Specific versus nonspecific binding of cationic PNAs to duplex DNA

Although peptide nucleic acids (PNAs) are neutral by themselves, they are usually appended with positively charged lysine residues to increase their solubility and binding affinity for nucleic acid targets. Thus obtained cationic PNAs very effectively interact with the designated duplex DNA targets in a sequence-specific manner forming strand-invasion complexes. We report on the study of the nonspecific effects in the kinetics of formation of sequence-specific PNA-DNA complexes. We find that in a typical range of salt concentrations used when working with strand-invading PNAs (10-20 mM NaCl) the PNA binding rates essentially do not depend on the presence of nontarget DNA in the reaction mixture. However, at lower salt concentrations (<10 mM NaCl), the rates of PNA binding to DNA targets are significantly slowed down by the excess of unrelated DNA. This effect of nontarget DNA arises from depleting the concentration of free PNA capable of interacting with DNA target due to adhesion of positively charged PNA molecules on the negatively charged DNA duplex. As expected, the nonspecific electrostatic effects are more pronounced for more charged PNAs. We propose a simple model quantitatively describing all major features of the observed phenomenon. This understanding is important for design of and manipulation with the DNA-binding polycationic ligands in general and PNA-based drugs in particular.