Yamamoto Kazutoshi, Soong Ronald, Ramamoorthy Ayyalusamy
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
Langmuir. 2009 Jun 16;25(12):7010-8. doi: 10.1021/la900200s.
Bicelles of various lipid/detergent ratios are commonly used in nuclear magnetic resonance (NMR) studies of membrane-associated molecules without the need to freeze the sample. While a decrease in the size (defined at a low temperature or by the q value) of a bicelle decreases its overall order parameter, the variation of lipid dynamics with a change in the lipid/detergent ratio is unknown. In this study, we report a thorough atomistic level analysis on the variation of lipid dynamics with the size and hydration level of bicelles composed of a phospholipid, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and a detergent, 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC). Two-dimensional (2D) separated-local-field NMR experiments were performed on magnetically aligned bicelles to measure (1)H-(13)C dipolar couplings, which were used to determine order parameters at various (head-group, glycerol, and acyl chain) regions of lipids in the bilayer. From our analysis, we uncover the extreme sensitivity of the glycerol region to the motion of the bicelle, which can be attributed to the effect of viscosity because of an extensive network of hydrogen bonds. As such, the water-membrane interface region exhibits the highest order parameter values among all three regions of a lipid molecule. Our experimental results demonstrate that the laboratory-frame 2D proton-detected-local-field pulse sequence is well-suited for the accurate measurement of motionally averaged (or long-range) weak and multiple (1)H-(13)C dipolar couplings associated with a single carbon site at the natural abundance of (13)C nuclei.
各种脂质/去污剂比例的双分子层常用于膜相关分子的核磁共振（NMR）研究，无需冷冻样品。虽然双分子层尺寸减小（在低温下定义或通过q值定义）会降低其整体序参数，但脂质动力学随脂质/去污剂比例变化的情况尚不清楚。在本研究中，我们报告了对由磷脂1,2-二肉豆蔻酰-sn-甘油-3-磷酸胆碱（DMPC）和去污剂1,2-二己酰-sn-甘油-3-磷酸胆碱（DHPC）组成的双分子层的脂质动力学随尺寸和水合水平变化的全面原子水平分析。对磁取向的双分子层进行了二维（2D）分离局部场NMR实验，以测量（1）H-（13）C偶极耦合，用于确定双层中脂质不同区域（头部基团、甘油和酰基链）的序参数。通过我们的分析，我们发现甘油区域对双分子层运动具有极高的敏感性，这可归因于广泛氢键网络导致的粘度效应。因此，水-膜界面区域在脂质分子的所有三个区域中表现出最高的序参数值。我们的实验结果表明，实验室坐标系二维质子检测局部场脉冲序列非常适合在（13）C核的自然丰度下，精确测量与单个碳位点相关的运动平均（或长程）弱且多个（1）H-（13）C偶极耦合。
