Formation of micelles and liposomes from carnitine amphiphiles.

Esterification of the carboxy and/or the hydroxy groups of (R)-carnitine (3-hydroxy-4-trimethylammonium butanoate) produces interesting classes of (cationic or zwitterionic) surfactants whose CMC values are in general predictable from their molecular structure. In fact similar relationships between CMC and the number of carbon atoms, Cn, have been found for three classes of such surfactants. However the sensitivity of CMC to Cn for the diesters is considerably lower than that calculated from literature values for the monoesters (either in their cationic or zwitterionic forms). The CMC values for the diesters have been determined by tensiometric, conductimetric and spectrophotometric methods, both in pure water and in 0.154 M NaCl solutions, at 25 degrees C. In particular the tensiometric results provide evidence that double-chain diesters undergo self assembly into structures more complex than simple micelles if the two chains are of comparable length. EPR and electron microscopy experiments show that the aggregates spontaneously formed by these surfactants are a mixture of multilamellar vescicles.

Effect of the addition of electrolytes on the partition coefficients, activity coefficients, and acid dissociation constants of carnitine and its acetyl and propionyl derivatives.

The partition coefficients, P, between n-octanol and water of carnitine, acetylcarnitine, propionylcarnitine, and tetraethylammonium pentacyanopropenides have been determined spectrophotometrically at 20 degrees C. The value of P increases upon addition of electrolytes, the increase produced by LiCl being particularly high. The carnitine cations are 'salted-out' by electrolytes as shown by the variation of their 'single-ion' activity coefficients (relative to that of the tetraethylammonium ion) with electrolyte concentration. These cations are appreciably more hydrated than the reference tetraethylammonium ion. The considered carnitine cations are at least one pKa (where Ka is the acid dissociation constant) unit more acidic than butanoic acid in water at 25 degrees C. Electrolyte addition somewhat increases the pKa of these cationic acids.