Self-Aggregation of Spin-Labeled Alamethicin in ePC Vesicles Studied by Pulsed Electron−Electron Double Resonance

The pulsed electron−electron double resonance technique was used to study the dipole−dipole interactions between 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid spin labels located at position 16 of an alamethicin analogue in ePC vesicles that were frozen to 77 K. We show that under these conditions the alamethicin molecules tend to form aggregates over the range of peptide concentrations 3.5 × 10^-3 to 1 × 10^-2 M. The number of molecules in the aggregate is found to be 4.2 ± 0.2. A spin-label distance distribution function is also obtained with a maximum at a distance of 2.3 nm and a half-height width of 1.3 nm. We envisage that these data will permit us to generate a molecular model of cellular ion channels