Structural Biology using Spin Probes

A DEER trace from a P450 enzyme measuring the electron-electron dipolar interaction and thus distance (ca. 25 Å, yellow dashed line) and orientation between the Fe3+ of the heme and the nitroxide spin label, MTSL, attached to a cysteine residue.

Distance measurements in the range of ca. 15 – 100 Å are obtained by measuring the dipolar coupling between two or more unpaired electrons using pulse EPR techniques, i.e. DEER (or PELDOR) and DQ EPR. For work with diamagnetic biomolecules, usually nitroxide spin labels (i.e. MTSL) are attached covalently using site-directed spin labelling. A number of such DEER based distance constraints, along with modelling techniques, allows for 3D structure determination. At the centre, we have pulse X- and Q-band spectrometers to carry out these studies.

For electron-electron distances below ca. 15 Å, the magnetic interaction between the spins is typical resolved in Continuous Wave (CW) EPR measurements and provides distance and additionally electronic information on the unpaired electron orbitals via the exchange coupling. Examples include our work on copper dimers.

If EPR experiments are carried out under conditions where the paramagnetic are mobile of the EPR timescale, information on the dynamics and relaxation of the system is obtainable. For example EPR experiments at room temperature on a nitroxide spin label attached to a biomolecule will be sensitive to the dynamics of the protein. For dynamic studies, we have multi-frequency CW and pulse EPR capabilities at the CAI.

DEER – double electron-electron resonance
PELDOR – pulsed electron double resonance
ESR – electron spin resonance