Teng Ching-Ling, Hinderliter Brian, Bryant Robert G
The Biophysics Program and Chemistry Department, University of Virginia, Charlottesville, Virginia 22904-4319, USA.
J Phys Chem A. 2006 Jan 19;110(2):580-8. doi: 10.1021/jp0526593.
The nuclear spin relaxation induced by a freely diffusing paramagnetic center provides a direct measure of intermolecular accessibility. A number of factors are involved in a quantitative interpretation of relaxation data including excluded volume effects, solvation differences, and the details of the electron spin relaxation in the paramagnetic center. In the case where the electron relaxation time is short compared with correlation times describing the electron-nuclear coupling, the nuclear spin relaxation rates may be related to the effective local concentration of the paramagnetic center at different locations about the solute of interest. The local concentrations may in turn be related to differences in the local free energies of interaction between the diffusing paramagnet and the cosolute. We demonstrate this approach for the case of ribonuclease A and deduce surface free energy differences for a large number of protein proton sites. We find that the oxygen accessibility is poorly represented by hard-sphere models such as computed solvent or steric accessibility. There is a distribution of local intermolecular interactions with a width of the order of RT that dominates the report of the intermolecular exploration of the protein by this simple solute.
由自由扩散的顺磁中心引起的核自旋弛豫提供了分子间可及性的直接测量方法。在对弛豫数据进行定量解释时涉及许多因素，包括排除体积效应、溶剂化差异以及顺磁中心中电子自旋弛豫的细节。在电子弛豫时间比描述电子 - 核耦合的相关时间短的情况下，核自旋弛豫率可能与顺磁中心在感兴趣溶质周围不同位置的有效局部浓度有关。局部浓度又可能与扩散顺磁体和共溶质之间局部相互作用自由能的差异有关。我们针对核糖核酸酶A的情况展示了这种方法，并推导了大量蛋白质质子位点的表面自由能差异。我们发现，诸如计算得到的溶剂可及性或空间可及性等硬球模型不能很好地表示氧可及性。存在一个局部分子间相互作用的分布，其宽度约为RT量级，这主导了这种简单溶质对蛋白质分子间探索的报告。
