Modified Bis-pyrimidine Clamps for Triplex Formation and Their Use in SARS-CoV-2 Detection

The formation of nucleic acid triple helices ("triplexes") is an area of great interest due to their potential role in the natural and artificial regulation of gene expression or for use in analytical, diagnostic, or synthetic methods. During the coronavirus pandemic, a large search for novel methods for the detection of SARS-CoV-2 was undertaken. Based on triplex affinity capture and using polypurine reverse-Hoogsteen hairpins, a method known as Triplex Enhanced Nucleic Acid Detection Assay (TENADA) was developed for the rapid detection of SARS-CoV-2 without the need for polymerase chain reaction (PCR) amplification. In this work, to expand the targeting scope of this method, we explored triplex-forming bis-pyrimidine clamps targeting a polypurine sequence in the ORF1a region of SARS-CoV-2. To enhance parallel triplex stability, 2'-sugar and 5-methylpyrimidine modifications were incorporated into both strands of the clamps, and their effect on the triplexes formed was assessed via NMR and other biophysical methods. The results revealed distinct stabilizing effects of the modifications, influenced by their size, sugar puckering, and capacity to form short contacts with neighboring residues. The dual ability of clamps to simultaneously form Watson-Crick and Hoogsteen hydrogen bonds offers a novel perspective on the effect of modifications on triplex stability, previously unexplored with triplex-forming oligonucleotides (TFOs). Finally, the bis-pyrimidine clamps that formed the most stable parallel triplexes were applied in a thermal lateral flow (TLF) sensing device, demonstrating their potential as biosensing probes. These clamps effectively detected the synthetic DNA target with limits of detection (LoDs) ranging from 0.05 to 0.001 nM. Understanding the best modification strategies and their impact on the triplex structure will advance the development of clamps as biosensing and therapeutic agents.