Erilov Denis A, Bartucci Rosa, Guzzi Rita, Marsh Derek, Dzuba Sergei A, Sportelli Luigi
Dipartimento di Fisica and Unità Instituto Nazionale per la Fisica della Materia, Università della Calabria, Arcavacata di Rende (CS), Italy.
Biophys J. 2004 Dec;87(6):3873-81. doi: 10.1529/biophysj.104.046631. Epub 2004 Sep 17.
Two-pulse, echo-detected (ED) electron paramagnetic resonance (EPR) spectroscopy was used to study the librational motions of spin-labeled lipids in membranes of dipalmitoylphosphatidylcholine + 50 mol % cholesterol. The temperature dependence, over the range 77-240 K, and the dependence on position of spin-labeling in the sn-2 chain (n=5, 7, 10, 12, and 14) of the phospholipid, were characterized 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, alpha2tauc, were constructed for diagnostic spectral line-height 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, alpha2, 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, tauc, which is deduced from combination of the conventional and ED-EPR results, lies in the subnanosecond 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 upon 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.
采用双脉冲、回波检测(ED)电子顺磁共振(EPR)光谱法研究了二棕榈酰磷脂酰胆碱 + 50 mol%胆固醇膜中自旋标记脂质的摆动运动。详细表征了77 - 240 K温度范围内的温度依赖性以及磷脂sn - 2链(n = 5、7、10、12和14)中自旋标记位置的依赖性。通过使用在77 K记录的光谱对由静态自旋 - 自旋相互作用引起的瞬时自旋扩散进行校正,在77 K时运动贡献可忽略不计。根据围绕方向随机分布的轴进行快速、小幅度摆动的模型进行模拟,能够描述实验光谱。针对不同回波延迟时间的诊断光谱线高比以及从在两个不同回波延迟下记录的ED光谱之比获得的弛豫光谱,构建了以振幅 - 相关时间乘积α₂τc表示的校准。根据传统EPR光谱中部分运动平均超精细分裂确定了脂质链14 - C位置自旋标记的摆动幅度α₂。从传统和ED - EPR结果的组合推导得出的摆动相关时间τc处于亚纳秒范围,并且仅微弱地依赖于温度。ED - EPR光谱的温度依赖性主要源于摆动幅度随温度升高以及沿脂质链向下位置的增加。在较高温度下会发生逐渐转变,从片段扭转摆动是累积的情况,即各个片段的贡献在沿链向下时逐渐累加,转变为仅微弱依赖于链位置的协同运动情况。这种摆动运动对于类玻璃态很重要,并且通常与高脂质堆积密度相关,例如在含胆固醇的筏结构域和凝聚复合物中。