Apparent pKa of the fatty acids within ordered mixtures of model human stratum corneum lipids.
OBJECTIVE
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.
Lieckfeldt
R
R
Department of Pharmaceutical Technology, Erlangen University, Germany.
Villalaín
J
J
Gómez-Fernández
J C
JC
Lee
G
G
D002518:Ceramides / Q000737:chemistry; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002784:Cholesterol / Q000737:chemistry; D004817:Epidermis / Q000737:chemistry*; D005227:Fatty Acids / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008055:Lipids / Q000737:chemistry*; D008956:Models, Chemical; D010169:Palmitic Acids / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared
D002518:Ceramides; D005227:Fatty Acids; D008055:Lipids; D010169:Palmitic Acids; D002784:Cholesterol