Cut-off effects in biological activities of surfactants

In the homologous series of long hydrocarbon chain surface active compounds, their various biological activities increase progressively with increasing chain length up to a critical point, beyond which the compounds cease to be active. The paper reviews several hypotheses of this cut-off effect in biological activities and experimental evidences supporting them. It is suggested that the lateral expansion of the phospholipid bilayer of biological membranes caused by the intercalation of long-chain amphiphile molecules between the phospholipid molecules and the mismatch between their hydrocarbon chain lengths results in the creation of free volume in the bilayer hydrophobic region. The elimination of the free volume via the hydrocarbon chain trans-gauche isomerisation or interdigitation results in the bilayer thickness change or in its destabilisation and formation of non-bilayer phase(s). In combination with the partition and ionisation equilibria of amphiphiles in the lipid/aqueous phase systems, the free volume predicts similar chain length and pH dependencies as observed in biological experiments. It is suggested that the free volume mechanism, in combination with other mechanisms, could be responsible for the cut-off effects in biological activities of amphiphiles.