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Backbone Dynamics of Alamethicin Bound to Lipid Membranes: Spin-Echo Electron Paramagnetic Resonance of TOAC-Spin Labels

Rosa Bartucci ^*, Rita Guzzi ^*, Marta De Zotti , Claudio Toniolo , Luigi Sportelli ^* and Derek Marsh 

^* Dipartimento di Fisica, Università della Calabria, 87036 Arcavacata di Rende (CS), Italy; Institute of Biomolecular Chemistry, Padova Unit, Consiglio Nazionale delle Ricerche, Department of Chemistry, University of Padova, 35131 Padova, Italy; and Max-Planck-Institut für biophysikalische Chemie, Abt. Spektroskopie, 37077 Göttingen, Germany

Correspondence: Address reprint requests to Derek Marsh, Tel.: +49-551-201-1285; Fax: +49-551-201-1501; E-mail: dmarsh{at}gwdg.de.

Alamethicin F50/5 is a hydrophobic peptide that is devoid of charged residues and that 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 analogs in which a selected Aib 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 association with 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.