Thursday, September 18, 2008

Solid-State NMR - Volume 34 Issues 1-2

Combined NMR and computational study for azide binding to human manganese superoxide dismutase
Th. Emmler, I. Ayala, D. Silverman, S. Hafner, A.S. Galstyan, E.W. Knapp and G. Buntkowsky
Human manganese superoxide dismutase (MnSOD) labeled with 3-fluorotyrosine (Tyf) was complexed with the 15N-labeled inhibitor azide ([15N3−]). The sample was characterized by solid-state NMR (SSNMR) spectroscopy (19F-MAS and 15N-CPMAS). Employing 19F-15N-REDOR spectroscopy, we determined the distances between the fluorine label in Tyrosine-34 and the three 15N-nuclei of the azide and the relative orientation of the azide in the binding pocket of the MnSOD. A distance of R1=4.85 Å between the 19F-label of Tyf34 and the nearest 15N of the azide and an azide–fluorotyrosine Tyf34 angle of 90° were determined. These geometry data are employed as input for molecular modeling of the location of the inhibitor in the active site of the enzyme. In the computations, several possible binding geometries of the azide near the Mn-complex were assumed. Only when the azide replaces the water ligand at the Mn-complex we obtained a geometry of the azide–Mn-complex, which is consistent with the present NMR data. This indicates that the water molecule ligating to the Mn-complex is removed and the azide is placed at this position. As a consequence the azide forms an H bond with Gln143 instead with Tyf34, in contrast to non-19F-labeled MnSOD, where the azide is hydrogen bonded to the hydroxy group of Tyr34.

A new application for an old concept: Constant time (CT)-REDOR for an accurate determination of second moments in multiple spin systems with strong heteronuclear dipolar couplings
Thomas Echelmeyer, Leo van Wüllen, and Sebastian Wegner
In this contribution we present a constant time version of the well known REDOR pulse sequence which enables us to determine the second moments in multiple spin systems with strong dipolar couplings. From the resulting dipolar evolution curves, accurate values for the second moments can be obtained without the need to incorporate the full information about the detailed spin geometry of the multiple spin systems into the simulation protocol.

Characterization of local environments in crystalline borophosphates using single and double resonance NMR
Devidas B. Raskar, Hellmut Eckert, Bastian Ewald and Rüdiger Kniep
11B and 31P magic-angle spinning as well as 11B{31P} and 31P{11B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K3[BP3O9(OH)3], NH4[ZnBP2O8] and Rb3[B2P3O11(OH)2]. Dipolar second moment values extracted from the REDOR curves at short evolution times (ΔS/S00.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B–O–P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.

Lithium ionic jump motion in the fast solid ion conductor Li5La3Nb2O12
Barbara Koch and Michael Vogel
Using 7Li NMR line-shape analysis, spin–lattice relaxation measurements and stimulated-echo spectroscopy, we investigate the lithium ionic jump motion in the garnet Li5La3Nb2O12. Results for two samples are compared, which were annealed at (GR-850) and at (GR-900), respectively. All 7Li NMR data consistently show that two lithium species with distinguishable dynamical behaviors coexist in each of the samples. While the less mobile species is the majority component in GR-850, the more mobile species is the majority component in GR-900. 7Li NMR stimulated-echo spectroscopy provides straightforward access to the correlation functions describing the jumps of the respective majority component in both samples. From the temperature-dependent correlation times, we obtain activation energies of 56 and for GR-850 and GR-900, respectively. For both samples, the correlation functions substantially deviate from simple exponential behavior, indicating a high complexity of the lithium ionic motion in Li5La3Nb2O12.

13C CPMAS NMR and DFT calculations of anthocyanidins
M. Wolniak and I. Wawer
Anthocyanidins, red dyes from flower petals and fruits, are beneficial to human health. They attract considerable attention owing to their strong antioxidant and radical scavenging properties, however they are unstable in solution and available in small amounts only. 13C CP MAS NMR spectra were recorded to characterize solid-state conformation of nine anthocyanidins: apigenidin, pelargonidin, cyanidin, delphinidin, peonidin, malvidin robinetidin, luteolinidin and diosmetinidin chlorides. For some carbons, the solid-state chemical shifts were different from those obtained for solutions, indicating differences in conformation and intermolecular interactions. The principal elements of the 13C chemical shift tensor were measured for pelargonidin, cyanidin, delphinidin and malvidin chlorides using PASS-2D NMR technique. DFT GIAO calculations of shielding constants were performed for apigenidin and several geometric isomers of pelargonidin. Comparison of experimental 13C δii with the theoretical shielding parameters was helpful in predicting the most reliable geometry in the solid state. The cross-polarization parameters were obtained from variable-contact time experiments; TCH are longer and the values of T1ρH are shorter in the order: pelargonidin cyanidin delphinidin. It is probable that solid anthocyanidins become less ordered as the number of OH groups increases.

51V solid-state NMR investigations and DFT studies of model compounds for vanadium haloperoxidases
Annika Schweitzer, Torsten Gutmann, Maria Wächtler, Hergen Breitzke, Axel Buchholz, Winfried Plass and Gerd Buntkowsky
Three cis-dioxovanadium(V) complexes with similar N-salicylidenehydrazide ligands modeling hydrogen bonding interactions of vanadate relevant for vanadium haloperoxidases are studied by 51V solid-state NMR spectroscopy. Their parameters describing the quadrupolar and chemical shift anisotropy interactions (quadrupolar coupling constant CQ, asymmetry of the quadrupolar tensor ηQ, isotropic chemical shift δiso, chemical shift anisotropy δσ, asymmetry of the chemical shift tensor ησ and the Euler angles α, β and γ) are determined both experimentally and theoretically using DFT methods. A comparative study of different methods to determine the NMR parameters by numerical simulation of the spectra is presented. Detailed theoretical investigations on the DFT level using various basis sets and structural models show that by useful choice of the methodology, the calculated parameters agree to the experimental ones in a very good manner.

Spin-diffusion NMR at low field for the study of multiphase solids
M. Mauri, Y. Thomann, H. Schneider and K. Saalwächter
The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials is becoming increasingly popular, in particular for the study of heterogeneous polymers. Under conditions where T1 relaxation can be neglected, which is mostly the case at high field, analytical and approximate solutions to the evolution of spin diffusion are available. In order to extend the technique to more general conditions, we performed a comprehensive study of the diffusion of magnetization in a model copolymer at low field, where T1 tends to be of the same order of magnitude as the typical spin-diffusion time. In order to study the effects of T1 and to delineate the optimal T1 values for back correction prior to applying the initial-rate approximation, we developed a numerical simulation based on the diffusion equation and including longitudinal relaxation. We present and discuss the limits of simple correction strategies for initial-slope analysis based on apparent relaxation times from saturation-recovery experiments or the spin-diffusion experiments themselves. Our best strategy faithfully reproduces domain sizes obtained by both TEM investigations and full simultaneous fitting of spin-diffusion and saturation-recovery curves. Full fitting of such independent data sets not only yields correct domain sizes, but also the true longitudinal relaxation times, as well as spin-diffusion coefficients. Effects of interphases with distinct mobility on spin-diffusion curves, as well as practical hints concerning the reliable component decomposition of the detected low-resolution FID signal by help of different magnetization filters are also discussed in detail.

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