Optical Rotatory Dispersion and Circular Dichroism of DNA from Various Sources at Alkaline pH

The effect of deprotonation and alkaline denaturation on the conformation of DNA from various sources has been investigated by optical rotatory dispersion and circular dichroism measurements. Upon transition to alkaline pH, changes in the optical rotatory dispersion at the characteristic peaks around 228 and 290 nm are observed. The variation of the optical rotations in case of A. T‐rich DNA is more pronounced than in case of G . C‐rich DNA. For G . C‐rich DNA, changes of rotations are mainly observed at pH values beyond 11.5; the transition ranging between pH 11.6 and 11.7. The transition is accompanied by a shift of the “cross‐over” point towards longer wavelengths presumably due to deprotonation of guanine residues. Native DNAs show characteristic differences in their circular dichroism spectra dependent on the G · C content. Increasing the temperature (below the melting region) reveals enhancement of the circular dichroism band of native DNA around 270 nm whereas the opposite effect is found in the denatured state. As a possible explanation an increase of the chirality in the native helical DNA structure or a structure of intermediary character is considered. Titration up to pH 12 or higher produces a drastic increase of the rotational strength around 228 nm as well as changes in the other two circular dichroism‐maxima which may be correlated to a diminuition of the chiral and helical structure.