PREFERENTIAL PARTITIONING OF MELITTIN INTO THE AIR WATER INTERFACE - STRUCTURAL AND THERMODYNAMIC IMPLICATIONS/

The membrane active agent melittin has been investigated with regard t o the formation of a Langmuir monolayer and the accordingly induced su rface activities, We show that in spite of its considerable solubility in an aqueous medium, this peptide nevertheless largely accumulates i n the air/water interface unless the lateral pressure is raised beyond a certain threshold value depending on the pH in the subphase. The tr ue surface concentrations have been determined by means of a recently developed novel method based on thermodynamic principles. It affords a n access to the partitioning equilibrium between the surface and subph ase domains, provided the latter surrounding is not excessively prefer red, In the present case this approach was used to derive quantitative information on the pertinent interfacial structure and thermodynamics , In particular, the apparent molecular area and the Gibbs energy of m utual interaction in the monolayer could be evaluated as a function of the applied surface pressure, The data suggest the existence of two s tructural conversions in the course of an increasing lateral compressi on. The surface-associated peptide accordingly assumes three different states of successively reduced area requirements, supposedly owing to an orientational transition involving a straightening up of a helical conformation. This conclusion is corroborated by surface potential me asurements reflecting corresponding changes of the effective dipole mo ment perpendicular to the surface.