Category Archives: PRE

Reversal of Paramagnetic Effects by Electron Spin Saturation #DNPNMR

Jain, Sheetal K., Ting A. Siaw, Asif Equbal, Christopher B. Wilson, Ilia Kaminker, and Songi Han. “Reversal of Paramagnetic Effects by Electron Spin Saturation.” The Journal of Physical Chemistry C 122, no. 10 (March 15, 2018): 5578–89.

https://doi.org/10.1021/acs.jpcc.8b00312.

We present a study in which both significant dynamic nuclear polarization (DNP) enhancement of 7Li NMR and reversal of the paramagnetic effects (PEs) are achieved by microwave (μw) irradiation-induced electron spin saturation of nitroxide radicals at liquid-helium temperatures. The reversal of the PE was manifested in significant narrowing of the 7Li NMR line and reversal of the paramagnetic chemical shift under DNP conditions. The extent of the PE was found to decrease with increased saturation of the electron paramagnetic resonance line, modulated as a function of microwave (μw) power, frequency, duration of irradiation, and gating time between μw irradiation and NMR detection. The defining observation was the shortening of the electron phase memory time, Tm, of the excited observer spins with increasing μw irradiation and concurrent electron spin saturation of the electron spin bath. This and a series of corroborating studies reveal the origin of the NMR line narrowing to be the reversal of paramagnetic relaxation enhancement (PRE), leading us to debut the term REversal of PRE by electron Spin SaturatION (REPRESSION). The shortening of electron Tm of any paramagnetic system as a function of electron spin saturation has not been reported to date, making REPRESSION a discovery of this study. The reversal of the paramagnetic dipolar shift is due to the decrease in electron spin order, also facilitated by electron spin saturation. This study offers new fundamental insights into PE under DNP conditions and a method to detect and identify NMR signal proximal to paramagnetic sites with reduced or minimal line broadening.

Shortening spin-lattice relaxation using a copper-chelated lipid at low-temperatures – A magic angle spinning solid-state NMR study on a membrane-bound protein

This article is not about DNP. However, the authors describe how to use paramagnetic relaxation enhancers to speed up the data acquisition and with this increase the sensitivity. A similar effect happens when a paramagnetic polarization agent is used in a DNP-NMR experiment and often the only reason why it is actually possible to run 1H-DNP-NMR experiments with recycling delays of several seconds is because of the relaxation enhancing nature of the polarizing agent.

Yamamoto, K., et al., Shortening spin-lattice relaxation using a copper-chelated lipid at low-temperatures – A magic angle spinning solid-state NMR study on a membrane-bound protein. J Magn Reson, 2013. 237(0): p. 175-81.

http://www.ncbi.nlm.nih.gov/pubmed/24246881

Inherent low sensitivity of NMR spectroscopy has been a major disadvantage, especially to study biomolecules like membrane proteins. Recent studies have successfully demonstrated the advantages of performing solid-state NMR experiments at very low and ultralow temperatures to enhance the sensitivity. However, the long spin-lattice relaxation time, T1, at very low temperatures is a major limitation. To overcome this difficulty, we demonstrate the use of a copper-chelated lipid for magic angle spinning solid-state NMR measurements on cytochrome-b5 reconstituted in multilamellar vesicles. Our results on multilamellar vesicles containing as small as 0.5mol% of a copper-chelated lipid can significantly shorten T1 of protons, which can be used to considerably reduce the data collection time or to enhance the signal-to-noise ratio. We also monitored the effect of slow cooling on the resolution and sensitivity of (13)C and (15)N signals from the protein and (13)C signals from lipids.

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