New Spin Label Designed for Double Electron-Electron Resonance Distance Measurements in the Liquid Nitrogen Temperature Range

Abstract

Double electron-electron resonance (DEER) measurements of interspin distances in biomolecules are restricted by the spin echo dephasing rates of the paramagnetic centers. Faster dephasing rates limit the longest distance that can be measured and the precision with which the distribution of distances can be measured. Because of the temperature dependence of dephasing rates for currently-used spin labels, the optimum temperature for DEER measurements is 50 to 60 K, which requires the use of liquid helium as the cryogen. A new spin label has been synthesized and characterized, with a structure that is analogous to the commonly-used synthetic peptide TOAC, except that the gem-dimethyl groups are replaced by spirocyclohexyl groups. Because of the absence of methyl groups, the spin echo dephasing rates for this new spin label in 1:1 water glycerol remains approximately independent of temperature up to about 130 K. The spin lattice relaxation rates for the new probe are sufficiently faster at 130 K to compensate for the changes in Boltzmann populations, so the signal-to-noise, longest distance that can be measured, and accuracy of determination of distance distributions in DEER experiments with this probe will be as favorable at 130 K as for currently-used probes at 50 to 60 K. These results indicate that it will be possible to perform DEER experiments with the substantially less expensive liquid nitrogen. Supported by NIH NIBIB EB002807(Denver), NSF CHE-0718117 (Nebraska), NSF/DOE under Grant No. CHE-0087817 and DOE under Contract No. W-31-109-Eng-38 (Indiana).