Zhang H, Bryant R G
Department of Chemistry, University of Virginia, Charlottesville 22901, USA.
Biophys J. 1995 Jun;68(6):2556-65. doi: 10.1016/S0006-3495(95)80438-8.
Nuclear magnetic resonance spectra for nuclei with spin l > 1 are considered in cases in which the observed nucleus may sample a rotationally immobilized and an isotropic environment that are coupled by a chemical exchange process. Spectra are simulated for the central (1/2, -1/2) transition for a 3/2 nucleus as a function of the concentrations of the two environments and as a function of the exchange rate between them. It is shown that a crucial feature determining the shape of the observable spectra is the spatial extent or the local order in the immobilized phase. In the case for which all rotationally immobilized sites sampled by the exchanging nucleus are identically oriented but where there is a distribution of these microdomain orientations with respect to the magnetic field direction, a powder pattern for the central transition is observed that carries whatever dynamic information may be derived from it. In the fast exchange limit, the width of the powder pattern scales inversely with the concentration of the isotropic environment as usual. In the intermediate exchange regimes, a complex line shape results that may mask the anisotropic character of the spectrum. In the slow exchange limit, superposition of the spectral contributions results; however, if the isotropic environment concentration is significantly larger than the anisotropic environment concentration, the anisotropic contribution is very difficult to detect because of the dynamic range problem and the possibly large difference in the effective line widths. In the case for which the exchanging nucleus samples a considerable distribution of rotationally immobilized site orientations, the anisotropic character of the spectrum is lost and a super-Lorentzian line shape results. These effects are demonstrated experimentally by 35Cl nuclear magnetic resonance spectra obtained on a lamellar liquid crystal that is modified with the addition of a thiolmercurate to provide a site of large quadrupole coupling constant and with cross-linked bovine serum albumin gels.
对于自旋(l>1)的原子核，当观测到的原子核可能处于通过化学交换过程耦合的旋转固定环境和各向同性环境中时，会考虑其核磁共振谱。针对一个(3/2)原子核的中心((1/2, -1/2))跃迁，模拟了谱线随两种环境浓度以及它们之间交换速率的变化情况。结果表明，决定可观测谱线形状的一个关键特征是固定相中空间范围或局部有序性。在交换原子核所采样的所有旋转固定位点方向相同，但这些微区方向相对于磁场方向存在分布的情况下，会观察到中心跃迁的粉末图样，从中可以获取任何可能的动态信息。在快速交换极限下，粉末图样的宽度通常与各向同性环境的浓度成反比。在中间交换区域，会产生复杂的线形，可能掩盖谱线的各向异性特征。在慢速交换极限下，会出现谱线贡献的叠加；然而，如果各向同性环境浓度显著大于各向异性环境浓度，由于动态范围问题以及有效线宽可能存在的巨大差异，各向异性贡献很难被检测到。在交换原子核采样的旋转固定位点方向存在相当大分布的情况下，谱线的各向异性特征消失，会得到超洛伦兹线形。通过在层状液晶上获得的(35Cl)核磁共振谱实验验证了这些效应，该层状液晶通过添加硫醇汞酸盐进行改性以提供具有大四极耦合常数的位点，并与交联牛血清白蛋白凝胶一起使用。
