AHA- heterodimer of a class-2 uncoupler: pentachlorophenol

Area of Science:

• Biochemistry
• Physical Chemistry
• Membrane Biophysics

Background:

• AHA- heterodimers, formed by weak acid (HA) and its conjugate anion (A-), are hypothesized as charged, membrane-permeable species for class-2 uncouplers.
• Previous attempts to isolate and identify AHA- heterodimers have been unsuccessful.

Purpose of the Study:

• To investigate the formation and characteristics of AHA- heterodimers using pentachlorophenol (PCP) as a model compound.
• To determine the formation constant (Kf) and molar absorptivity of AHA- heterodimers in various environments, including lipid membranes.

Main Methods:

• Optical spectroscopy (UV-Vis) was used to measure spectra of PCP solutions (HA+A- in 1:1 ratio) in different solvents and with liposomes.
• Nuclear magnetic resonance (NMR) spectroscopy supplemented optical studies for solutions in dichloroethane.
• Membrane electrical conductivity and adsorbed PCP surface density data were used to estimate Kf in lipid bilayers.

Main Results:

• Evidence for AHA- heterodimer formation was found in non-hydrogen-bonding solvents like dichloroethane and acetonitrile.
• The formation constant (Kf) decreased with increasing dielectric constant and water concentration.
• AHA- heterodimers were not optically detected in hydrogen-bonding solvents (octanol, methanol) or in liposomes.
• Estimated Kf for PCP in lipid bilayers (0.005 < Kf < 0.5 M-1) aligns with the lack of optical detection.

Conclusions:

• AHA- heterodimer formation is favored in non-polar, non-hydrogen-bonding environments.
• Water penetration into lipid bilayers likely inhibits AHA- heterodimer formation within the membrane.
• The study provides crucial insights into the behavior of uncoupler species at membrane interfaces.