2H-13C HETCOR MAS NMR for indirect detection of 2H quadrupole patterns and spin-lattice relaxation rates

Carbon-detected deuterium solid-state NMR rotating frame relaxation measurements for protein methyl groups under magic angle spinning

Deuterium rotating frame solid-state NMR relaxation measurements (2H R1ρ) are important tools in quantitative studies of molecular dynamics. We demonstrate how 2H to 13C cross-polarization (CP) approaches under 10-40 kHz magic angle spinning rates can be combined with the 2H R1ρ blocks to allow for extension of deuterium rotating frame relaxation studies to methyl groups in biomolecules. This extension permits detection on the 13C nuclei and, hence, for the achievement of site-specific resolution. The measurements are demonstrated using a nine-residue low complexity peptide with the sequence GGKGMGFGL, in which a single selective -13CD3 label is placed at the methionine residue. Carbon-detected measurements are compared with the deuterium direct-detection results, which allows for fine-tuning of experimental approaches. In particular, we show how the adiabatic respiration CP scheme and the double adiabatic sweep on the 2H and 13C channels can be combined with the 2H R1ρ relaxation rates measurement. Off-resonance 2H R1ρ measurements are investigated in addition to the on-resonance condition, as they extent the range of effective spin-locking field.

Calcium interactions in amelogenin-derived peptide assembly

Phosphorylation of serine residues has been recognized as a pivotal event in the evolution of mineralized tissues in many biological systems. During enamel development, the extracellular matrix protein amelogenin is most abundant and appears to be critical to the extreme high aspect ratios (length:width) of apatite mineral fibers reaching several millimeters in larger mammalian teeth. A 14-residue peptide (14P2, residues Gly8 to Thr21) was previously identified as a key sequence mediating amelogenin assembly formation, the domain also contains the native single phosphoserine residue (Ser16) of the full-length amelogenin. In this research, 14P2 and its phosphorylated form (p14P2) were investigated at pH 6.0 with various calcium and phosphate ion concentrations, indicating that both peptides could self-assemble into amyloid-like conformation but with differences in structural details. With calcium, the distance between ³¹P within the p14P2 self-assemblies is averaged to be 4.4 ± 0.2Å, determined by solid-state NMR ³¹P PITHIRDS-CT experiments. Combining with other experimental results, solid-state Nuclear Magnetic Resonance (SSNMR) suggests that the p14P2 self-assemblies are in parallel in-register β-sheet conformation and divalent calcium ions most likely connect two adjacent peptide chains by binding to the phosphate group of Ser16 and the carboxylate of Glu18 side-chain. This study on the interactions between calcium ions and amelogenin-derived peptides provides insights on how amelogenin may self-assemble in the presence of calcium ions in early enamel development.

Deuteration for High-Resolution Detection of Protons in Protein Magic Angle Spinning (MAS) Solid-State NMR

Proton detection developed in the last 20 years as the method of choice to study biomolecules in the solid state. In perdeuterated proteins, proton dipolar interactions are strongly attenuated, which allows yielding of high-resolution proton spectra. Perdeuteration and backsubstitution of exchangeable protons is essential if samples are rotated with MAS rotation frequencies below 60 kHz. Protonated samples can be investigated directly without spin dilution using proton detection methods in case the MAS frequency exceeds 110 kHz. This review summarizes labeling strategies and the spectroscopic methods to perform experiments that yield assignments, quantitative information on structure, and dynamics using perdeuterated samples. Techniques for solvent suppression, H/D exchange, and deuterium spectroscopy are discussed. Finally, experimental and theoretical results that allow estimation of the sensitivity of proton detected experiments as a function of the MAS frequency and the external B₀ field in a perdeuterated environment are compiled.

Spontaneous Keto-Enol Tautomerization in the Crystal Lattice Visualized with the Help of Water Encapsulated in Hydrophilic Reservoirs

Keto-enol tautomerism in the solid phase is a process that is particularly difficult to follow. In this work we demonstrate how it can be done by introducing deuterium into the crystal lattice of organic compounds which tend to form hydrates. In our studies we explored H-D exchange in the crystals stored in contact with deuterium oxide vapors. Employing barbituric acid (BA) and (+)-catechin (CAT) as model samples and by using advanced solid-state NMR spectroscopy and mass spectrometry, we revealed that not only OH and NH protons of these chemicals undergo exchange to deuterium in a crystal lattice, but also usually immobile protons, that is, (Ar)CH (in CAT) and CH₂ (in BA) are exchanged as a result of keto-enol tautomerism.

Applications of Solid-State NMR Spectroscopy for the Study of Lipid Membranes with Polyphilic Guest (Macro)Molecules

The incorporation of polymers or smaller complex molecules into lipid membranes allows for property modifications or the introduction of new functional elements. The corresponding molecular-scale details, such as changes in dynamics or features of potential supramolecular structures, can be studied by a variety of solid-state NMR techniques. Here, we review various approaches to characterizing the structure and dynamics of the guest molecules as well as the lipid phase structure and dynamics by different high-resolution magic-angle spinning proton and 13C NMR experiments as well as static 31P NMR experiments. Special emphasis is placed upon the incorporation of novel synthetic polyphilic molecules such as shape-persistent T- and X-shaped molecules as well as di- and tri-block copolymers. Most of the systems studied feature dynamic heterogeneities, for instance those arising from the coexistence of different phases; possibilities for a quantitative assessment are of particular concern.

