Intramolecular Hydrogen Bonding Restricts Gd-Aqua-Ligand Dynamics.

Aqua ligands can undergo rapid internal rotation about the M-O bond. For magnetic resonance contrast agents, this rotation results in diminished relaxivity. Herein, we show that an intramolecular hydrogen bond to the aqua ligand can reduce this internal rotation and increase relaxivity.

Zero field splitting fluctuations induced phase relaxation of Gd3+ in frozen solutions at cryogenic temperatures.

Distance measurements using double electron-electron resonance (DEER) and Gd(3+) chelates for spin labels (GdSL) have been shown to be an attractive alternative to nitroxide spin labels at W-band (95GHz). The maximal distance that can be accessed by DEER measurements and the sensitivity of such measurements strongly depends on the phase relaxation of Gd(3+) chelates in frozen, glassy solutions. In this work, we explore the phase relaxation of Gd(3+)-DOTA as a representative of GdSL in temperature and concentration ranges typically used for W-band DEER measurements.

Pulsed electron paramagnetic resonance spectroscopy of (33)S-labeled molybdenum cofactor in catalytically active bioengineered sulfite oxidase.

Molybdenum enzymes contain at least one pyranopterin dithiolate (molybdopterin, MPT) moiety that coordinates Mo through two dithiolate (dithiolene) sulfur atoms. For sulfite oxidase (SO), hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of magnetic nuclei (I ≠ 0) near the Mo(V) (d(1)) center have been measured using high-resolution pulsed electron paramagnetic resonance (EPR) methods and interpreted with the help of density functional theory (DFT) calculations. These have provided important insights about the active site structure and the reaction mechanism of the enzyme.

Optimization of pulsed DEER measurements for Gd-based labels: choice of operational frequencies, pulse durations and positions, and temperature.

In this work, the experimental conditions and parameters necessary to optimize the long-distance (≥ 60 Å) Double Electron-Electron Resonance (DEER) measurements of biomacromolecules labeled with Gd(III) tags are analyzed. The specific parameters discussed are the temperature, microwave band, the separation between the pumping and observation frequencies, pulse train repetition rate, pulse durations and pulse positioning in the electron paramagnetic resonance spectrum. It was found that: (i) in optimized DEER measurements, the observation pulses have to be applied at the maximum of the EPR spectrum; (ii) the optimal temperature range for K-band measurements is 14-17 K, while in W-band the optimal temperatures are between 6-9 K; (iii) W-band is preferable to K-band for DEER measurements.