Fatty carboxyl group, dissociation

The names and structures of some fatty acids are summarized in Table 8-1. Notice that these acids have straight carbon chains and may contain one or more double bonds. Except for the smallest members of the series, which are soluble in water, fatty acids are strongly hydrophobic. However, they are all acids with pKa values in water of 4.8. To the extent that free fatty acids occur in nature, they are likely to be found in interfaces between lipid and water with the carboxyl groups dissociated and protruding into the water. However, most naturally occurring fatty acids... 

The interfacial surface tension may be markedly affected by a surface reaction occurring in the presence of traces of impurities. A good example of such an effect is to be noted in the alteration of the interfacial surface tension between an oil containing a fatty acid and water containing acid or alkali. For acid solutions the interfacial surface tension remains constant and almost independent of the Ph of the aqueous phase. As the solution becomes more alkaline the carboxyl groups of the acid commence to dissociate and the interfacial surface tension falls rapidly. (Hartridge and Peters, Proc. Boy. Soc. A, Cl. 348, 1922, see p. 252.)... 

Fig. 1. The Relation Between the Logarithm of the Acid Dissociation Constant, pKi, in Series of Substituted Fatty Acids, and the Reciprocal of the Number of Carbon Atoms, d, between the Substituent and the Carboxyl Group.

It seems quite natural to ascribe the lesser polarisabxlities of the carboxyl group of the latter to a certain constitutional influence. They are derived from polymer carbohydrates and thus the presence of hydroxyl groups in the neighbourhood of the carboxyl group could be the cause of the decreased polarisability (cf. the well known action of hydroxyl substitution on the dissociation contants of fatty acids). 

Pyruvate carboxytransphosphorylase contains about 1.5 pg of biotin per mg of protein. Under low ionic strength and alkaline conditions the enzyme dissociates spontaneously into the inactive subunits. There are two species of subunits, one contains biotin, and the other contains metals, Co and Zn (Wood et al., 1963 Northrop and Wood, 1969 Gerwin et al., 1969). The transcarboxylase activity with succinyl-CoA is 10-15 times lower than with methylmalonyl-CoA. Specificity to carboxyl donors is fairly broad, with the Km values for propionyl-CoA of 1.0 mM for acetyl-CoA, 0.5 mM for butyryl-CoA, 0.1 mM and for acetoacetyl-CoA, 0.025 mM. The broad specificity to the CoA esters enables the carboxyl group of oxaloacetate to enter the citric acid cycle or to be used in the synthesis of fatty acids. 

Dissociated fatty acids (soaps) have quite different features. Delocalisation of the C=0 double bond across the CO2 group depresses the frequency of the stretching vibration. Stretching of the two C-O bonds is coupled (i.e. the vibration of one influences the vibration of the other). Both asymmetric and symmetric vibrations occur and are infrared active. Bands appear between 1600 and 1560 cm (asymmetric), and 1420 and 1340 cm (symmetric), with short-chain carboxylates absorbing at the higher end of the range. 

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