Change in conformation of various DNAs on melting as followed by circular dichroism

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Abstract
The intensities of the CD bands at about 275 and 190 nm were monitored for DNAs with different G + C contents as a function of temperature. The 190‐nm bands showed a nearly complete and cooperative collapse on melting of the DNA, demonstrating that the CD arises from base–base interactions. The small cooperative change on melting shown by the 275‐nm bands indicates that base–base interactions do not contribute much CD intensity here. No significant difference in melting temperature was found between the two wavelengths, but the lack of premelting in the 190‐nm bands contrasted with the significant premelting in the 275 nm bands. Since the 190‐nm bands are particularly sensitive to base–base interactions, the relative positions of the bases must not change much during premelting. Still, changes in such interactions would be noticeable on top of the low intensity of the 275‐nm bands. Premelting is discussed in the light of recent studies on DNA conformation.