Abstract
Double electron-electron resonance (DEER) is a pulse electron paramagnetic resonance (EPR) technique that measures distances between paramagnetic centres. It utilizes a four-pulse sequence based on the refocused Hahn spin echo. The echo decays with increasing pulse sequence length , where and are the two time delays. In DEER, the value of is determined by the longest inter-spin distance that needs to be resolved, and is adjusted to maximize the echo amplitude and, thus, sensitivity. We show experimentally that, for typical spin centres (nitroxyl, trityl, and Gd(III)) diluted in frozen protonated solvents, the largest refocused echo amplitude for a given is obtained neither at very short (which minimizes the pulse sequence length) nor at (which maximizes dynamic decoupling for a given total sequence length) but rather at values smaller than . Large-scale spin dynamics simulations based on the coupled cluster expansion (CCE), including the electron spin and several hundred neighbouring protons, reproduce the experimentally observed behaviour almost quantitatively. They show that electron spin dephasing is driven by solvent protons via the flip-flop coupling among themselves and their hyperfine couplings to the electron spin.
Highlights
Double electron–electron resonance (DEER) spectroscopy is a highly effective method for nanometer-scale distance measurements in bio-macromolecules such as proteins, nucleic acids, and their complexes (Jeschke, 2013; Jeschke and Polyhach, 2007)
To investigate the refocused echo decay, we acquired its amplitude as a function of both τ1 and τ2 for frozen solutions of a 3-maleimido-proxyl radical, the trityl OXO063, and Gd(III)
The data show that the echo decay is not a function of only the total pulse length, 2(τ1 + τ2), which is in contrast to the two-pulse echo decay
Summary
DEER (double electron–electron resonance) spectroscopy is a highly effective method for nanometer-scale distance measurements in bio-macromolecules such as proteins, nucleic acids, and their complexes (Jeschke, 2013; Jeschke and Polyhach, 2007). This method measures the magnetic dipolar coupling between two spin labels attached at well-defined, specific locations in the bio-macromolecules. This sequence suffers from instrumental artefacts near t = 0 due to pulse overlap between the first pulse and the pump pulse To eliminate these artefacts, the dead-time-free four-pulse DEER experiment was introduced by Spiess and co-workers (Fig. 1b; Martin et al, 1998; Pannier et al, 2000). The four-pulse DEER sequence is the most widely used DEER experiment
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