Librational Motion of Spin-labelled Lipids in High-Cholesterol Containing Membranes from Echo-Detected EPR Spectra
Denis A. Erilov 1, Rosa Bartucci 1*, Rita Guzzi 1, Derek Marsh 2, Sergei A. Dzuba 3 and Luigi Sportelli 1
1 Dipartimento di Fisica, Universitá della Calabria, Italy
2 Max-Planck-Institut für biophysikalische Chemie, Abteilung Spektroskopie
3 Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Novosibirsk, Russia
* To whom correspondence should be addressed. E-mail: bartucci{at}fis.unical.it.
Submitted on May 27, 2004
Revised on July 21, 2004
Accepted on 19 August 2004
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Abstract |
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Two-pulse, echo-detected (ED) EPR spectroscopy was used to study the librational motions of spin-labelled lipids in membranes of dipalmitoyl phosphatidylcholine + 50 mol% cholesterol. The temperature dependence, over the range 77 240 K, and the dependence on position of spin-labelling in the sn-2 chain (n = 5, 7, 10, 12 and 14) of the phospholipid, were characterised in detail. The experimental ED-spectra were corrected for instantaneous spin diffusion arising from static spin-spin interactions, by using spectra recorded at 77 K, where motional contributions are negligible. Simulations according to a model of rapid, small-amplitude librations about an axis whose direction is randomly distributed are able to describe the experimental spectra. Calibrations, in terms of the amplitude-correlation time product, <a2>tc, were constructed for diagnostic spectral lineheight ratios at different echo delay times, and for relaxation spectra obtained from the ratio of ED spectra recorded at two different echo delays. The librational amplitude, <a2>, was determined for a spin label at the 14-C position of the lipid chain from the partially motionally averaged hyperfine splitting in the conventional EPR spectra. The librational correlation time, tc, which is deduced from combination of the conventional and ED-EPR results, lies in the sub-nanosecond regime and depends only weakly on temperature. The temperature dependence of the ED EPR spectra arises mainly from an increase in librational amplitude with increasing temperature, and position down the lipid chain. A gradual transition takes place at higher temperatures, from a situation in which segmental torsional librations are cumulative, i.e., the contributions of the individual segments add-up progressively on going down the chain, to one of concerted motion only weakly dependent on chain position. Such librational motions are important for glass-like states and are generally relevant to high lipid packing densities, e.g., in cholesterol-containing raft domains and condensed complexes.
Key Words:
Cholesterol, Echo Detected Spectra, Librational Motion, Lipid Membranes, Pulsed Electron Spin Resonance, Spin Label
