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

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.