Journal
Journal of Physical Chemistry Letters
Volume 7, Issue 23, Pages 4815-4818Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpclett.6b02417
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Funding
- Australian Research Council
- Angus Nicholson Honours scholarship
- Engineering and Physical Sciences Research Council [EP/N006895/1] Funding Source: researchfish
- EPSRC [EP/N006895/1] Funding Source: UKRI
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Paramagnetic metal ions accelerate nuclear spin relaxation; this effect is widely used for distance measurement and called paramagnetic relaxation enhancement (PRE). Theoretical predictions established that, under special circumstances, it is also possible to achieve a reduction in nuclear relaxation rates (negative PRE). This situation would occur if the mechanism of nuclear relaxation in the diamagnetic state is counterbalanced by a paramagnetic relaxation mechanism caused by the metal ion. Here we report the, first experimental evidence for such a cross-correlation effect. Using a uniformly N-15-labeled mutant of calbindin D-9k loaded with either Tm3+ or Tb3+, reduced R-1 and R-2 relaxation rates of backbone N-15 spins were observed compared with the diamagnetic reference (the same protein loaded with Y3+). The effect arises from the compensation of the chemical shift anisotropy tensor by the anisotropic dipolar shielding generated by the unpaired electron spin.
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