The linear dichroism spectra of complexes of tetrakis(N-methyl-4-pyridinio)prophine (H2TMpyP) and its
zinc(II) derivative (
ZnTMpyP) with
DNA oriented in a flow gradient have been investigated. The dichroism of H2TMpyP determined within the Soret band and the Qy band system is consistent with an intercalative conformation in which the plane of the
porphyrin ring system is nearly parallel to the planes of the
DNA bases. In the case of
ZnTMpyP on the other hand, the
porphyrin ring system is inclined at angles of 62-67 degrees with respect to the axis of the
DNA helix. The pyridyl groups in both cases are characterized by a low degree of orientation with respect to the axis of the helix. In contrast to H2TMpyP which does not significantly affect the degree of alignment of the
DNA in the flow gradient, the binding of
ZnTMpyP causes a significant decrease (about 50% for a base pair/
ZnTMpyP ratio of 20) in the intrinsic dichroism at 260 nm due to the oriented
DNA bases; the binding of
ZnTMpyP to
DNA either gives rise to regions of higher flexibility or causes
bends or kinks at the binding sites. Increasing the ionic strength has little influence on the linear dichroism of the
ZnTMpyP-
DNA complexes, but the number of molecules bound at intercalation sites diminishes in the case of the H2TMpyP-DNA complexes; the accompanying changes in the linear dichroism characteristics suggest that external H2TMpyP complexes are formed at the expense of intercalation complexes.(ABSTRACT TRUNCATED AT 250 WORDS)