Koenig S H, Hallenga K, Shporer M
Proc Natl Acad Sci U S A. 1975 Jul;72(7):2667-71. doi: 10.1073/pnas.72.7.2667.
Previous studies of the magnetic field dependence of the magnetic relaxation rate of solvent protons in protein solutions have indicated that this dependence (called relaxation dispersion) is related to the rotational Brownian motion of the solute proteins. In particular, the dispersion of the longitudinal (spin-lattice) relaxation rate 1/T1 shows a monotonic decrease with increasing field, with an inflection point corresponding to a proton Larmor frequency which is inversely proportional to the orientational relaxation time of the protein. We have now compared the relaxation dispersion of solvent 1H, 2H, and 17O In aqueous solutions of lysozyme (molecular weight 14,700) and 1H and 2H in solutions of hemocyanin (molecular weight 14,7 00) and 1H and 2H in solutions of hemocyanin (molecular weight 9 x 10(6)). The main experimental observation is that the dispersion of the relaxation rates of the three solvent nuclei in lysozyme solutions, normalized to their respective rates in pure water, is essentially the same. This is also true for 1H and 2H relaxation in hemocyanin solutions. These results confirm that entire solvent water molecules, rather than exchanging protons, are involved in the interaction. We have been unable to deduce the correct mechanism to explain the data, but we can eliminate several interaction mechanisms from consideration. For example, all observations combined cannot be explained by a simple two-site model of exchange, in which water molecules are either in sites on the protein with a relaxation rate characteristic of these sites, or else in the bulk solvent (the observed relaxation rate being the weighted average of the two). Also eliminated is the class of models in which the protein molecules induce a preferential partial alignment of neighboring solvent molecules, for example by electrostatic interaction of the electric dipole moments of the water with the electric fields produced by surface charges of the protein molecules. In addition, the idea that relaxation of solvent nuclei is due, in the main, to interactions with protein protons is precluded. Rather, it appears that the protein molecules influence the dynamics of the motion of solvent water molecules in their neighborhood in a manner that imposes on all the solvent molecules a correlation time for their orientational relaxation which equals that of the solute proteins.
先前对蛋白质溶液中溶剂质子磁弛豫率的磁场依赖性研究表明，这种依赖性（称为弛豫色散）与溶质蛋白质的旋转布朗运动有关。特别地，纵向（自旋 - 晶格）弛豫率1/T1的色散随磁场增加呈单调下降，拐点对应的质子拉莫尔频率与蛋白质的取向弛豫时间成反比。我们现在比较了溶菌酶（分子量14,700）水溶液中溶剂1H、2H和17O的弛豫色散，以及血蓝蛋白（分子量14,700）溶液中1H和2H的弛豫色散，还有血蓝蛋白（分子量9×10⁶）溶液中1H和2H的弛豫色散。主要的实验观察结果是，溶菌酶溶液中三种溶剂核的弛豫率色散，相对于它们在纯水中各自的速率进行归一化后，基本相同。血蓝蛋白溶液中1H和2H的弛豫情况也是如此。这些结果证实，参与相互作用的是整个溶剂水分子，而非交换质子。我们无法推导出解释这些数据的正确机制，但可以排除几种相互作用机制。例如，所有观察结果综合起来无法用简单的双位点交换模型解释，在该模型中，水分子要么处于蛋白质上具有这些位点特征弛豫率的位点，要么处于本体溶剂中（观察到的弛豫率是两者的加权平均值）。一类模型也被排除，在这类模型中，蛋白质分子诱导相邻溶剂分子产生优先的部分排列，例如通过水的电偶极矩与蛋白质分子表面电荷产生的电场之间的静电相互作用。此外，溶剂核弛豫主要归因于与蛋白质质子相互作用的观点也被排除。相反，似乎蛋白质分子以一种方式影响其附近溶剂水分子的运动动力学，这种方式使所有溶剂分子的取向弛豫相关时间等于溶质蛋白质的相关时间。
