NUCLEAR MAGNETIC RESONANCE--NEW METHODS AND MOLECULAR STRUCTURE DETERMINATION

已经开发出用于测量同核和异核的新方法 同位素富集蛋白质中的 J 耦合。这些方法允许这些 J 联轴器以约 0.1 Hz 的异常高精度进行测量。 大约四十年前，卡普拉斯证明了以下两者之间的经验关系： 三键 J 耦合的大小和中间的二面角。 除了二面角之外，J 联轴器还受到以下因素的影响： 许多其他因素，例如取代基效应、电场 梯度和键合应变。蛋白质中有四个三键 J 描述主干角 phi 的联轴器。所有四个 J 联轴器均具有 在小模型中测量了大部分氨基酸残基 蛋白质、泛素。测量的 J 联轴器与 phi 的比较 根据该蛋白质的 X 射线结构计算出的角度允许 重新参数化以前的“Karplus 关系” 基于约束小模型中 J 联轴器的测量 化合物。尽管在质量上相似，但新的参数化 与常用的文献值有很大偏差。一致 对于给定的残基，使用所有四个 J 联轴器可以使 4) 角度为 高精度测定。结果表明，不确定性 NMR 得出的 phi 角约为 2 度，与 X 射线中 phi 角观测到的不确定性约为 3.2 度 碱性胰蛋白酶抑制剂的结构，在 1 埃处解析 分辨率。

New methods have been developed for measurement of homo- and heteronuclear J couplings in isotopically enriched proteins. These methods permit these J couplings to be measured at an unusually high precision of ca 0.1 Hz. Nearly forty years ago, Karplus demonstrated an empirical relation between the size of the three-bond J coupling and the intervening dihedral angle. Besides the dihedral angle, J couplings are also known to be influenced by a number of other factors, such as substituent effects, electric field gradients, and bond strain. In proteins, there are four three-bond J couplings that describe the backbone angle phi. All four J couplings have been measured for most of the amino acid residues in a small model protein, ubiquitin. Comparison of the measured J couplings with the phi angles calculated from the X-ray structure of this protein allowed reparametrization of the "Karplus relationships" which previously had been based on the measurement of J couplings in constrained small model compounds. Although qualitatively similar, the new parametrizations deviate considerably from the commonly used literature values. Concerted use of all four J couplings for a given residue allows the 4) angle to be determined at high precision. Results indicate that the uncertainty in the NMR-derived phi angle is ca 2 degrees, which compares favorably with the uncertainty of ca 3.2 degrees observed for the phi angle in the X-ray structure of basic pancreatic trypsin inhibitor, solved at 1-Angstroms resolution.