Double Electron−Electron Resonance Measured Between Gd³⁺ Ions and Nitroxide Radicals

Abstract

Double electron−electron resonance has attracted growing attention as a technique to study structure and conformational changes of biomacromolecules. Here, a new combination of paramagnetic labels is experimentally tested, one being a commonly used nitroxide radical, and the other being a Gd3+ ion. The Gd3+−nitroxide spin pair can serve as a good substitute for the nitroxide−nitroxide pair of spin labels and potentially provides a link to other experimental approaches dealing with structural information.

Gd(III)–Gd(III) distance measurements with chirp pump pulses

“…For all the other Gd-rulers, the sample volume was 40 µl with a concentration of 50 µM. In the case of Gd-ruler 1 3 , also different concentrations were prepared. These concentrations are found in Fig.…”

“…The temperature was 10 K and the repetition rate of the pulse sequences was 342 µs in the case of Gd-ruler 1 3 and 716 µs for all the other Gd-rulers. An exception is the acquisition of resonator profiles ν 1 ( f ) based on transient nutation experiments, which were all acquired at a repetition time of 342 µs.…”

“…The performance of distance measurements with a single chirp pump pulse was evaluated with Gd-ruler 1 3 , whose echodetected EPR spectrum cast in frequency scale is shown in red in Fig. 2a.…”

“…Moreover, the consecutive chirp pulses presented below in Section 3.4 do typically not require such small offsets. 3 . Black: same data as in Fig.…”

“…The majority of experiments to date rely on the four-pulse DEER (double electronelectron resonance) sequence to access distance information between a pair of Gd(III) centers [2]. Modifications of this approach involve the use of non-identical spin labels, in particular a Gd(III) center and a nitroxide [3,4], as well as different experimental schemes, such as relaxation-induced dipolar modulation (RIDME) [5] or continuous-wave EPR [6]. Distance measurements involving Gd spin labels have been successfully demonstrated on a number of systems, including model compounds, peptides, nanoparticles, proteins and DNAs [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”