The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) moiety tethered to the headgroup of phosphatidylcholine (PC) lipid is employed in spin labeling electron paramagnetic resonance spectroscopy to probe the water dynamics near lipid bilayer interfaces. Due to its amphiphilic character, however, TEMPO spin label could partition between aqueous and lipid phases, and may even be stabilized in the lipid phase. Accurate assessment of the TEMPO-PC configuration in bilayer membranes is essential for correctly interpreting the data from measurements. Here, we carry out all-atom molecular dynamics (MD) simulations of TEMPO-PC probe in single-component lipid bilayers at varying temperatures, using two standard MD force fields. We find that, for a dipalmitoylphosphatidylcholine (DPPC) membrane whose gel-to-fluid lipid phase transition occurs at 314 K, while the TEMPO spin label is stabilized above the bilayer interface in the gel phase, there is a preferential location of TEMPO below the membrane interface in the fluid phase. For bilayers made of unsaturated lipids, 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), which adopt the fluid phase at ambient temperature, TEMPO is unequivocally stabilized inside the bilayers. Our finding of membrane phase-dependent positioning of the TEMPO moiety highlights the importance of assessing the packing order and fluidity of lipids under a given measurement condition.

与磷脂酰胆碱 (PC) 脂质头基相连的 (2,2,6,6-四甲基哌啶-1-基)氧基 (TEMPO) 部分用于自旋标记电子顺磁共振波谱探测脂质双层界面附近的水动力学。然而，由于其两亲特性，TEMPO 自旋标记可以在水相和脂相之间分配，甚至可以稳定在脂相中。准确评估双层膜中的 TEMPO-PC 配置对于正确解释测量数据至关重要。在这里，我们使用两个标准 MD 力场，在不同温度下对单组分脂质双层中的 TEMPO-PC 探针进行全原子分子动力学 (MD) 模拟。我们发现，对于二棕榈酰磷脂酰胆碱 (DPPC) 膜，其凝胶到流体的脂质相变发生在 314 K，虽然 TEMPO 自旋标记在凝胶相中稳定在双层界面上方，但在流体相中 TEMPO 优先位于膜界面下方。对于由不饱和脂质、1,2-二油酰-sn-甘油-3-磷酸胆碱 (DOPC) 和 1-棕榈酰-2-油酰-sn-甘油-3-磷酸胆碱 (POPC) 制成的双层，采用室温下的液相在一定温度下，TEMPO 在双层内明确稳定。我们对 TEMPO 部分的膜相依赖性定位的发现强调了在给定测量条件下评估脂质的堆积顺序和流动性的重要性。2-二油酰-sn-甘油-3-磷酸胆碱 (DOPC) 和 1-棕榈酰-2-油酰-sn-甘油-3-磷酸胆碱 (POPC) 在环境温度下采用液相，TEMPO 在双层内明确稳定。我们对 TEMPO 部分的膜相依赖性定位的发现强调了在给定测量条件下评估脂质的堆积顺序和流动性的重要性。2-二油酰-sn-甘油-3-磷酸胆碱 (DOPC) 和 1-棕榈酰-2-油酰-sn-甘油-3-磷酸胆碱 (POPC) 在环境温度下采用液相，TEMPO 在双层内明确稳定。我们对 TEMPO 部分的膜相依赖性定位的发现强调了在给定测量条件下评估脂质的堆积顺序和流动性的重要性。