaction 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 (-958) ethanol percentage. The 670% 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 --t n* transitions to the CD spectrum.
Flow linear dichroism is used to measure specific inclinations for each of the four bases in poly[d(AC)].poly[d(GT)] and poly[d(AG)].poly[d(CT)] in both the B and A forms. For the B form in solution the bases are found to have a sizable inclination. Inclination is increased in the A form, as expected. In all cases the pyrimidines are more inclined than the purines.
SYNOPSISThe Z-form of poly(dG-m5dC)~poly(dG-m5dC) in 85% ethanol/MOPS and the Z-form poly ( rG-rC ) . poly ( rG-rC) in 4.8 M NaClO,, 20% ethanol are studied by flow linear dichroism. Simultaneous analysis of the isotropic absorption and linear dichroism data yields the angle of inclination of the base normal relative to the helix axis, and the orientation of the axis around which the bases incline. For the Z-form of poly(dGm5dC) .poly(dG-m5dC) the inclination angles are 31.3" for guanine, and 27.9" for cytosine; for the Z-form of poly(rG-rC).poly(rG-rC) they are 25.0" for guanine, and 23.3" for cytosine. The inclination angles for the Z-form DNA are similar to those for the Zform, and the angles for Z-form RNA are somewhat smaller. The axes of inclination for guanine and cytosine are similar in all three left-handed forms. 0 1995 John Wiley & Sons, Inc.
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