EVIDENCE FOR MULTILAYER FORMATION OF MELITTIN ON SOLID-SUPPORTED PHOSPHOLIPID-MEMBRANES BY SHEAR-WAVE RESONATOR MEASUREMENTS

The adsorption of the peptide melittin from Apis mellifera on solid-su pported membranes (SSM) consisting of octanethiol and 1-palmitoyl-2-ol eoyl-sn-glycero-3-phosphocholine (POPC) was investigated by quartz cry stal microbalance (QCM) measurements in a wide concentration range. Th e SSM were prepared by fusion of unilamellar phospholipid-vesicles on hydrophobic octanethiol monolayers previously chemisorbed on the gold electrode of a polished 5 MHz AT-cut quartz crystal, The bilayer forma tion was monitored by impedance spectroscopy revealing a mean specific capacitance of (1.0 +/- 0.1) mu F/cm(2) for the bilayer. The adsorpti on of melittin on these SSM up to a concentration of 20 mu M in soluti on was followed by QCM-measurements. The adsorption isotherm exhibits three distinct regions. Up to a melittin concentration of 8-10 mu M a submonomolecular coverage of the POPC-monolayer with melittin without any damage of the membrane structure was observed. Increasing the meli ttin concentration up to 20 mu M leads evidently to a multilayer adsor ption which is discussed either in terms of monomer or tetramer stacki ng to the surface. In this concentration-range of 8-20 mu M the shift in the resonant frequency of the quartz crystal increases almost linea r with the concentration of the peptide in solution. Higher concentrat ions than 20 mu M result in a partial detachment of the POPC-monolayer confirmed by an increase of the specific capacitance and a dramatic i ncrease of the resonant frequency. In order to evaluate the first part of the isotherm an electrostatic adsorption model was applied due to the positively charged peptide. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.