Monday, January 31, 2011

Solid-State NMR and Density Functional Theory Studies of Ionization States of Thiamin

J. Phys. Chem. B, 2011, 115 (4), pp 730–736

Thiamin diphosphate (ThDP) is a key coenzyme in sugar metabolism. The 4′-aminopyrimidine ring of ThDP cycles through several ionization and tautomeric states during enzyme catalysis, but it is not fully understood which states are adopted during the individual steps of the catalytic cycle. Thiamin has been synthesized with labels selectively inserted into the C2 and C6′ positions, as well as into the amino group, creating [C2, C6′-13C2] thiamin and [N4′-15N] thiamin. Magic-angle spinning (MAS) NMR spectroscopy has been employed to record the 13C and 15N chemical shift anisotropy (CSA) tensors for C2, C6′, and N4′ atoms. Our results indicate that the isotropic chemical shifts as well as the principal components of the 13C and 15N CSA tensors are very sensitive to the protonation states in these compounds and, therefore, permit differentiating between the two ionization states, 4-aminopyrimidine and 4-aminopyrimidinium. Using density functional theory (DFT), we have calculated the magnetic shielding anisotropy tensors of C2, C6′, and N4′ and found excellent agreement between the computed and the experimental tensors. Our findings indicate that MAS NMR spectroscopy in conjunction with DFT calculations is a sensitive probe of ionization states in the thiamin cofactor. The results of this study will serve as a guide for characterization of ionization and tautomeric states of thiamin in complexes with thiamin-dependent enzymes.

Siting and Mobility of Deuterium Absorbed in Cosputtered Mg0.65Ti0.35. A MAS 2H NMR Study

J. Phys. Chem. C, 2011, 115 (1), pp 288–297

Nanostructured magnesium titanium alloys are interesting lightweight materials for chemical hydrogen storage. We have therefore investigated the siting and dynamics of deuterium absorbed in a Mg0.65Ti0.35 alloy generated by magnetron cosputtering, and made a comparison to the corresponding features in bulk samples of deuterium-loaded Mg0.65Ti0.35 and Mg0.65Sc0.35 prepared by ball-milling and melt-casting, respectively. Magic-angle spinning 2H NMR of cosputtered Mg0.65Ti0.35D1.1 shows partly resolved signals of deuterium located in nonconductive domains at tetrahedral Mg4 and mixed MgnTi4−n sites (4 ppm) and deuterium at Ti4 sites in conducting TiD2 nanodomains (−29 and −68 ppm). No bulk TiD2 signal at −150 ppm is observed, in contrast to what we find in ball-milled Mg0.65Ti0.35D0.65, which is largely phase separated. The deuterium species with shift values of 4 and −29 ppm undergo complete exchange at a subsecond time scale in one- and two-dimensional exchange NMR and must therefore be close together in the lattice. In contrast, deuterium resonating at −68 ppm does not show deuterium exchange and thus appears to be located at more stable sites. The observed deuterium exchange and the reduced Knight shift compared to bulk TiD2 are explained using a model with TiD2 nanoslabs.

Towards Portable High-Resolution NMR Spectroscopy†

Angew. Chem. Int. Ed. 2011, 50, 354 – 356

Analysis on the go: Portable high-resolution NMR spectroscopy is of great interest for many applications. Recent advances in magnet design, spectrometer stability, and acquisition schemes have placed the realization of low-field spectrometers based on room-temperature permanent magnets and that can deliver chemical shift resolution within reach.

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