DNA interaction with DAPI fluorescent dye: Force spectroscopy decouples two different binding modes.

Area of Science:

• Biophysics
• Molecular Biology
• Physical Chemistry

Background:

• The interaction between small molecules and DNA is crucial for understanding DNA function and developing therapeutics.
• DAPI (4',6-diamidino-2-phenylindole) is a fluorescent dye commonly used to stain DNA.
• Understanding the binding mechanisms of DAPI to DNA provides insights into DNA-protein interactions and drug design.

Purpose of the Study:

• To investigate the binding modes of DAPI to lambda DNA (λ-DNA) using force spectroscopy.
• To determine the physicochemical parameters governing DAPI-DNA interactions at varying ionic strengths.
• To decouple and characterize distinct binding mechanisms of DAPI to DNA.

Main Methods:

• Force spectroscopy was employed to measure the mechanical properties of DNA-DAPI complexes.
• Persistence length and contour length of DNA were analyzed as a function of DAPI concentration.
• Advanced models were utilized to correlate mechanical property changes with binding thermodynamics.

Main Results:

• Two primary binding modes of DAPI to λ-DNA were identified: minor groove binding and intercalation.
• Minor groove binding saturates at low DAPI concentrations (∼0.50 μM) with high binding constants (∼10^7 M⁻¹).
• Intercalation becomes significant at higher DAPI concentrations, exhibiting lower binding constants (∼10^5 M⁻¹).

Conclusions:

• DAPI exhibits a sequential binding mechanism to DNA, starting with minor groove binding followed by intercalation.
• The binding affinity and mechanism are dependent on DAPI concentration and influenced by ionic strength.
• This study provides a quantitative understanding of DAPI-DNA interactions, relevant for molecular biology and drug development.