Gd(III)-Gd(III) Relaxation-Induced Dipolar Modulation Enhancement for In-Cell Electron Paramagnetic Resonance Distance Determination

In-cell distance determination by electron paramagnetic resonance (EPR) spectroscopy reveals essential structural information about biomacromolecules under native conditions. We demonstrate that the pulsed EPR technique RIDME (relaxation induced dipolar modulation enhancement) can be utilized for such distance determination. The performance of in-cell RIDME has been assessed at Q-band using stiff molecular rulers labeled with Gd(III)-PyMTA and microinjected into Xenopus laevis oocytes. The overtone coefficients are determined to be the same for protonated aqueous solutions and inside cells. As compared to in-cell DEER (double electron-electron resonance, also abbreviated as PELDOR), in-cell RIDME features approximately 5 times larger modulation depth and does not show artificial broadening in the distance distributions due to the effect of pseudosecular terms

Distance Measurements on Orthogonally Spin-Labeled Membrane Spanning WALP23 Polypeptides

EPR-based Gd(III)-nitroxide distance measurements were performed on a series of membrane-incorporated orthogonally labeled WALP23 polypeptides. The obtained distance distributions were stable upon the change of detection frequency from 10 GHz (X-band) to 35 GHz (Q-band). The alpha-helical pitch of WALP23 polypeptide could be experimentally observed, despite the flexibility of the two spin labels. The spectroscopic properties of Gd(III) ions and nitroxide radicals allow detecting both types of paramagnetic species selectively in different EPR experiments. In particular, this spectroscopic selectivity allows for supplementing Gd(III)-nitroxide distance measurements with independent checks of polypeptide aggregation and with measurements of the local environment of the nitroxide spin labels. All mentioned additional checks do not require preparation of further samples, as it is the case in the experiments with pairs of identical nitroxide spin labels

Multiple pathway relaxation enhancement in the system composed of three paramagnetic nitroxide radical-Ln3+-O2

Longitudinal relaxation of nitroxide spin-labels has been measured for a membrane-incorporated alpha-helical polypeptide in the presence and absence of residual amounts of membrane-dissolved O-2 and paramagnetic Dy3+ ions. Such a model system, containing three different types of paramagnetic species, provides an important example of nonadditivity of two different relaxation channels for the nitroxide spins

Binding of single gold atoms on thin MgO(001) films

In the present Letter the first electron paramagnetic resonance spectra of single metal atoms on a single crystalline oxide surface are presented. For Au atoms on a MgO(001) film investigated here an analysis of the angular dependent resonance positions and the hyperfine coupling to 17O shows that the atoms are bound on top of oxygen ions on the terrace of the film. This result is in perfect agreement with scanning tunneling microscopy measurements at 5 K presented here. The measured hyperfine matrix components allow an experimental verification of the theoretically proposed binding mechanism of Au atoms on MgO. In particular, the large reduction of the isotropic hyperfine coupling constant of supported Au as compared to free atoms is not due to a charge transfer at the interface but a hybridization of orbitals and a resulting polarization of the unpaired electron