Biomedical subjects

Flemming H Larsen

Publications and source records attributed to Flemming H Larsen.

3 recordsLinked to original sources

PubMed2005

Separation of 47Ti and 49Ti solid-state NMR lineshapes by static QCPMG experiments at multiple fields.

Experimental procedures are proposed and demonstrated that separate the spectroscopic contribution from both (47)Ti and (49)Ti in solid-state nuclear magnetic resonance spectra. These take advantage of the different nuclear spin quantum numbers of these isotopes that lead to different "effective" radiofrequency fields for the central transition nutation frequencies when these nuclei occur in sites with a significant electric field gradient. Numerical simulations and solid-state NMR experiments were performed on the TiO(2) polymorphs anatase and rutile. For anatase, the separation of the two isotopes at high field (21.1T) facilitated accurate determination of the electric field gradient (EFG) and chemical shift anisotropy (CSA) tensors. This was accomplished by taking advantage of the quadrupolar interaction between the EFG at the titanium site and the different magnitudes of the nuclear quadrupole moments (Q) of the two isotopes. Rutile, having a larger quadrupolar coupling constant (C(Q)), was examined by (49)Ti-selective experiments at different magnetic fields to obtain spectra with different scalings of the two anisotropic tensors. A small chemical shielding anisotropy (CSA) of -30 ppm was determined.

Journal Article↗

PubMed2004

Structure and biochemical function of a prototypical Arabidopsis U-box domain.

U-box proteins, as well as other proteins involved in regulated protein degradation, are apparently over-represented in Arabidopsis compared with other model eukaryotes. The Arabidopsis protein AtPUB14 contains a typical U-box domain followed by an Armadillo repeat region, a domain organization that is frequently found in plant U-box proteins. In vitro ubiquitination assays demonstrated that AtPUB14 functions as an E3 ubiquitin ligase with specific E2 ubiquitin-conjugating enzymes. The structure of the AtPUB14 U-box domain was determined by NMR spectroscopy. It adopts the betabetaalphabeta fold of the Prp19p U-box and RING finger domains. In these proteins, conserved hydrophobic residues form a putative E2-binding cleft. By contrast, they contain no common polar E2 binding site motif. Two hydrophobic cores stabilize the AtPUB14 U-box fold, and hydrogen bonds and salt bridges interconnect the residues corresponding to zinc ion-coordinating residues in RING domains. Residues from a C-terminal alpha-helix interact with the core domain and contribute to stabilization. The Prp19p U-box lacks a corresponding C-terminal alpha-helix. Chemical shift analysis suggested that aromatic residues exposed at the N terminus and the C-terminal alpha-helix of the AtPUB14 U-box participate in dimerization. Thus, AtPUB14 may form a biologically relevant dimer. This is the first plant U-box structure to be determined, and it provides a model for studies of the many plant U-box proteins and their interactions. Structural insight into these interactions is important, because ubiquitin-dependent protein degradation is a prevalent regulatory mechanism in plants.

Amino Acid Sequence↗

PubMed2004

Molecular dynamics of half-integer quadrupolar nuclei studied by QCPMG solid-state NMR experiments on static and rotating samples. Theory and simulations.

Simulations of QCPMG NMR type experiments have been used to explore dynamic processes of half-integer quadrupolar nuclei in solids. By setting up a theoretical approach that is well suited for efficient numerical simulations the QCPMG type experiments have been analyzed regarding the effect of the magnitude of the EFG- and CSA-tensors, the spin-quantum number, different dynamical processes and MAS. Compared to the QE experiment the QCPMG experiment offers not only intensity gain by an order of magnitude and changes in overall lineshape as a function of the kinetic rate constant but the lineshape of the individual spin-echo sidebands is also very sensitive towards dynamics. Hereby a visual identification of the dynamics is obtained. In common for all the simulations the spin-echo sidebands are narrow in the slow (k< or =10(2) Hz) and the fast (k> or =10(7) Hz) dynamic regime whereas they are broadened in the intermediate regime 10(3)< or =k< or =10(7) Hz. The maximum intensity of the spin-echo sidebands for two-site jumps is highly dependent on the type of anisotropic interactions involved and the type of QCPMG experiment. Hence, in the fast limit the maximum intensity was 140% of the initial intensity when significant CSA was present or under the QCPMG-MAS experiment compared to 89 or 71% for the static experiment influenced by the quadrupolar interaction only. For 3-, 4-, and 6-site jumps the maximum intensity in the fast limit reached up to 339% of the intensity in the static limit.

Journal Article↗