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通过化学位移各向异性(CSA)与偶极自旋偶合(DSA)交叉相关对顺磁弛豫增强的距离依赖性进行调制。

Modulation of the distance dependence of paramagnetic relaxation enhancements by CSA x DSA cross-correlation.

作者信息

Pintacuda Guido, Kaikkonen Andrei, Otting Gottfried

机构信息

Research School of Chemistry, Australian National University, Canberra, ACT 2605, Australia.

出版信息

J Magn Reson. 2004 Dec;171(2):233-43. doi: 10.1016/j.jmr.2004.08.019.

Abstract

Paramagnetic metal ions with fast-relaxing electronic spin and anisotropic susceptibility tensor provide a rich source of structural information that can be derived from pseudo-contact shifts, residual dipolar couplings, dipole-dipole Curie spin cross-correlation, and paramagnetic relaxation enhancements. The present study draws attention to a cross-correlation effect between nuclear relaxation due to anisotropic chemical shielding (CSA) and due to the anisotropic dipolar shielding (DSA) caused by the electronic Curie spin. This CSA x DSA cross-correlation contribution seems to have been overlooked in previous interpretations of paramagnetic relaxation enhancements. It is shown to be sufficiently large to compromise the 1/r6 distance dependence usually assumed. The effect cannot experimentally be separated from auto-correlated DSA relaxation. It can increase or decrease the observed paramagnetic relaxation enhancement. Under certain conditions, the effect can dominate the entire paramagnetic relaxation, resulting in nuclear resonances narrower than in the absence of the paramagnetic center. CSAxDSA cross-correlation becomes important when paramagnetic relaxation is predominantly due to the Curie rather than the Solomon mechanism. Therefore the effect is most pronounced for relaxation by metal ions with large magnetic susceptibility and fast-relaxing electron spin. It most strongly affects paramagnetic enhancements of transverse relaxation in macromolecules and of longitudinal relaxation in small molecules.

摘要

具有快速弛豫电子自旋和各向异性磁化率张量的顺磁性金属离子提供了丰富的结构信息来源,这些信息可从赝接触位移、剩余偶极耦合、偶极-偶极居里自旋交叉相关和顺磁弛豫增强中获得。本研究关注了由各向异性化学屏蔽(CSA)和由电子居里自旋引起的各向异性偶极屏蔽(DSA)导致的核弛豫之间的交叉相关效应。这种CSA×DSA交叉相关贡献在先前对顺磁弛豫增强的解释中似乎被忽视了。结果表明,它足够大,足以破坏通常所假设的1/r6距离依赖性。该效应在实验上无法与自相关的DSA弛豫区分开来。它可以增加或减少观察到的顺磁弛豫增强。在某些条件下,该效应可以主导整个顺磁弛豫,导致核共振比没有顺磁中心时更窄。当顺磁弛豫主要由居里机制而非所罗门机制引起时,CSA×DSA交叉相关变得很重要。因此,对于具有大磁化率和快速弛豫电子自旋的金属离子引起的弛豫,该效应最为明显。它对大分子中横向弛豫和顺磁增强以及小分子中纵向弛豫的顺磁增强影响最大。

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