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

Abstract

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.

Manipulating DNA Conformation Using Intertwined Conducting Polymer Chains

“…29 However, in this case the negative peak at 245 nm does not change in intensity, as it did when polyaniline is present. Additional changes in the 245 nm band of the DNApolyaniline complex indicate a transition similar to what is observed for a "B" to "C" polymorph transition 38 and is perhaps evidence of a new type of DNA polymorph. This significant conformational change in DNA induced by the polymerization of aniline supports the formation of a charged, emeraldine salt form of polyaniline that is complexed to the DNA template.…”

“…These changes are indicative of the polymorphic transition in DNA from a loosely wound "B" form (containing 10.4 base pairs per turn) to the over-wound form (containing less than 10 base pairs per turn). 27 The formation of polyaniline on the DNA template is concomitant with the shielding of charges on the phosphate groups of the DNA. This shielding of charge on the phosphate groups by the polyaniline reduces the electrostatic repulsion between successive phosphate groups, leading to the formation of the over-wound polymorph.…”

On structural transitions, thermodynamic equilibrium, and the phase diagram of DNA and RNA duplexes under torque and tension

“…Soon after its publication, the P-DNA structure appeared to be untenable in the light of the Watson-Crick model (28) for DNA under physiological conditions. Subsequently, some evidence existed to assume the presence of double-stranded P-DNA, but only under very particular conditions in dry DNA (29) or ethanol solutions (30) (see also ref. 31).…”

Molecular modeling and spectroscopic studies of semustine binding with DNA and its comparison with lomustine–DNA adduct formation

“…Subsequently, alteration in these native properties of B-DNA (winding angle and propeller twist) causes small change in number of base pairs per turn in DNA duplex (Patil & Rhodes, 2000;Portugal & Subirana, 1985). Therefore, altogether, these spectral alterations make a prospect for the transition of native DNA conformation from B-form (10.4 base pair/helical turn) to C-form (9.2 base pair/helical turn) (Bokma, Curtis, & Blok, 1987;Portugal & Subirana, 1985;Zhang, Huang, Tang, Wang, & Dong, 2002). However, this transition appears to be limited up to few base pairs of DNA as for the complete transformation from B-to C-form, ellipticity at 248 nm must be in the exact ratio of two-third (66%) to that of B-DNA band, while here, we observed 70% intensification (Bokma et al, 1987;Brahms, Pilet, Lan, & Hill, 1973;Portugal & Subirana, 1985).…”