Researches on plant growth regulators XV. The influence of fatty acids on soapcoacervates

Abstract

1. 1. The complex theory of permeability (Bungenberg De Jong) affords a possibility to unite the lipoid theory and the filter theory. If the protoplasmic membrane is conceived as a tricomplex system, composed of phosphatides, proteins and cations, the differences between these well-known theories vanish, when considered in molecular dimensions. 2. 2. Oleate coacervates may serve very well as a model for the non-polar part of the protoplasmic membrane. The influence of many on this model organic compounds has been determined previously; these investigations were now extended by experiments on the influence of fatty acid anions. 3. 3. The influence of fatty acid anions proved to be strongly dependent on the length of the carbon chain, very remarkable differences being found (Fig. 7). 4. 4. In general the fatty acid anions demonstrate a turgescent effect (the water content, and so the volume, of the coacervate augments) at low concentrations. This influence becomes perceptible with six carbon atoms in the molecule and, in the homologous series, strongly increases up to the anion with eleven carbon atoms. After a minimal activity at fifteen C-atoms it rises again. 5. 5. The first increase of activity can be explained by the distribution of the added fatty acid anions between the equilibrium liquid and the soap micells; with a growing number of C-atoms increasing amounts are adsorbed by these micells. On that occasion the added fatty acid anion exerts a disturbing action upon the orderly soap micells. 6. 6. This disturbing action is minimal if the added fatty acid anion fits into the order of the oleate micells (in the case of 15 carbon atoms). 7. 7. With another substrate (e.g. stearate coacervates) the minimum appears at a different point (with 18 C-atoms). 8. 8. If there is but little similarity between the substrate and the added fatty acid anion (as in the case of sodium desoxycholate coacervate), a pronounced minimum is not observed any longer. 9. 9. This obvious difference in activity of compounds from a homologous series, dependent on structural relations between “substrate” and “agent”, may be of primary importance to the general problem of the influence of chemical structure on physiological action.