A study of the interaction of bee venom melittin with model biological membranes using both total and polarized fluorescence measurements

Polarized fluorescence intensity measurements consisting of vertical and horizontal components were taken using a stopped-flow fluorometer in order to study the kinetics of melittin binding to dimyristoylphosphatidylcholine (DMPC) small unilamellar vesicles and dipalmitoylphosphatidylcholine (DPPC) small unilamellar vesicles over a range of temperatures. The data were analyzed within the framework of a sophisticated model proposed by Bradrick (1994). The results for DMPC small unilamellar vesicles showed that the rates of melittin-vesicle association and dissociation increased with increasing temperature, as expected. The number of lipid molecules per binding site did not show any noticeable trend although at the transition temperature of 21 °C, it was lower. An Arrhenius plot gave an activation energy greatCT than that required for melting the acyl chains, showing that more than the melting of the acyl chains is required for binding to take place. Most probably, some contribution goes into an ordering/disordering of the membrane so as to accommodate the melittin molecule. To determine whether there may exist more than one species of bound melittin, the effective quantum yield and the anisotropy of bound melittin were taken as indicators. A constant effective quantum yield for bound melittin at different temperatures was obtained which by itself may not be conclusive since it is possible for say, two different modes of binding to take place and yet result in effective quantum yields that may not be distinguishable. The anisotropy, a sensitive indicator of rotational dynamics, was then used to further explore the question of whether there may be more than one species of bound melittin. The resulting anisotropies for bound melittin did not show any variation at the different temperatiu-es used in the study, indicating that for these temperatxu-es binding took place in one class of binding site. For DPPC small unilamellar vesicles, the results of the analysis are less conclusive due to the &ct that the change in the fluorescence intensity for some concentrations was too small and could not be read by the numerical technique employed in the analysis. These concentrations were, thus, excluded from the analysis. The results, which may not be the unique solutions being sought in the global analysis, seem to indicate a tighter binding which, if true, agree with observations by Bradrick et al. (1995).