Specific binding of cinnamycin (Ro 09-0198) to phosphatidylethanolamine : Comparison between micellar and membrane environments

Cinnamycin (Ro 09-0198) is a tetracyclic peptide antibiotic that binds specifically to phosphatidylethanolamine (PE). Formation of a complex with phosphatidylethanolamine follows a 1:1 stoichiometry. Using high-sensitivity isothermal titration calorimetry (ITC), we have measured the thermodynamic parameters of complex formation for two different PE environments, namely, PE dissolved either in octyl glucoside (OG) micelles or in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer membrane. We have compared diacyl-PE with lyso-PE and have varied the carbon chain length from 6 to 18. Binding requires both a PE headgroup and at least one fatty acyl chain. The optimum chain length for complex formation (n) is eight. Longer chains do not enhance the binding affinity; for shorter chains, the interaction is weakened. The cinnamycin-PE complex has a binding constant K(0) of approximately 10(7)-10(8) M(-1) in the POPC membrane and only approximately 10(6) M(-1) in the octyl glucoside micelle. The difference can be attributed to the nonspecific hydrophobic interaction of cinnamycin with the lipid membrane. Complex formation is enthalpy-driven in OG micelles, whereas enthalpy and entropy make equal contributions in bilayer membranes. However, for the optimum chain length (n) of eight, the binding reaction is also completely enthalpy-driven for the bilayer membrane.