Molecular design for reversing the photoswitching mode of turning ON and OFF DNA hybridization.

A new photoswitch for DNA hybridization involving para-substituted azobenzenes (such as isopropyl- or tert-butyl-substituted derivatives) with L-threoninol as a linker was synthesized. Irradiation of the modified DNA with visible light led to dissociation of the duplex owing to the destabilization effect of the bulky substituent on the trans-azobenzene. In contrast, trans-to-cis isomerization (UV light irradiation) facilitated duplex formation. The direction of this photoswitching mode was entirely reversed relative to the previous system with an unmodified azobenzene on D-threoninol whose trans form turned on the hybridization, and cis form turned it off. Such reversed and reversible photoswitching of DNA hybridization was directly demonstrated by using fluorophore- and quencher-attached oligonucleotides. Furthermore, it was revealed that the cis-to-trans thermal isomerization was greatly suppressed in the presence of the complementary strand owing to the formation of the more-stable duplex in the cis form.