Counterion association with native and denatured nucleic acids: an experimental approach

Area of Science:

• Biophysical Chemistry
• Nucleic Acid Chemistry
• Solution Thermodynamics

Background:

• The poly(dA) . poly(dT) double helix exhibits a sharp melting transition highly sensitive to salt concentration.
• This sensitivity allows for precise characterization of solution properties even at low duplex concentrations.

Purpose of the Study:

• To utilize poly(dA) . poly(dT) melting as a sensitive probe for detecting and characterizing the influence of other solutes on solution properties.
• To investigate solvent-mediated crosstalk between non-contacting biopolymers in solution by measuring shifts in poly(dA) . poly(dT) melting temperature.

Main Methods:

• Calorimetric analysis of poly(dA) . poly(dT) melting temperature (Tm) in the presence of various nucleic acid solutes (DNA, RNA duplexes, and triplexes).
• Quantification of apparent changes in bulk solution cation concentration based on Tm shifts.
• Comparison of experimental results with counterion condensation theory.

Main Results:

• Addition of other nucleic acid polymers (DNA, RNA duplexes, triplexes) caused a proportional shift in poly(dA) . poly(dT) Tm, dependent on the added polymer's concentration and conformational state.
• The magnitude of the Tm shift was primarily dependent on the number of strands in the added helix, not significantly on whether it was RNA or DNA.
• Observed Tm shifts were attributed to increased ionic strength from added nucleic acids and their counterions, indicating solvent-mediated crosstalk.

Conclusions:

• Poly(dA) . poly(dT) serves as an effective probe for quantifying ionic strength changes and characterizing solute interactions in solution.
• Current counterion condensation theories predict the direction of Tm shifts but overestimate their magnitude.
• The findings have implications for understanding nucleic acid behavior in complex solutions and potential regulatory mechanisms.