Abstract
Purpose. The apparent pKa of the fatty acids within hydrated (30 % w/w) model human stratum corneum (SC) lipid mixtures should be measured.
Methods. The degree of ionisation of the fatty acids was calculated as a function of pH using Fourier transform infra-red spectroscopy. The relative intensity of the stretching bands of the unionized and ionized carboxylic groups was determined and fitted to the relevant expression for ionic equilibrium of a monoprotic acid. The pKa was then calculated for increasing proportion of unsaturated fatty acid in the lipid mixture.
Results. Values for pKa in the range 6.2-7.3 were found, increasing with greater proportion of oleic acid. These are some 1.5-3 pH units higher than the pKas of fatty acids in molecular solution.
Conclusions. As there exists a pH-gradient across the SC, the degree of ionisation will also vary. In the innermost SC layers, a pH of 7 will produce 90% ionization of the fatty acids and head-group repulsion will be great. At the SC surface, the pH of 5 will cause almost minimal head-group repulsion, tending to increase crystallinity and promote a bilayer structure.
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Lieckfeldt, R., Villalaín, J., Gómez-Fernández, JC. et al. Apparent pKa of the Fatty Acids Within Ordered Mixtures of Model Human Stratum Corneum Lipids. Pharm Res 12, 1614–1617 (1995). https://doi.org/10.1023/A:1016280714593
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DOI: https://doi.org/10.1023/A:1016280714593