Evidence from 900 MHz ¹H MAS NMR of Displacive Behavior of the Model Order–Disorder Antiferroelectric NH₄H₂AsO₄

NH₄H₂AsO₄ (ADA) is a model compound for understanding the mechanism of phase transitions in the KH₂PO₄ (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at <i>T</i>_N ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (¹H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, <i>T</i>₁, of the O–H···O and NH₄⁺ protons through the <i>T</i>_N. As <i>T</i> → <i>T</i>_N, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its <i>T</i>_N, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at <i>T</i>_N, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH₄⁺ protons. Both sets of protons exhibited order–disorder characteristics in their <i>T</i>₁ data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into <i>order–disorder</i> and <i>displacive</i> ones, should rather be “<i>order–disorder cum displacive</i>” type

¹⁷O MAS NMR Correlation Spectroscopy at High Magnetic Fields

The structure of two protected amino acids, FMOC-l-leucine and FMOC-l-valine, and a dipeptide, <i>N</i>-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing ¹⁷O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6–35.2 T/750–1500 MHz for ¹H) the ¹⁷O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected amino acids and the three distinct oxygen environments of the dipeptide. The one- and two-dimensional, ¹⁷O, ¹⁵N–¹⁷O, ¹³C–¹⁷O, and ¹H–¹⁷O double-resonance correlation experiments performed on the uniformly ¹³C,¹⁵N and 70% ¹⁷O-labeled dipeptide prove the attainability of ¹⁷O as a probe for structure studies of biological systems. ¹⁵N–¹⁷O and ¹³C–¹⁷O distances were measured via one-dimensional REAPDOR and ZF-TEDOR experimental buildup curves and determined to be within 15% of previously reported distances, thus demonstrating the use of ¹⁷O NMR to quantitate interatomic distances in a fully labeled dipeptide. Through-space hydrogen bonding of N-Ac-VL was investigated by a two-dimensional ¹H-detected ¹⁷O R³-R-INEPT experiment, furthering the importance of ¹⁷O for studies of structure in biomolecular solids

Structural Model of the Tubular Assembly of the Rous Sarcoma Virus Capsid Protein

The orthoretroviral capsid protein (CA) assembles into polymorphic capsids, whose architecture, assembly, and stability are still being investigated. The N-terminal and C-terminal domains of CA (NTD and CTD, respectively) engage in both homotypic and heterotypic interactions to create the capsid. Hexameric turrets formed by the NTD decorate the majority of the capsid surface. We report nearly complete solid-state NMR (ssNMR) resonance assignments of Rous sarcoma virus (RSV) CA, assembled into hexamer tubes that mimic the authentic capsid. The ssNMR assignments show that, upon assembly, large conformational changes occur in loops connecting helices, as well as the short 3₁₀ helix initiating the CTD. The interdomain linker becomes statically disordered. Combining constraints from ssNMR and cryo-electron microscopy (cryo-EM), we establish an atomic resolution model of the RSV CA tubular assembly using molecular dynamics flexible fitting (MDFF) simulations. On the basis of comparison of this MDFF model with an earlier-derived crystallographic model for the planar assembly, the induction of curvature into the RSV CA hexamer lattice arises predominantly from reconfiguration of the NTD–CTD and CTD trimer interfaces. The CTD dimer and CTD trimer interfaces are also intrinsically variable. Hence, deformation of the CA hexamer lattice results from the variable displacement of the CTDs that surround each hexameric turret. Pervasive H-bonding is found at all interdomain interfaces, which may contribute to their malleability. Finally, we find helices at the interfaces of HIV and RSV CA assemblies have very different contact angles, which may reflect differences in the capsid assembly pathway for these viruses

HIV‑1 Capsid Function Is Regulated by Dynamics: Quantitative Atomic-Resolution Insights by Integrating Magic-Angle-Spinning NMR, QM/MM, and MD

HIV-1 CA capsid protein possesses intrinsic conformational flexibility, which is essential for its assembly into conical capsids and interactions with host factors. CA is dynamic in the assembled capsid, and residues in functionally important regions of the protein undergo motions spanning many decades of time scales. Chemical shift anisotropy (CSA) tensors, recorded in magic-angle-spinning NMR experiments, provide direct residue-specific probes of motions on nano- to microsecond time scales. We combined NMR, MD, and density-functional-theory calculations, to gain quantitative understanding of internal backbone dynamics in CA assemblies, and we found that the dynamically averaged ¹⁵N CSA tensors calculated by this joined protocol are in remarkable agreement with experiment. Thus, quantitative atomic-level understanding of the relationships between CSA tensors, local backbone structure, and motions in CA assemblies is achieved, demonstrating the power of integrating NMR experimental data and theory for characterizing atomic-resolution dynamics in biological systems