Peptide and Protein Dynamics and Low-Temperature/DNP Magic Angle Spinning NMR

Abstract

In DNP MAS NMR experiments at ∼80–110 K, the structurally important −¹³CH₃ and −¹⁵NH₃⁺ signals in MAS spectra of biological samples disappear due to the interference of the molecular motions with the ¹H 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-Cβ peak disappears completely at 112 K, concomitant with the attenuation of CP signals from other ¹³C’s and ¹⁵N’s. Similarly, the ¹⁵N signal from Ala-NH₃⁺ disappears at ∼173 K, concurrent with the attenuation in CP experiments of other 15N’s as well as 13C’s. In bR and PI3-SH3, the methyl groups are attenuated at ∼95 K, while all other 13C’s remain unaffected. However, both systems exhibit substantial losses of intensity at ∼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, 2H 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.