Conformation and reactivity of DNA in the complex with proteins. III. Helix-coil transition and conformational studies of model complexes of DNA's with poly-L-histidine.

Differences in the interaction of poly-L-histidine with DNA of various base composition have been demonstrated using melting and CD measurements. The two types of complexes formed with DNA at pH values below the pK of 5.9 and in the region of pH 6.5 are very different in their CD spectral properties. The binding effects with highly protonated poly-L-histidine are AT-dependent as reflected by large negative CD spectra indicating the formation of psi-DNA as a condensed state of the double helix. GC-rich DNA may, however, also form psi-DNA structures with poly-L-histidine under certain conditions. At pH 6.5 complex formation with the weakly protonated polypeptide is GC-dependent. From the results it is concluded that protonated poly-L-histidine interacts more specifically at AT base pairs, prabably along the small groove while the weakly protonated poly-L-histidine tends to interact preferentially with GC regions which seems to occur rather in the large groove.