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The Ionization Behavior of Fatty Acids and Bile Acids in Micelles and Membranes
Author(s) -
Small Donald M.,
Cabral Donna J.,
Cistola David P.,
Parks John S.,
Hamilton James A.
Publication year - 1984
Publication title -
hepatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.488
H-Index - 361
eISSN - 1527-3350
pISSN - 0270-9139
DOI - 10.1002/hep.1840040814
Subject(s) - chemistry , micelle , cholic acid , vesicle , phospholipid , biological membrane , membrane , bile acid , organic chemistry , biochemistry , aqueous solution
The ionization behavior of carboxylic acids including aliphatic chain fatty acids and bile acids in solutions, micelles, membranes and proteins is of considerable biological interest. The 13 C nuclear magnetic resonance chemical shift of carboxyl carbon of a variety of acids has been shown to be a linear function of the state of ionization of the carboxyl group. Thus, by measuring the chemical shift as a function of the amount of acid or base added to a solution, the state of ionization and the apparent pKa of the carboxyl group may be determined. The method is illustrated in this paper using butyric acid, and results (given as apparent pKa values) are tabulated for a variety of fatty acids and cholic acid in different environments such as monomeric solutions, micelles, membranes and bound to albumin. The apparent pKa varies greatly depending upon the environment. For instance, the apparent pKa of oleic acid is about 4.2 when bound to albumin whereas it is about 7.5 when incorporated in the bilayer of a phospholipid vesicle. Cholic acid shows similar changes in the apparent pKa, being approximately 5.2 in sodium taurocholate micelles and at least 6.8 in phospholipid vesicles. The use of 13 C nuclear magnetic resonance (NMR) and, in particular, I3 C NMR of 13 C‐enriched carboxyl groups provides a method for direct observation of the state of ionization of the carboxyl group in biologically interesting acids. Such techniques may be adapted for use in biological systems, such as cells or tissues.