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Permeation of aromatic carboxylic acids across lipid bilayers: the pH-partition hypothesis revisited.

Biophysical journal (2005-06-14)
Anita V Thomae, Heidi Wunderli-Allenspach, Stefanie D Krämer
ABSTRACT

According to the pH-partition hypothesis the charged species of organic compounds do not contribute to lipid bilayer permeation as they generally show negligible partitioning into n-octanol. With this assumption, membrane permeation is related to the molar fraction of the neutral species at a particular pH. A recently developed permeation assay permits us to directly determine pH-dependent permeation of aromatic carboxylic acids. Tb(3+)-loaded liposomes are incubated with aromatic carboxylic acids and upon excitation at the absorption wavelength of the acid, permeation kinetics can be measured as an increase in Tb(3+) luminescence. The anions of the tested acids permeated egg phosphatidylcholine membranes only 12 (2-hydroxynicotinic acid), 66 (salicylic acid), and 155 (dipicolinic acid) times slower than the net neutral species. The anions, therefore, controlled the total permeation already at 1-2 pH units above their pK(a). These results indicate that in contrast to the expectations of the pH-partition hypothesis, lipid bilayer permeation of an acidic compound can be completely controlled by the anion at physiological pH.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
2-Hydroxypyridine-3-carboxylic acid, 98%
Sigma-Aldrich
MOPS, BioUltra, for molecular biology, ≥99.5% (T)