Solution structure of the duplexes between phosphorothioate DNA and target DNA/RNA

Area of Science:

• Biochemistry
• Molecular Biology
• Organic Chemistry

Background:

• Phosphorothioate modifications in oligonucleotides are crucial for therapeutic applications, influencing duplex stability.
• Understanding the stereochemical effects of phosphorothioate linkages on DNA/RNA duplex stability is essential for designing effective nucleic acid-based drugs.

Purpose of the Study:

• To investigate the impact of phosphorothioate linkage stereochemistry (Sp vs. Rp isomers) on the melting temperature (Tm) of DNA and RNA duplexes.
• To explore the relationship between duplex stability and the structural characteristics of these modified oligonucleotides.

Main Methods:

• Duplex stability was measured using melting temperature (Tm) analysis.
• Nuclear Magnetic Resonance (NMR) and Circular Dichroism (CD) spectroscopy were employed to analyze duplex structures.
• Temperature-dependent NMR was used to observe changes in proton signals.

Main Results:

• For DNA targets, Sp isomers generally exhibited higher Tm values than Rp isomers.
• For RNA targets, specific sequences (5'-dGCTsTCG-3' and 5'-dGCAsTCG-3') showed higher stability with Rp isomers compared to Sp isomers.
• NMR and CD spectra revealed minimal differences in base proton environments around the phosphorothioate linkage, insufficient to explain Tm variations.
• Temperature-dependent NMR showed characteristic signal broadening for specific protons (AH8 for Rp, TH6 for Sp), suggesting distinct melting behaviors.

Conclusions:

• The stereochemistry of phosphorothioate linkages significantly influences DNA/RNA duplex stability in a sequence- and target-dependent manner.
• NMR spectral broadening provides insights into the differential melting dynamics of Sp and Rp phosphorothioate isomers within duplexes.
• Further investigation is needed to fully elucidate the structural underpinnings of these observed stability differences.