Journal of Physical Chemistry B, Vol.121, No.19, 4997-5006, 2017
Peptide and Protein Dynamics and Low-Temperature/DNP Magic Angle Spinning NMR
In DNP MAS NMR experiments at similar to 80-110 K, the structurally important -(CH3)-C-13 and -(NH3+)-N-15 signals in MAS spectra of biological samples disappear due to the interference of the molecular motions with the H-1 decoupling. Here we investigate the effect of these dynamic processes on the NMR line shapes and signal intensities in several typical systems: (1) microcrystalline APG, (2) membrane protein bR, (3) amyloid fibrils PI3-SH3, (4) monomeric alanine-CD3, and (5) the protonated and deuterated dipeptide N-Ac-VL over 78-300 K. In APG, the three-site hopping of the Ala-C beta peak disappears completely at 112 K, concomitant with the attenuation of CP signals from other C-13's and N-15's. Similarly, the N-15 signal from Ala-NH3+ disappears at similar to 173 K, concurrent with the attenuation in CP experiments of other N-15's as well as C-13's. In bR and PI3-SH3, the methyl groups are attenuated at similar to 95 K, while all other C-13's remain unaffected. However, both systems exhibit substantial losses of intensity at similar to 243 K. Finally, with spectra of Ala and N-Ac-VL, we show that it is possible to extract site specific dynamic data from the temperature dependence of the intensity losses. Furthermore, H-2 labeling can assist with recovering the spectral intensity. Thus, our study provides insight into the dynamic behavior of biological systems over a wide range of temperatures, and serves as a guide to optimizing the sensitivity and resolution of structural data in low temperature DNP MAS NMR spectra.