DNA base-pair flipping with fluorescent perylenediimide pincers

The synthesis, structure, and electronic spectra of a series of DNA hairpins possessing two perylenediimide (PDI) base pair surrogates are reported. The PDI chromophores are located in opposite strands of the hairpin base pair domain opposite abasic sites and are either adjacent to each other or separated by a variable number of AT or GC base pairs. Molecular modeling of the conjugate having adjacent PDI chromophores shows that they adopt a slipped, pi-stacked geometry with an angle of 40. between the PDI long axes. The electronic absorption, fluorescence, and circular dichroism of this conjugate are consistent with a stacked PDI structure. Conjugates having one or two GC base pairs between the PDI chromophores display spectra that are consistent with isolated PDIs. Conjugates having 1-4 AT base pairs have more complex spectra, suggestive of an equilibrium between base paired and flipped structures having stacked PDIs. Heating of the conjugates possessing isolated PDI chromophores results in base pair flipping. The free energy for PDI stacking is greater than that for a single AT base pair and comparable to that for a single GC base pair or two AT base pairs.