[Interaction of ethidium bromide with synthetic double-stranded polyribonucleotides]

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysical Chemistry

Background:

• Ethidium bromide (EtBr) is a fluorescent intercalating agent used to stain nucleic acids.
• Understanding the binding interactions of EtBr with different nucleic acid structures is crucial for molecular biology applications.

Purpose of the Study:

• To investigate the binding characteristics of ethidium bromide (EtBr) with synthetic polyribonucleotides: poly(G).poly(C), poly(A).poly(U), and poly(I).poly(C).
• To determine the selectivity and thermodynamic parameters of EtBr binding to these polynucleotides.

Main Methods:

• Isothermal microcalorimetry
• Circular dichroism spectroscopy
• Absorption spectroscopy
• Fluorescence spectroscopy

Main Results:

• EtBr exhibited selective binding, with a higher binding constant for poly(A).poly(U) (2.5 x 10^6 M^-1) compared to poly(G).poly(C) (4 x 10^3 M^-1) and poly(I).poly(C) (1.5 x 10^4 M^-1).
• The binding stoichiometry varied, with approximately one EtBr molecule per 2 base pairs for poly(A).poly(U).
• The average binding enthalpy for EtBr with poly(A).poly(U) and poly(G).poly(C) was -7.5 Kcal/mol.
• The selectivity of EtBr binding was found to be primarily of entropic origin.

Conclusions:

• Ethidium bromide displays significant selectivity in its binding to synthetic polyribonucleotides.
• The entropic contribution plays a key role in the observed selectivity of EtBr binding.
• Thermodynamic data provides insights into the molecular interactions between EtBr and different polyribonucleotide structures.