DNA-Binding Studies of a Tetraalkyl-Substituted Porphyrin and the Mutually Adaptive Distortion Principle

This
investigation explores DNA-binding interactions of various
forms of an alkyl-substituted cationic porphyrin, H2TC3
(5,10,15,20-tetra­[3-(3′-methylimidazolium-1′-yl)]­porphyrin).
The motivating idea is that incorporating alkyl rather than aryl substituents
in the meso positions will enhance the prospects for intercalative
as well as external binding to DNA hosts. The ligands may also be
applicable for photodynamic and/or anticancer therapy. Methods employed
include absorbance, circular dichroism, and emission spectroscopies,
as well as viscometry and X-ray crystallography. By comparison with
the classical H2T4 system, H2TC3 exhibits a
higher molar extinction coefficient but is more prone to self-association.
Findings of note include that the copper­(II)-containing form Cu­(TC3)
is adept at internalizing into single-stranded as well as B-form DNA,
regardless of the base composition. Surprisingly, however, external
binding of H2TC3 occurs within domains that are rich in
adenine–thymine base pairs. The difference in the deformability
of H2TC3 versus Cu­(TC3) probably accounts for the reactivity
difference. Finally, Zn­(TC3) binds externally, as the metal center
remains five-coordinate.