Nitrogen-15 Nuclei Research Articles

Hyperpolarization of nitrogen-15 nuclei by cross polarization and dissolution dynamic nuclear polarization.

Dynamic Nuclear Polarization (DNP) is often achieved by the direct transfer of polarization from electrons to nuclei such as 13C, induced by microwave saturation of the wings of narrow EPR lines of radicals like trityl. In the indirect approach on the other hand, DNP is used to transfer the polarization from the electrons of radicals such as nitroxides that have broad EPR lines to nuclear spins I = 1H, followed by cross-polarization (CP) from I = 1H to S = 13C or other nuclei with low gyromagnetic ratios. This approach is particularly attractive for S = 15N, since direct DNP yields modest polarizations P(15N) < 4% with build-up times that can be as long as τDNP(15N) > 2 h. In this paper, we show that CP from 1H to 15N at 1.2 K can yield P(15N) = 25% with τCP-DNP(15N) = 10-15 min. After rapid dissolution and transfer to a solution-state NMR spectrometer, a polarization P(15N) = 20% was observed at 300 K. The longitudinal relaxation times in solution can be as long as T1(15N) > 800 s in favorable cases.

Apparent transverse relaxation rates in systems with coupled carbon-13 spins

In systems with homonuclear scalar couplings, the envelopes of spin echoes obtained with simple refocusing pulses or trains of such pulses are normally modulated so that it is difficult to extract transverse relaxation rates. It has been shown recently that echo modulations can be quenched by cumulative pulse errors that arise after applying a large number of refocusing pulses with moderate rf amplitudes. The resulting unmodulated decays allow one to extract apparent transverse relaxation rates. Early work on systems comprising only two nitrogen-15 nuclei or two carbon-13 spins has recently been extended to systems with coupled protons. This work focuses on systems with three coupled carbon-13 spins, which in turn are coupled to several neighbouring protons. Unmodulated echo trains can be obtained by optimizing the pulse interval, the carrier frequency and the rf amplitude of the refocusing pulses.

Compensating increases in protein backbone flexibility occur when the Dead ringer AT‐rich interaction domain (ARID) binds DNA: A nitrogen‐15 relaxation study

AT-rich interaction domains (ARIDs) are found in a large number of eukaryotic transcription factors that regulate cell proliferation, differentiation, and development. Previously we elucidated how ARIDs recognize DNA by determining the solution structure of the Drosophila melanogaster Dead ringer protein in both its DNA-free and -bound states. In order to quantitatively determine how ARIDs alter their mobility to recognize DNA, we have measured the relaxation parameters of the backbone nitrogen-15 nuclei of Dead ringer in its free and bound forms, and interpreted these data using the model-free approach. We show that Dead ringer undergoes significant changes in its mobility upon binding, with residues in the loop connecting helices H5 and H6 becoming immobilized in the major groove and contacts to the minor groove slowing down the motion of residues at the C terminus. A DNA-induced rotation and displacement of the N-terminal subdomain of the protein increases the mobility of helix H1 located distal to the DNA interface and may partially negate the entropic cost of immobilizing interfacial residues. Elevated motions on the micro- to millisecond timescale in the N-terminal domain prior to DNA binding appear to foreshadow the DNA-induced conformation change.

Evidence for slow motion in proteins by multiple refocusing of heteronuclear nitrogen/proton multiple quantum coherences in NMR.

A novel NMR method characterizes slow motions in proteins by multiple refocusing of double- and zero-quantum coherences of amide protons and nitrogen-15 nuclei. If both nuclei experience changes in their isotropic chemical shifts because of internal motions on slow time scales (mus - ms), this leads to a difference in the relaxation rates of double- and zero-quantum coherences. This is due to CSM/CSM (chemical shift modulation) cross-correlation effects that are related to the well-known chemical exchange contribution Rex to the decay rate R2 = 1/T2 of nitrogen-15 nuclei. The CSM/CSM contributions can be distinguished from other mechanisms through their dependence on the repetition rate of a Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing sequence. In ubiquitin, motional processes can be identified that could hitherto not be observed by conventional CPMG nitrogen-15 NMR.

