Backbone dynamics of alamethicin in lipid membranes: spin-echo EPR of TOAC spin labels

¹ Universita della Calabria
² University of Padova, 35131 Padova, Italy
³ University of Padova
⁴ Max-Planck-Institut für biophysikalische Chemie

^* To whom correspondence should be addressed. E-mail: dmarsh{at}gwdg.de.

Submitted on June 15, 2007
Revised on August 4, 2007
Accepted on 29 November 2007

	Abstract

Alamethicin F50/5 is a hydrophobic peptide that is devoid of charged residues and which induces voltage-dependent ion channels in lipid membranes. The peptide backbone is likely to be involved in the ion conduction pathway. Electron spin-echo spectroscopy of alamethicin F50/5 analogues in which a selected -aminoisobutyric acid residue (at position n = 1, 8 or 16) is replaced by the TOAC amino-acid spin label was used to study torsional dynamics of the peptide backbone in phosphatidylcholine bilayer membranes. Rapid librational motions of limited angular amplitude were observed at each of the three TOAC sites by recording echo-detected spectra as a function of echo delay time, 2. Simulation of the time-resolved spectra, combined with conventional EPR measurements of the librational amplitude, shows that torsional fluctuations of the peptide backbone take place on the subnanosecond to nanosecond timescale, with little temperature dependence. Associated fluctuations in polar fields from the peptide could facilitate ion permeation.

Key Words: 22,66,tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid, TOAC, alamethicin, echo-detected EPR, librations, spin-label, torsional dynamics