ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Time-dependent uv absorption and CD spectrum changes in salt-induced conformational B → Z and Z → B transitions of poly(dG—dC) · poly(dG—dC) were measured. This polynucleotide does not convert directly from a right-handed double-helical B form to a left-handed double-helical Z form, but goes through an intermediate, B* form, with the B → B* transition proceeding nearly instantaneously, and then transforms gradually to the Z form. Uv absorption spectra of these B and B* forms are nearly identical, but their CD spectra are quite different. The CD spectrum of the B* form is identical with that obtained for DNA in high salt solutions and is similar to a spectrum which for some time was thought to be a C form. These B and B* forms have the same number of base pairs per turn [Sprecher, C.A., Baase, W.A. & Johnson, Jr., W.C. (1979) Biopolymers 18, 1009-1019]. Kinetic measurements showed that uv absorption and CD intensities at fixed wavelengths do not change in a simple exponential curve. However, both the uv absorption spectrum change in the B → Z transition and the CD spectrum change in the B* → Z transition, respectively, have isosbestic points. In the B → Z transition, no hyperchromicity can be observed. These results suggest that this B* form unfolding or premelting process is a rate-determining step in the B* → Z transition and makes it easy for the unfolded or premelted polynucleotide to almost immediately fold into the Z form. The double helix does not dissociate into single strands and transforms from the B* form to the Z form point-by-point along the chain in a soliton-like manner of with a small amount of open states in which the bases are unpaired. Also, in the Z → B transition, the polynucleotide does not convert directly from the Z to the B form, but goes through a B*-like form. In this transition, the uv-absorption spectra also have an isosbestic point. The reaction velocity in the Z → B transition is much faster than that in the B → Z transition. Possibly, the positive CD band between 265 and 310 nm in the B form comes from a n-π* transition due to an interaction of the bases with sugarphosphate groups.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.360231108
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