Physical Chemistry, Lund University, Lund, Sweden.
J Magn Reson. 2013 Jan;226:13-8. doi: 10.1016/j.jmr.2012.10.015. Epub 2012 Nov 6.
When PGSE NMR is applied to water in microheterogeneous materials such as liquid crystals, foodstuffs, porous rocks, and biological tissues, the signal attenuation is often multi-exponential, indicating the presence of pores having a range of sizes or anisotropic domains having a spread of orientations. Here we modify the standard PGSE experiment by introducing low-amplitude harmonically modulated gradients, which effectively make the q-vector perform magic-angle spinning (MAS) about an axis fixed in the laboratory frame. With this new technique, denoted q-MAS PGSE, the signal attenuation depends on the isotropic average of the local diffusion tensor. The capability of q-MAS PGSE to distinguish between pore size and domain orientation dispersion is demonstrated by experiments on a yeast cell suspension and a polydomain anisotropic liquid crystal. In the latter case, the broad distribution of apparent diffusivities observed with PGSE is narrowed to its isotropic average with q-MAS PGSE in a manner that is analogous to the narrowing of chemical shift anisotropy powder patterns using magic-angle sample spinning in solid-state NMR. The new q-MAS PGSE technique could be useful for resolving size/orientation ambiguities in the interpretation of PGSE data from, e.g., water confined within the axons of human brain tissue.
当 PGSE NMR 应用于微不均匀材料中的水时，例如液晶、食品、多孔岩石和生物组织，信号衰减通常是多指数的，表明存在具有一系列尺寸的孔或具有取向分布的各向异性域。在这里，我们通过引入低幅度谐波调制梯度来修改标准 PGSE 实验，这有效地使 q-矢量在实验室框架中固定的轴上执行魔角旋转（MAS）。使用这种新技术，称为 q-MAS PGSE，信号衰减取决于局部扩散张量的各向同性平均值。通过对酵母悬浮液和多畴各向异性液晶的实验，证明了 q-MAS PGSE 能够区分孔径和域取向分散。在后一种情况下，与 PGSE 相比，q-MAS PGSE 将观察到的表观扩散率的宽分布窄化为各向同性平均值，这类似于在固态 NMR 中使用魔角样品旋转对化学位移各向异性粉末图进行窄化。这种新的 q-MAS PGSE 技术可用于解决从例如人脑组织轴突内受限的水的 PGSE 数据解释中的大小/取向歧义。
