Effect of Molecular Organization in Micelles and Bilayers on Binding and Conformation of Biologically Active Peptides

Abstract

Amphiphiles form different types of aggregates, such as micelles and bilayers, depending on their shape and hydrophilic-hydrophobic balance. While bilayers form vesicles containing an inner aqueous compartment, micelles are smaller, approximately spherically-shaped, and have no internal aqueous compartment. Thus, molecular packing and mobility vary in these aggregates, and EPR spectra of spin probes can be used to examine these properties. EPR spectra evince tighter molecular packing and slower rate of motion in bilayers than in micelles. Such differences affect binding of peptides, both qualitatively and quantitatively. Fluorescence, CD, and EPR were employed to investigate interactions of micelles and vesicles with antimicrobial peptides, as well as fragments of GPCR and cytolytic toxins. EPR was also used for peptide analogues containing the paramagnetic amino acid TOAC. Two-component spectra indicated slow exchange between bound peptide and peptide tumbling fast in aqueous solution, allowing the calculation of binding constants. Peptide-membrane interaction was also monitored by changes in peptide fluorescence intensity and emission wavelengths, as well as accessibility to a water soluble quencher. CD spectra showed that upon binding the peptides acquired secondary structure due to formation of intramolecular hydrogen bonds, favored by the decreased polarity at the lipid interface. While in most cases, bilayer binding was only observed when electrostatic interactions occurred between positively charged peptides and negatively charged phospholipids, electrostatic effects played a less important role in peptide-micelle interaction. These differences were ascribed to differences in molecular packing and curvature in both types of aggregates. The positive curvature of micelles is proposed to mimic the lipid organization of toroidal pores. Thus, the conformational behavior in the presence of micelles would correspond to that of peptides forming toroidal pores in bilayer membranes.Supported by FAPESP, CNPq, CAPES.