Category Archives: Methyl Group Rotors

Methyl dynamics in amino acids modulate heteronuclear cross relaxation in the solid state under MAS DNP #DNPNMR

Aladin, Victoria, and Björn Corzilius. “Methyl Dynamics in Amino Acids Modulate Heteronuclear Cross Relaxation in the Solid State under MAS DNP.” Solid State Nuclear Magnetic Resonance 99 (July 2019): 27–35.

https://doi.org/10.1016/j.ssnmr.2019.02.004

Dynamic Nuclear Polarization (DNP) is a wide-spread technique for sensitivity enhancement of MAS NMR. During a typical MAS DNP experiment, several mechanisms resulting in polarization transfer may be active at the same time. One such mechanism which is most commonly active but up to now mostly disregarded is SCREAM-DNP (Specific Cross Relaxation Enhancement by Active Motions under DNP). This effect is generally observed in direct DNP experiments if molecular dynamics are supporting heteronuclear cross relaxation similar to the nuclear Overhauser effect. We investigate this effect for the CH3 groups of all methyl-bearing amino acids (i.e., alanine, valine, leucine, isoleucine, threonine, and methionine). At the typical DNP temperature of ∼110 K the three-fold reorientation dynamics are still active, and efficient SCREAM-DNP is observed. We discuss variations in enhancement factors obtained by this effect in context of sample temperature and sterical hindrance of the methyl group. Next to the direct transfer to the methyl carbon, we also find evidence for much weaker transfer from the methyl protons directly to other carbons in the amino acid molecule and succeed to correlate build-up dynamics with the CH dipole coupling which is modulated by the CH3 orientation. Besides methyl dynamics we also identify ring dynamics within proline as a source of SCREAM-DNP. Our results are the first step towards utilization of this effect as a specific probing techniqueusing methyl groups in protein systems.

Quantum-rotor-induced polarization

Meier, Benno. “Quantum-Rotor-Induced Polarization.” Magnetic Resonance in Chemistry 0, no. 0 (2018).

https://doi.org/10.1002/mrc.4725

Quantum-rotor-induced polarization is closely related to para-hydrogen-induced polarization. In both cases, the hyperpolarized spin order derives from rotational interaction and the Pauli principle by which the symmetry of the rotational ground state dictates the symmetry of the associated nuclear spin state. In quantum-rotor-induced polarization, there may be several spin states associated with the rotational ground state, and the hyperpolarization is typically generated by hetero-nuclear cross-relaxation. This review discusses preconditions for quantum-rotor-induced polarization for both the 1-dimensional methyl rotor and the asymmetric rotor H217O@C60, that is, a single water molecule encapsulated in fullerene C60. Experimental results are presented for both rotors.

Dynamic Nuclear Polarization of Long-Lived Nuclear Spin States in Methyl Groups

Dumez, J.-N., et al., Dynamic Nuclear Polarization of Long-Lived Nuclear Spin States in Methyl Groups. The Journal of Physical Chemistry Letters, 2017. 8(15): p. 3549-3555.

http://dx.doi.org/10.1021/acs.jpclett.7b01512

We have induced hyperpolarized long-lived states in compounds containing 13C-bearing methyl groups by dynamic nuclear polarization (DNP) at cryogenic temperatures, followed by dissolution with a warm solvent. The hyperpolarized methyl long-lived states give rise to enhanced antiphase 13C NMR signals in solution, which often persist for times much longer than the 13C and 1H spin–lattice relaxation times under the same conditions. The DNP-induced effects are similar to quantum-rotor-induced polarization (QRIP) but are observed in a wider range of compounds because they do not depend critically on the height of the rotational barrier. We interpret our observations with a model in which nuclear Zeeman and methyl tunnelling reservoirs adopt an approximately uniform temperature, under DNP conditions. The generation of hyperpolarized NMR signals that persist for relatively long times in a range of methyl-bearing substances may be important for applications such as investigations of metabolism, enzymatic reactions, protein–ligand binding, drug screening, and molecular imaging.

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