Lazo N D, Hu W, Cross T A
Department of Chemistry, Florida State University, Tallahassee 32306-3006, USA.
J Magn Reson B. 1995 Apr;107(1):43-50. doi: 10.1006/jmrb.1995.1056.
The local molecular dynamics of gramicidin A in dimyristoylphosphatidylcholine bilayers were probed by a combination of low-temperature solid-state 15N NMR and rapid freezing of samples using liquid propane. It has previously been shown that this approach leads to sharp discontinuities in powder-pattern spectra of the gramicidin channel in lipid bilayers, and hence, an accurate determination of tensor element magnitudes is achieved. The static-tensor-element magnitudes determined here using hydrated bilayer samples at low temperatures are different from the values obtained from dry-powder samples at 308 K. However, for the one site studied, the orientation of the 15N chemical-shift tensor in the molecular frame is the same for both types of samples. Averaging of the chemical-shift tensor between 200 and 263 K is shown to be anisotropic and consistent with a motional axis parallel with the C alpha-C alpha axis for adjacent residues. The librational amplitudes determined here range from +/- 14 degrees to +/- 22 degrees at 263 K and are significantly larger than the nanosecond librations characterized by relaxation studies. The nanosecond motions are thought to represent correlated motions, while the librations described here are the sum of correlated (nanosecond) and uncorrelated (picosecond) motions. The picosecond librational amplitudes become considerably larger and more isotropic above 0 degree for the residues near the bilayer surface.
通过低温固态15N核磁共振和使用液态丙烷快速冷冻样品相结合的方法，对短杆菌肽A在二肉豆蔻酰磷脂酰胆碱双层膜中的局部分子动力学进行了探测。此前已经表明，这种方法会导致脂质双层膜中短杆菌肽通道的粉末图谱光谱出现明显的不连续性，因此，可以准确测定张量元素的大小。这里在低温下使用水合双层膜样品确定的静态张量元素大小与在308K下从干粉样品获得的值不同。然而，对于所研究的一个位点，两种类型的样品在分子框架中15N化学位移张量的方向是相同的。结果表明，在200至263K之间化学位移张量的平均是各向异性的，并且与相邻残基的Cα - Cα轴平行的运动轴一致。这里确定的摆动幅度在263K时范围为±14度至±22度，并且明显大于通过弛豫研究表征的纳秒级摆动。纳秒级运动被认为代表相关运动，而这里描述的摆动是相关（纳秒级）和不相关（皮秒级）运动的总和。对于双层膜表面附近的残基，皮秒级摆动幅度在高于0度时变得相当大且更具各向同性。
