Photoinduced [2+2] and [4+4] Cycloaddition and Cycloreversion Reactions for the Development of Photocontrollable DNA Binders

In the current field of photopharmacology, molecular photoswitches are applied whose interactions with DNA can be triggered or controlled by light. And although several photochromic reactions have been shown to serve this purpose well, the reversible photocycloaddition and photocycloreversion reactions have been largely neglected. This absence of research is surprising because especially the photodimerization of a DNA ligand leads to products with significant change of the size and shape which, in turn, leads to strongly diminished or even suppressed DNA association. Therefore, photocycloaddition-cycloreversion sequences have a huge potential for the photoinduced, reversible deactivation and activation of ligand-DNA interactions, as will be shown with selected examples in this Concept Article. Specifically, heterostyryl and -stilbene derivatives are presented whose DNA-binding properties are efficiently switched in reversible [2+2] photocycloaddition reactions. In addition, the photocontrolled DNA-binding of anthracene derivatives and their heterocyclic benzo[b]quinolizinium analogues in a [4+4] photocycloaddition, as well as the use of this reaction as part of dual-mode switches in combination with redox-active functionalities, are highlighted. Furthermore, examples of conjugates are provided, in which the photochromic unit is bound covalently to nucleic acids or proteins, such that the photocycloaddition reaction can be used for reversible photoinduced crosslinking, ligation, or inhibition of gene expression. In this Concept Article the scope, the limits, and the potential of the photoinduced [2+2] and [4+4] cycloaddition-cycloreversion sequence in the development of photoswitchable DNA binders are presented. image