Single-stranded DNA oligoaptamers: molecular recognition and LPS antagonism are length- and secondary structure-dependent.

In Gram-negative bacterial infection, lipopolysaccharide (LPS) readily overwhelms the host innate immune system, which could result in inflammation and sepsis in severe cases. Therefore, developing anti-LPS molecules would confer an efficient antibacterial strategy. We used SELEX (Systemic Evolution of Ligands by EXponential enrichment) to isolate single-stranded DNA (ssDNA) aptamers. By immobilizing and exposing different orientations of the LPS molecule on hydrophobic and hydrophilic surfaces, two populations of aptamers were captured from a library of 10(14-15) ssDNA oligonucleotides. Progressive SELEX enriched the aptamers towards thymidine residues. The more hydrophobic aptamers with T-rich loops showed strong molecular recognition for the lipid A moiety of LPS, binding at affinity of up to K(D) of 10(-9)M, and eliciting 95% neutralization of endotoxicity. The longer ssDNAs exhibited greater avidity for LPS and conferred more efficacious antagonism against LPS. The nucleotide composition imposes subtle influence on the aptamer folding and affinity for LPS.