ADSORPTION PROPERTIES OF POLAR APOLAR INDUCERS AT A CHARGED INTERFACEAND THEIR RELEVANCE TO LEUKEMIA-CELL DIFFERENTIATION

The interfacial adsorption properties of polar/apolar inducers of cell differentiation (PAIs) were studied on a mercury electrode. This stud y, on a clean and reproducible charged surface, unraveled the purely p hysical interactions among these compounds and the surface, apart from the complexity of the biological membrane. The interfacial behavior o f two classical inducers, hexamethylenebisacetamide (HMBA) and dimethy lsulfoxide, was compared with that of a typical apolar aliphatic compo und, 1-octanol, that has a similar hydrophobic moiety as HMBA but a mu ch smaller dipolar moment. Both HMBA and Octanol adsorb flat in contac t with the surface because of hydrophobic forces, with a very similar free energy of adsorption. However, the ratio of polar to apolar moiet ies in PAIs turned out to be crucial to drive the adsorption maximum t oward physiological values of surface charge density, where octanol is desorbed. The electrostatic effects in the interfacial region reflect ed the adsorption properties: the changes in the potential drop across the interfacial region as a function of the surface charge density, i n the physiological range, were opposite in PAIs as compared with apol ar aliphatic compounds, as exemplified by octanol. This peculiar elect rostatic effect of PAIs has far-reaching relevance for the design of i nducers with an adequate therapeutic index to be used in clinical tria ls.