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首页> 外文期刊>Journal of magnetic resonance >Mechanism of H-1-N-14 cross-relaxation in immobilized proteins
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Mechanism of H-1-N-14 cross-relaxation in immobilized proteins

机译：H-1-N-14交叉松弛在固定蛋白中的作用机理

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摘要

A resonant enhancement of the water-H-1 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 H-1 nuclei to rapidly relaxing amide N-14 nuclei in rotationally immobilized proteins. While the Q-peak frequencies conform to the known amide N-14 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 N-14 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 N-14 by resonant dipolar relaxation of the second kind', driven by N-14 spill 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 H-1 -> N-14 polarization transfer followed by fast N-14 relaxation. Using independent structural and kinetic information, we show that the Q peaks from these two proteins involve similar to 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.
机译：在各种水性生物系统中，已观察到在0.5-3 MHz范围内三个不同频率处水H-1弛豫速率的共振增强。这些所谓的四极峰（Q）已与从旋转固定蛋白中的H-1核到快速松弛的酰胺N-14核的偶极触发器极化转移相关。尽管Q峰频率符合已知的酰胺N-14四极偶合参数，但尚无用于解释Q峰强度和形状的分子模型。在这里，我们提出了这样一个模型，并在温度，pH和H / D同位素组成可变的条件下，针对来自两种完全水合的交联蛋白的大量Q峰数据进行了测试。我们建议极化过程从本体水到酰胺N-14的转移分为三个步骤：通过材料扩散/交换从本体水到所谓的中间质子，通过交换介导的取向随机化驱动的交叉弛豫从中间质子到酰胺质子它们之间的偶极偶合，并通过第二种共振偶极弛豫从酰胺质子到N-14，这是由N-14溢出波动驱动的，而该波动又是由蛋白质刚性体运动的限制引起的。可以用相干的H-1-> N-14极化转移接着快速的N-14弛豫来表示最后一步的基本等同描述。使用独立的结构和动力学信息，我们显示这两种蛋白质的Q峰涉及内部水分子和侧链羟基的7个中间质子，停留时间约为10（-5）s。该模型不仅定量地说明了广泛的数据集，而且解释了为什么从明胶中几乎看不到Q峰。

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