Site-Specific Internal Motions in GB1 Protein Microcrystals Revealed by 3D ²H-¹³C-¹³C Solid-State NMR Spectroscopy

(2)H quadrupolar line shapes deliver rich information about protein dynamics. A newly designed 3D (2)H-(13)C-(13)C solid-state NMR magic angle spinning (MAS) experiment is presented and demonstrated on the microcrystalline β1 immunoglobulin binding domain of protein G (GB1). The implementation of (2)H-(13)C adiabatic rotor-echo-short-pulse-irradiation cross-polarization (RESPIRATION CP) ensures the accuracy of the extracted line shapes and provides enhanced sensitivity relative to conventional CP methods. The 3D (2)H-(13)C-(13)C spectrum reveals (2)H line shapes for 140 resolved aliphatic deuterium sites. Motional-averaged (2)H quadrupolar parameters obtained from the line-shape fitting identify side-chain motions. Restricted side-chain dynamics are observed for a number of polar residues including K13, D22, E27, K31, D36, N37, D46, D47, K50, and E56, which we attribute to the effects of salt bridges and hydrogen bonds. In contrast, we observe significantly enhanced side-chain flexibility for Q2, K4, K10, E15, E19, N35, N40, and E42, due to solvent exposure and low packing density. T11, T16, and T17 side chains exhibit motions with larger amplitudes than other Thr residues due to solvent interactions. The side chains of L5, V54, and V29 are highly rigid because they are packed in the core of the protein. High correlations were demonstrated between GB1 side-chain dynamics and its biological function. Large-amplitude side-chain motions are observed for regions contacting and interacting with immunoglobulin G (IgG). In contrast, rigid side chains are primarily found for residues in the structural core of the protein that are absent from protein binding and interactions.

Revisiting NMR composite pulses for broadband (2)H excitation

Quadrupolar echo NMR spectroscopy of static solids often requires RF excitation that covers spectral widths exceeding 100 kHz, which is difficult to obtain due to instrumental limitations. In this work we revisit four well-known composite pulses (COM-I, II, III and IV) for broadband excitation in deuterium quadrupolar echo spectroscopy. These composite pulses are combined with several phase cycling schemes that were previously shown to decrease finite pulse width distortions in deuterium solid-echo experiments performed with two single pulses. The simulations and experiments show that COM-II and IV composite pulses combined with an 8-step phase cycling aid in achieving broadband excitation with limited pulse width distortions.

Structural hysteresis in dragline spider silks induced by supercontraction: An x-ray fiber micro-diffraction study

Interaction with water causes shrinkage and significant changes in the structure of spider dragline silks, which has been referred to as supercontraction in the literature. Preferred orientation or alignment of protein chains with respect to the fiber axis is extensively changed during this supercontraction process. Synchrotron x-ray micro-fiber diffraction experiments have been performed on Nephila clavipes and Argiope aurantia major and minor ampullate dragline spider fibers in the native dry, contracted (by immersion in water) and restretched (from contracted) states. Changes in the orientation of β-sheet nanocrystallites and the oriented component of the amorphous network have been determined from wide-angle x-ray diffraction patterns. While both the crystalline and amorphous components lose preferred orientation on wetting with water, the nano-crystallites regain their orientation on wet-restretching, whereas the oriented amorphous components only partially regain their orientation. Dragline major ampullate silks in both the species contract more than their minor ampullate silks.

Correlation 2D-NMR experiments involving both 13C and 2H isotopes in oriented media: methodological developments and analytical applications

Correlation 2D-NMR experiments for (13)C and (2)H isotopes turn out to be powerful methods for the assignment of the quadrupolar doublets in the (2)H NMR spectra of isotopically modified (polydeuterated or perdeuterated) or unmodified solutes in homogeneously oriented solvents, such as thermotropic systems or lyotropic liquid crystals. We review here the different pulse sequences, which have been employed, their properties, and their most salient applications. These 2D-NMR sequences have been used for (i) (13)C-(2)H correlation with and without (1)H relay and (ii) (2)H-(2)H correlation with (13)C relay. The (13)C-(2) H correlation experiments without (1)H relay have been achieved for specifically deuterated or non-selectively deuterated analytes, but also more recently for isotopically unmodified ones thanks to the high sensitivity of very high-field NMR spectrometers (21.1 T) equipped with cryogenic probes. The (13)C-(2)H correlation 2D-NMR experiments are especially useful for the assignment of overcrowded deuterium spectra because the (2)H signals are correlated to (13)C signals, which benefit from a much larger dispersion of chemical shifts. In this contribution, particular attention will be paid to the use of correlation 2D-NMR experiments for (2)H and (13)C nuclei in weakly aligning, polypeptide oriented chiral solvents, because these methods are useful and original tools for enantiomeric and enantiotopic analyses.

Elucidating proline dynamics in spider dragline silk fibre using 2H–13C HETCOR MAS NMR

²H–¹³C HETCOR MAS NMR is performed on ²H/¹³C/¹⁵N-Pro enriched A. aurantia dragline silk. Proline dynamics are extracted from ²H NMR line shapes and T₁ in a site-specific manner to elucidate the backbone and side chain molecular dynamics for the MaSp2 GPGXX β-turn regions for spider dragline silk in the dry and wet, supercontracted states.