Photophysical and duplex-DNA-binding properties of distamycin dimers based on 4,4'- and 2,2'-dialkoxyazobenzenes as the core

Abstract

Distamycin-based tetrapeptide (1) was covalently tethered to both ends of the central dihydroxyazobenzene moiety at either the 2,2' or 4,4' positions. This afforded two isomeric, distamycin-azobenzene-distamycin systems, 2 (para) and 3 (ortho), both of them being photoisomerizable. Illumination of these conjugates in solution at approximately 360 nm induced photoisomerization and the time course of the process was followed by UV/Vis and ¹H NMR spectroscopy. The kinetics of the thermal reversion at various temperatures of cis to trans isomers of the conjugates obtained after photoillumination were also examined. This afforded the respective thermal-activation parameters. Both the molecular architecture and the location of the substituent around the core azobenzene determined the rate and activation-energy barrier for the cis-to-trans back-isomerization of these conjugates in solution. Duplex-DNA binding of the conjugates and the changes in DNA-binding efficiency upon photoisomerization was also examined by CD spectroscopy, thermal denaturation studies, and a Hoechst displacement assay. The conjugate 2 showed higher DNA-binding affinity and a greater change in the DNA-binding efficiency upon photoisomerization compared with its 2,2'-disubstituted counterpart. The experimental findings were substantiated by using molecular-docking studies involving each conjugate with a model duplex d[(GC(AT)₁₀CG)]₂ DNA molecule.