Ionic Strength Investigation on the Interaction Between miR-155 and a PNA-Based Probe by Atomic Force Spectroscopy

Peptide nucleic acids (PNAs) are synthetic analogues of DNA/RNA characterized by the absence of negative phosphate groups, which confer a low sensitivity to ionic strength for hybridization with respect to the canonical counterpart. PNAs are a suitable probe for miRNAs, as well as endogenous molecules of single-strand non-coding RNA whose dysregulation is often linked to several diseases. The interaction forces between PNA and microRNA-155 (miR-155), a multifunctional microRNA overexpressed in a variety of tumors, were investigated by Atomic Force Spectroscopy (AFS) in fluid under different conditions. We found that the unbinding forces acquired at the ionic strength of 150 mM for a rather wide range of loading rates (ΔF/Δt) can be described using the Bell-Evans model. This allows us to extract information on the kinetics and thermodynamic properties of the miR-155/PNA duplex. Additionally, we probed the unbinding forces and the target recognition times between miR-155 and PNA in the 50-300 mM ionic strength range. Our results indicate that both of these parameters are practically independent from the ionic strength in the analyzed range. The results provide information that is useful for a wider use of PNA in biosensors for diagnostics and therapeutics, even in situ.