The structure and unzipping behavior of dumbbell and hairpin DNA revealed by real-time nanopore sensing

Hairpin structures play an essential role in DNA replication, transcription, and recombination. Single-molecule studies enable the real-time measurement and observation of the energetics and dynamics of hairpin structures, including folding and DNA-protein interactions. Nanopore sensing is emerging as a powerful tool for DNA sensing and sequencing, and previous research into hairpins using an α-hemolysin (α-HL) nanopore suggested that hairpin DNA enters from its stem side. In this work, the translocation and interaction of hairpin and dumbbell DNA samples with varying stems, loops, and toeholds were investigated systematically using a Mycobacterium smegmatis porin A (MspA) nanopore. It was found that these DNA constructs could translocate through the pore under a bias voltage above +80 mV, and blockage events with two conductance states could be observed. The events of the lower blockage were correlated with the loop size of the hairpin or dumbbell DNA (7 nt to 25 nt), which could be attributed to non-specific collisions with the pore, whereas the dwell time of events with the higher blockage were correlated with the stem length, thus indicating effective translocation. Furthermore, dumbbell DNA with and without a stem opening generated different dwell times when driven through the MspA nanopore. Finally, a new strategy based on the dwell time difference was developed to detect single nucleotide polymorphisms (SNPs). These results demonstrated that the unzipping behaviors and DNA-protein interactions of hairpin and dumbbell DNA could be revealed using nanopore technology, and this could be further developed to create sensors for the secondary structures of nucleic acids. A model to understand the unzipping behavior of hairpin DNA and dumbbell DNA in MspA nanopores is proposed, and this strategy was used to successfully detect SNPs without labeling.