Multiple-spin effects in fast magic angle spinning Lee-Goldburg cross-polarization experiments in uniformly labeled compounds

The proton–carbon polarization exchange in Lee–Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly 13C13C-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon–proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear 13C–1H13C–1H spin pairs, it is difficult to unambiguously extract structural constraints from experimental data. To better understand the complexity of the LG-CP processes, experiments on simple three- and four-spin systems are simulated and analyzed. More specifically, it is shown that in 13CH–13CH13CH–13CH and 13CH2–13C13CH2–13C spin systems, a significant amount of the proton polarization can be transferred to both carbons, despite the fact that the individual proton–carbon heteronuclear couplings between each proton and the carbon spins are very different. The dependence of the polarization transfer on the position of the proton carrier frequency is analyzed and it is shown that by an appropriate choice of this frequency, specific polarization transfer pathways can be selected. Experimental results from [U–13C][U–13C] tyrosine.HCl and [U–13C,[U–13C, 15N]15N] histidine.HCl.H2Ohistidine.HCl.H2O samples are in satisfactory agreement with simulations.Article / Letter to editorLIC/ES/Biophysical Organic Chemistr

Multiple-spin effects in fast magic angle spinning Lee-Goldburg cross-polarization experiments in uniformly labeled compounds

The proton–carbon polarization exchange in Lee–Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly 13C13C-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon–proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear 13C–1H13C–1H spin pairs, it is difficult to unambiguously extract structural constraints from experimental data. To better understand the complexity of the LG-CP processes, experiments on simple three- and four-spin systems are simulated and analyzed. More specifically, it is shown that in 13CH–13CH13CH–13CH and 13CH2–13C13CH2–13C spin systems, a significant amount of the proton polarization can be transferred to both carbons, despite the fact that the individual proton–carbon heteronuclear couplings between each proton and the carbon spins are very different. The dependence of the polarization transfer on the position of the proton carrier frequency is analyzed and it is shown that by an appropriate choice of this frequency, specific polarization transfer pathways can be selected. Experimental results from [U–13C][U–13C] tyrosine.HCl and [U–13C,[U–13C, 15N]15N] histidine.HCl.H2Ohistidine.HCl.H2O samples are in satisfactory agreement with simulations.Solid state NMR/Biophysical Organic Chemistr

Magnetization steps in Zn_(1-x)Mn_xO: Four largest exchange constants and single-ion anisotropy

Magnetization steps (MST's) from Mn pairs in several single crystals of Zn_(1-x)Mn_xO (0.0056<=x<=0.030, and in one powder (x=0.029), were observed. The largest two exchange constants, J1/kB=-18.2+/-0.5K and J1'/kB=-24.3+/-0.6K, were obtained from large peaks in the differential susceptibility, dM/dH, measured in pulsed magnetic fields, H, up to 500 kOe. These two largest J's are associated with the two inequivalent classes of nearest neighbors (NN's) in the wurtzite structure. The 29% difference between J1 and J1' is substantially larger than 13% in CdS:Mn, and 15% in CdSe:Mn. The pulsed-field data also indicate that, despite the direct contact between the samples and a superfluid-helium bath, substantial departures from thermal equilibrium occurred during the 7.4 ms pulse. The third- and fourth-largest J's were determined from the magnetization M at 20 mK, measured in dc magnetic fields H up to 90 kOe. Both field orientations H||c and H||[10-10] were studied. (The [10-10] direction is perpendicular to the c-axis, [0001].) By definition, neighbors which are not NN's are distant neighbors (DN's). The largest DN exchange constant (third-largest overall), has the value J/kB=-0.543+/-0.005K, and is associated with the DN at r=c. Because this is not the closest DN, this result implies that the J's do not decrease monotonically with the distance r. The second-largest DN exchange constant (fourth-largest overall), has the value J/kB=-0.080 K. It is associated with one of the two classes of neighbors that have a coordination number z=12, but the evidence is insufficient for a definite unique choice. The dependence of M on the direction of H gives D/kB=-0.039+/-0.008K, in fair agreement with -0.031 K from earlier EPR work.Comment: 12 pages, 10 figures. Submitted to PR

Structural constraints for the Crh protein from solid-state NMR experiments

We demonstrate that short, medium and long-range constraints can be extracted from proton mediated, rare-spin detected correlation solid-state NMR experiments for the microcrystalline 10.4 × 2 kDa dimeric model protein Crh. Magnetization build-up curves from cross signals in NHHC and CHHC spectra deliver detailed information on side chain conformers and secondary structure for interactions between spin pairs. A large number of medium and long-range correlations can be observed in the spectra, and an analysis of the resolved signals reveals that the constraints cover the entire sequence, also including inter-monomer contacts between the two molecules forming the domain-swapped Crh dimer. Dynamic behavior is shown to have an impact on cross signals intensities, as indicated for mobile residues or regions by contacts predicted from the crystal structure, but absent in the spectra. Our work validates strategies involving proton distance measurements for large and complex proteins as the Crh dimer, and confirms the magnetization transfer properties previously described for small molecules in solid protein samples

Exploring Chromophore-Binding Pocket: High-Resolution Solid-State 1H–13C Interfacial Correlation NMR Spectra with Windowed PMLG Scheme

High-resolution two-dimensional (2D) 1H–13C heteronuclear correlation spectra are recorded for selective observation of interfacial 3–5.5 Å contacts of the uniformly 13C-labeled phycocyanobilin (PCB) chromophore with its unlabeled binding pocket. The experiment is based on a medium- and long-distance heteronuclear correlation (MELODI–HETCOR) method. For improving 1H spectral resolution, a windowed phase-modulated Lee–Goldburg (wPMLG) decoupling scheme is applied during the t1 evolution period. Our approach allows for identification of chromophore–protein interactions, in particular for elucidation of the hydrogen-bonding networks and charge distributions within the chromophore-binding pocket. The resulting pulse sequence is tested on the cyanobacterial (Cph1) phytochrome sensory module (residues 1–514, Cph1Δ2) containing uniformly 13C- and 15N-labeled PCB chromophore (u-[13C,15N]-PCB-Cph1Δ2) at 17.6 T