Hydrogen bonds lengths in nucleic acids estimated from longitudinal nitrogen-15 relaxation

A new NMR method has been designed for the measurement of the longitudinal relaxation rates of both donor and acceptor nitrogen-15 nuclei in Watson–Crick base pairs in 15N-enriched nucleic acids. The experiment was applied to a 22-nucleotide RNA hairpin. The lengths of four hydrogen bonds could be estimated from the longitudinal relaxation rates.

Quasi-Spectral-Density Function Analysis for Nitrogen-15 Nuclei in Proteins

Quasi-Spectral-Density Function Analysis for Nitrogen-15 Nuclei in Proteins

Overcoming Solvent Saturation-Transfer Artifacts in Protein NMR at Neutral pH. Application of Pulsed Field Gradients in Measurements of 1H- 15N Overhauser Effects

Artifacts due to solvent saturation-transfer effects result in incorrect measurements of 1H- 15N heteronuclear NOE (HNOE). These artifacts are commonly observed in aqueous protein solutions at neutral pH. We describe the application of pulsed field gradients (PFGs) together with long recycle delays in overcoming errors in HNOE measurements which arise from H 2O solvent preirradiation and solvent saturation transfer. Even in the absence of explicit solvent irradiation, the HNOE pulse sequence itself results in a nonequilibrium spin-state distribution of solvent nuclei which can then be transferred by chemical exchange into amide-proton sites. This effect can be avoided by using PFGs for suppression of solvent H 2O together with a recycle delay sufficiently long for the magnetization of water to relax back to equilibrium values during the preparation period. These effects were studied in 15N-enriched human type α transforming growth factor at pH 7.1. Comparisons of PFG-HNOE experiments with and without selective H 2O irradiation and with different recycle times provide estimates of the effects of solvent irradiation on HNOE measurements, which are different for different amide nitrogen-15 nuclei. The amplitudes of these artificial HNOE enhancements are roughly correlated with solvent accessibilities of amide sites in the three-dimensional structure of hTGFα.

NMR chemical shifts and coupling constants of various nuclei of N, N-diethylbenzenesulfenamide, N, N-diethylbenzeneseleneamide and some related sulfur amides

The 77Se, 15N, 13C and 1H NMR parameters have been determined for N, N-diethylbenzenesulfen- and N, N-diethylbenzeneseleneamide and the 1H 13C coupling constants additionally for N, N-dimethylmethanesulfinamide, N, N-dimethylbenzenesulfinamide, N, N-dimethylmethanesulfonamide, N, N-dimethylbenzenesulfonamide and diphenylsulfone. The nitrogen-15 nucleus is slightly more shielded in the sulfenamide than in the seleneamide. Compared to sulfin- and sulfonamides the nitrogen nuclei of these amides are more shielded. The solvent effects on the chemical shift of both selenium-77 and nitrogen-15 nuclei are observable. The nitrogen-15 chemical shifts of sulfur and seleneamides are less sensitive to the solvents than those of carboxamides. This change in the nitrogen-15 chemical shift in sulfen- and seleneamide is the same order of magnitude, but opposite in direction compared with that in sulfon- and sulfinamides. The 1H 13C direct aromatic coupling constants increase along with the increasing oxidation state of sulfur in sulfur amides, being greatest in sulfonamides.

Sensitivity-enhanced NMR detection of nonprotonated nitrogen-15 nuclei

Sensitivity-enhanced NMR detection of nonprotonated nitrogen-15 nuclei

Nitrogen-15 nuclear magnetic resonance spectroscopy of adsorbed molecules

Nitrogen-15 n.m.r. spectroscopy has been applied to the study of the interactions between solid surfaces and molecules adsorbed on them. Nitrogen-15 spectra of ammonia, trimethylamine, pyridine and acetonitrile molecules sorbed in various zeolites were measured at 9.12 MHz by means of the conventional Fourier-transform n.m.r. technique. In all measurements carried out, substances were employed which were enriched with nitrogen-15 nuclei (ca. 95 %).The resonance shifts depend strongly on the nature of adsorption sites which may occur in the zeolites (e.g. Na+ cations, Bronsted- and Lewis-acid sites).The results clearly reveal the advantage of nitrogen-15 n.m.r. investigations in characterizing acidic properties in comparison with the carbon-13 n.m.r. measurements performed until now on adsorbate–adsorbent systems and emphasize that nitrogen-15 spectroscopy may become a powerful tool for the study of surface phenomena.