Performance of PELDOR, RIDME, SIFTER, and DQC in measuring distances in trityl based bi- and triradicals: exchange coupling, pseudosecular coupling and multi-spin effects.

Electron paramagnetic resonance (EPR) based distance measurements in combination with site directed spin labeling is an important method for determining biomolecular structures and their conformational changes. Commonly used spin labels are nitroxides but trityl radicals are emerging as a new type of spin labels specifically for distance measurements. Here, a systematic evaluation is presented for the performance of different EPR pulse sequences for measuring inter-spin distances using trityl labels. The model systems used are two bistrityl and one tristrityl molecule. One bistrityl molecule has a long spin-spin separation and serves as a reference, the second one has a short inter-spin distance serving as a model case for strong and intermediate coupling and the third is a model for the influence of multi-spin effects. Experimental and theoretical approaches to cope with the effects are presented and evaluated.

Versatile Trityl Spin Labels for Nanometer Distance Measurements on Biomolecules In†Vitro and within Cells.

Structure determination of biomacromolecules under in-cell conditions is a relevant yet challenging task. Electron paramagnetic resonance (EPR) distance measurements in combination with site-directed spin labeling (SDSL) are a valuable tool in this endeavor but the usually used nitroxide spin labels are not well-suited for in-cell measurements. In contrast, triarylmethyl (trityl) radicals are highly persistent, exhibit a long relaxation time and a narrow spectral width. Here, the synthesis of a versatile collection of trityl spin labels and their application in in vitro and in-cell trityl-iron distance measurements on a cytochrome P450 protein are described. The trityl labels show similar labeling efficiencies and better signal-to-noise ratios (SNR) as compared to the popular methanethiosulfonate spin label (MTSSL) and enabled a successful in-cell measurement.

Di-copper(ii) DNA G-quadruplexes as EPR distance rulers.

Artificial metal-base tetrads composed of square-planar CuII(pyridine)4 complexes were covalently attached to both the 3' and 5' ends of tetramolecular DNA G-quadruplexes [Ld(G3-5)LdT]4 (L = pyridine ligand) of different lengths. Owing to the planar four-point attachment of the metal complexes, the magnetic orbitals (dx2-y2) of the d9-configured CuII cations are placed in a coplanar fashion, separated by the stacked guanine tetrads. Pulsed EPR-derived CuII-CuII distances and angles were found to be in agreement with those obtained from molecular dynamics simulations. DNA-confined transition metal spin labels open new ways to study oligonucleotide structure and DNA-protein complexes.

Efficient and General Aerobic Oxidative Cross-Coupling of THIQs with Organozinc Reagents Catalyzed by CuCl2: Proof of a Radical Intermediate.

A general new method for the highly concise synthesis of C-1-alkylated tetrahydroisoquinolines (THIQ) is reported. The CuCl2-catalyzed procedure is based on a coupling of nonfunctionalized THIQs with organozinc reagents under aerobic conditions. It proceeds in high yields and is broadly applicable to a wide range of substrates. It relies on a regioselective sp(3) C-H bond activation allowing for an sp(3)-sp(3) bond union under mild reaction conditions in a rapid and effective manner. Mechanistically it involves an iminium ion intermediate that is formed via an organic radical involving a single-electron-transfer process. For the first time for this type of reaction a radical intermediate has been proven by EPR spectroscopy.