Selectively Identifying Exposed-over-Unexposed C-C⁺ Pairs in Human Telomeric i-Motif Structures with Length-Dependent...

The i-motif structure (iM) has attracted much attention, because of its in vivo bioactivity and wide in vitro applications such as DNA-based switches. Herein, the length-dependent folding of cytosine-rich repeats of the human telomeric 5'-(CCCTAA)_n-1CCC-3' (iM-n, where n = 2-8) was fully explored. We found that iM-4, iM-5, and iM-8 mainly form the intramolecular monomer iM structures, while a tetramolecular structure populates only for iM-3. However, iM-6 and iM-7 have the potential to fold as well into the dimeric iM structures besides the monomer ones. The natural hypericin (Hyp) was used as the polymorphism-selective probe to recognize the iM structures. Interestingly, only iM-3, iM-6, and iM-7 can efficiently switch on the Hyp fluorescence by specifically binding with the outmost C-C⁺ base pairs that are exposed directly to solution. However, other iM structures that fold in a way with a coverage of the outmost C-C⁺ pairs by loop sequences are totally unavailable for the Hyp binding. Theoretical modeling indicates that adaptive π-π and cation-π interactions contribute to the Hyp recognition toward the exposed C-C⁺ pairs. This specific iM recognition can be boosted by a photocatalytic DNAzyme construct. Our work provides a reliable fluorescence method to selectively explore the polymorphism of iM structures.