Biophysical Chemistry, Center for Molecular Protein Science, Lund University, SE-22100 Lund, Sweden.
J Magn Reson. 2010 Apr;203(2):257-73. doi: 10.1016/j.jmr.2010.01.008. Epub 2010 Jan 25.
A resonant enhancement of the water-1H relaxation rate at three distinct frequencies in the range 0.5-3 MHz has been observed in a variety of aqueous biological systems. These so-called quadrupole (Q) peaks have been linked to a dipolar flip-flop polarization transfer from 1H nuclei to rapidly relaxing amide 14N nuclei in rotationally immobilized proteins. While the Q-peak frequencies conform to the known amide 14N quadrupole coupling parameters, a molecular model that accounts for the intensity and shape of the Q peaks has not been available. Here, we present such a model and test it against an extensive set of Q-peak data from two fully hydrated crosslinked proteins under conditions of variable temperature, pH and H/D isotope composition. We propose that polarization transfer from bulk water to amide 14N occurs in three steps: from bulk water to a so-called intermediary proton via material diffusion/exchange, from intermediary to amide proton by cross-relaxation driven by exchange-mediated orientational randomization of their mutual dipole coupling, and from amide proton to 14N by resonant dipolar relaxation 'of the second kind', driven by 14N spin fluctuations, which, in turn, are induced by restricted rigid-body motions of the protein. An essentially equivalent description of the last step can be formulated in terms of coherent 1H-->14N polarization transfer followed by fast 14N relaxation. Using independent structural and kinetic information, we show that the Q peaks from these two proteins involve approximately 7 intermediary protons in internal water molecules and side-chain hydroxyl groups with residence times of order 10(-5) s. The model not only accounts quantitatively for the extensive data set, but also explains why Q peaks are hardly observed from gelatin gels.
在各种水生物系统中，已经观察到在 0.5-3 MHz 范围内三个不同频率的水-1H 弛豫率的共振增强。这些所谓的四极(Q)峰与来自旋转固定蛋白质中的快速弛豫酰胺 14N 核的偶极翻转-翻转极化转移有关。虽然 Q 峰频率符合已知的酰胺 14N 四极偶合参数，但没有可以解释 Q 峰强度和形状的分子模型。在这里，我们提出了这样的模型，并根据两个完全水合的交联蛋白质在不同温度、pH 值和 H/D 同位素组成条件下的大量 Q 峰数据对其进行了测试。我们提出，从体相水到酰胺 14N 的极化转移分三个步骤进行：通过物质扩散/交换从体相水到所谓的中间质子，通过由交换介导的它们相互偶极耦合的取向随机化驱动的交叉弛豫从中介质子到酰胺质子，以及通过由 14N 自旋波动驱动的共振偶极弛豫“第二类”，从酰胺质子到 14N，这反过来又由蛋白质的受限刚性体运动引起。最后一步可以用基本上等效的描述来表示，即通过内部水分子和侧链羟基中的 7 个中间质子的相干 1H->14N 极化转移，然后是快速的 14N 弛豫。利用独立的结构和动力学信息，我们表明这两种蛋白质的 Q 峰涉及内部水分子和侧链羟基中的大约 7 个中间质子，其停留时间约为 10(-5) s。该模型不仅定量地解释了广泛的数据集合，还解释了为什么 Q 峰几乎不会从明胶凝胶中观察到。
