Schreiner L J, MacTavish J C, Pintar M M, Rupprecht A
Department of Physics, University of Waterloo, Ontario, Canada.
Biophys J. 1991 Jan;59(1):221-34. doi: 10.1016/S0006-3495(91)82213-5.
The NMR spin-grouping technique is applied to low hydration oriented fibers of NaDNA to study the role of exchange in determining the apparent (observed) spin relaxation of the system. The analysis proceeds in three steps: first, the apparent proton relaxation is measured at high fields, with both selective and nonselective inversion pulse sequences, and in the rotating frame. The spin-grouping technique is used in all spin-lattice relaxation measurements to provide the optimum apparent relaxation characterization of the sample. Next, all apparent results are analyzed for exchange. In this analysis the results from the high field and rotating frame experiments (which probe the exchange at two different time scales) are correlated to determine the inherent (or true) spin relaxation parameters of each of the proton groups in the system. The results of selective inversion T1 measurements are also incorporated into the exchange analysis. Finally, the dynamics of each spin group are inferred from the inherent relaxation characterization. The low hydration NaDNA structure is such that the exchange between the protons on the water and those on the NaDNA is limited, a priori, to dipolar mixing. The results of the exchange analysis indicate that the dipolar mixing between water and NaDNA protons is faster than the spin diffusion within the NaDNA proton group itself. The spin-diffusion on the macromolecule is the bottleneck for the exchange between the water protons and the NaDNA protons. The water protons serve as the relaxation sink both at high fields and in the rotating frame for the total NaDNA-water spin bath. The inherent relaxation of the water is characteristic of water undergoing anisotropic motion with a fast reorientational correlation time about one axis (5 X 10(-10) less than or equal to tau r less than or equal to 8 X 10(-9)S) which is about three orders of magnitude slower than that of water in the bulk; and a slow tumbling correlation time for this axis (1.5 x 10(-7) less than or equal to tau t less than or equal to 8 x 10(-7)S) which is two orders of magnitude slower yet.
核磁共振自旋分组技术应用于低水合取向的钠DNA纤维，以研究交换在确定该系统表观（观测到的）自旋弛豫中的作用。分析过程分三步进行：首先，在高场下，使用选择性和非选择性反转脉冲序列，并在旋转坐标系中测量表观质子弛豫。在所有自旋晶格弛豫测量中都采用自旋分组技术，以提供样品的最佳表观弛豫特征。其次，对所有表观结果进行交换分析。在该分析中，将高场和旋转坐标系实验的结果（它们在两个不同的时间尺度上探测交换）进行关联，以确定系统中每个质子组的固有（或真实）自旋弛豫参数。选择性反转T1测量的结果也纳入交换分析。最后，从固有弛豫特征推断每个自旋组的动力学。低水合钠DNA结构使得水和钠DNA上质子之间的交换，先验地，仅限于偶极混合。交换分析结果表明，水和钠DNA质子之间的偶极混合比钠DNA质子组自身内的自旋扩散更快。大分子上的自旋扩散是水质子和钠DNA质子之间交换的瓶颈。对于整个钠DNA - 水自旋体系，水质子在高场和旋转坐标系中均作为弛豫汇。水的固有弛豫特征是水经历各向异性运动，其围绕一个轴的快速重取向相关时间（5×10^(-10)≤τr≤8×10^(-9)秒），这比本体水中的水慢约三个数量级；以及该轴的缓慢翻滚相关时间（1.5×10^(-7)≤τt≤8×10^(-7)秒），这又慢两个数量级。
