Evidence for heterodimers of 2,4,5-trichlorophenol on planar lipid layers. A FTIR-ATR investigation.
Trichlorophenols are weak acids of high hydrophobicity and are able to transport protons across the mitochondrial membrane. Thus the proton motive force is dissipated and the ATP production decreased. In situ Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR) experiments with 2,4,5-trichlorophenol (TCP) adsorbed to model membranes resulted in good evidence for the formation of the TCP-heterodimer. Two surfaces were examined: a dipalmitoyl phosphatidic acid (DPPA) monolayer and a planar DPPA/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. TCP was adsorbed from 1 to 3 mM solutions at pH 6.0 to the lipid layers leading to surface layers at the water/lipid interface. Difference spectra showed an effect on DPPA acyl chains even when it was covered with POPC. Time-resolved measurements revealed two distinct adsorption processes, which were assigned to TCP and its deprotonated anion (phenoxide), respectively. For DPPA/POPC bilayers, the adsorption of TCP was faster than that of its phenoxide, whereas adsorption of both species to DPPA monolayers proceeded with similar velocity. In both cases, phenoxide formation at the membrane was found to be delayed with respect to phenol adsorption. Phenoxide and phenol were retained after replacing the TCP solution with buffer. For the retained species, we estimated a phenol/phenoxide molar ratio of 1 at pH 6.0 (pKa=6.94 for TCP), demonstrating strong evidence for heterodimer formation.
Siam
Monira
M
Institute of Physical Chemistry, University of Vienna, Althanstrasse 14, Vienna A-1090, Austria.
Reiter
Gerald
G
Hunziker
René
R
Escher
Beate
B
Karpfen
Alfred
A
Simperler
Alexandra
A
Baurecht
Dieter
D
Fringeli
Urs Peter
UP
D000327:Adsorption; D002733:Chlorophenols / Q000737:chemistry*; D019281:Dimerization; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D010712:Phosphatidic Acids / Q000737:chemistry; D011188:Potassium / Q000737:chemistry; D013053:Spectrophotometry; D013055:Spectrophotometry, Infrared; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*; D013696:Temperature; D013997:Time Factors; D014466:Ultraviolet Rays
D002733:Chlorophenols; D008051:Lipid Bilayers; D010712:Phosphatidic Acids; C031599:dipalmitoylphosphatidic acid; C009534:2,4,5-trichlorophenol; D011188:Potassium