Protonated polynucleotide structures : 14. Poly(dG). poly(dC) and its metastable form

Area of Science:

• Biophysics
• Molecular Biology
• Genetics

Background:

• Poly(dG).poly(dC) is a synthetic DNA polymer with a known propensity for forming unusual secondary structures.
• Understanding DNA structural transitions is crucial for comprehending genetic processes and developing novel therapeutics.

Purpose of the Study:

• To investigate the pH-dependent structural changes of poly(dG).poly(dC) using circular dichroism spectroscopy.
• To characterize the nature of pH-induced transitions and identify any intermediate or metastable forms.

Main Methods:

• Circular dichroism (CD) spectroscopy was employed to monitor structural changes.
• Acid and alkaline titrations were performed in a 0.15 M NaCl solution to cover a wide pH range.

Main Results:

• Two distinct transitions were observed during acid titration, occurring around pH 5 and below pH 3, indicative of disproportionation reactions.
• Alkaline back titration showed a single transition above pH 7, with no intermediate form reappearing.
• A metastable, likely protonated, form was identified at pH 7, exhibiting hysteresis and reverting to the neutral form upon heating or alkali treatment.

Conclusions:

• The pH-induced structural transitions in poly(dG).poly(dC) are complex disproportionation reactions.
• A hysteretic loop exists, leading to a metastable form that requires a complete titration cycle to be re-established.
• These findings contribute to the understanding of DNA structural dynamics and pH sensitivity.