Photoaddition to DNA by nonintercalated chlorpromazine molecules

Area of Science:

• Photochemistry
• Molecular Biology
• Pharmacology

Background:

• Chlorpromazine (CPZ) is known to form photoadducts with DNA, leading to DNA strand breaks.
• These reactions are implicated in CPZ's photomutagenicity and phototoxicity.
• The specific CPZ-DNA complex acting as the photoreceptor for photoaddition remains unclear.

Purpose of the Study:

• To determine if intercalated chlorpromazine (CPZ) molecules within double-stranded DNA (ds-DNA) are the primary absorbers of light in the photoaddition reaction.
• To elucidate the role of different CPZ-DNA complexes in the absorption spectrum relevant to photoaddition.

Main Methods:

• Investigated CPZ-DNA interactions using fluorescence quenching and nonlinear Stern-Volmer analysis.
• Analyzed CPZ-DNA complexes with linear dichroism spectroscopy.
• Determined the absorption spectrum of CPZ-DNA complexes via dialysis experiments.
• Measured the wavelength dependence of covalent binding of 3H-CPZ to ds-DNA upon irradiation at various wavelengths.

Main Results:

• Fluorescence quenching indicated the formation of multiple CPZ-DNA complexes.
• Linear dichroism suggested red-shifted absorption maxima for intercalated complexes compared to free CPZ.
• Dialysis experiments revealed that non-intercalated complexes dominate the absorption spectrum below 350 nm.
• The action spectrum for CPZ-DNA covalent binding closely matched the absorption spectrum of non-intercalated CPZ.

Conclusions:

• Intercalated CPZ within ds-DNA is not the primary chromophore for the photoaddition reaction.
• Non-intercalated CPZ molecules are the main absorbers responsible for CPZ-DNA photoadduct formation.
• This finding clarifies the mechanism underlying CPZ-induced phototoxicity and photomutagenicity.