High-sensitivity Gd<sup>3+</sup>–Gd<sup>3+</sup> EPR distance measurements that eliminate artefacts seen at short distances

Abstract

Abstract. Gadolinium complexes are attracting increasing attention as spin labels for EPR dipolar distance measurements in biomolecules and particularly for in-cell measurements. It has been shown that flip-flop transitions within the central transition of the high-spin Gd3+ ion can introduce artefacts in dipolar distance measurements, particularly when measuring distances less than 3 nm. Previous work has shown some reduction of these artefacts through increasing the frequency separation between the two frequencies required for the double electron–electron resonance (DEER) experiment. Here we use a high-power (1 kW), wideband, non-resonant system operating at 94 GHz to evaluate DEER measurement protocols using two stiff Gd(III) rulers, consisting of two bis-Gd3+–PyMTA complexes, with separations of 2.1 nm and 6.0 nm, respectively. We show that by avoiding the -12→12 central transition completely, and placing both the pump and the observer pulses on either side of the central transition, we can now observe apparently artefact-free spectra and narrow distance distributions, even for a Gd–Gd distance of 2.1 nm. Importantly we still maintain excellent signal-to-noise ratio and relatively high modulation depths. These results have implications for in-cell EPR measurements at naturally occurring biomolecule concentrations.

Dipolar-Coupled Entangled Molecular 4f Qubits

“…In this latter case, an example of a family of heterodinuclear Ln complexes exists where two-qubit gates were proposed within the same molecule, by exploiting the difference of g-factors of the two different Ln centers, and thus not between independent 4f qubits. However, there is a growing number of reports on pulsed-EPR of pairs of Gd III ions, particularly by DEER spectroscopy. − …”

Frequency swept pulses for the enhanced resolution of ENDOR spectra detecting on higher spin transitions of Gd(III)

“…As higher temperatures are generally easier to achieve and maintain on standard EPR setups, polarisation transfer from the CT can be seen as a means to mimic the gain in the Boltzmann distribution of the lower lying levels by thermal polarisation at high fields and low temperatures. Future applications of pre-polarisation away from the CT might include DEER with pump and detection frequencies set to be resonant either side of the CT in order to minimise spectral diffusion related to flip-flop transitions [40]. Additionally it may be possible to push the resolution of Gd(III)-19 F ENDOR based distance measurements to smaller coupling values, while avoiding the extremely low temperatures normally needed for efficient thermal polarisation of the corresponding EPR transitions.…”

Increasing the Modulation Depth of GdIII-Based Pulsed Dipolar EPR Spectroscopy (PDS) with Porphyrin–GdIII Laser-Induced Magnetic Dipole Spectroscopy

“…Although nitroxides and carbon-based trityl labels have received the most attention in the last years, − Gd III complexes with a chelating ligand (electron spin S = 7 / 2 ) are becoming increasingly popular. ,, Their advantages includes reduction stability, , the absence of orientation selection and high sensitivities at Q- and W-bands. − However, their broad EPR spectrum spans several GHz and exceeds the excitation bandwidth of most resonators and microwave pulses . Therefore, only a small fraction of spins are addressed by the microwave pump pulse used in Gd III –Gd III DEER, which results in a much lower modulation depth than those achieved with other PDS techniques or labels. , Another drawback of Gd III –Gd III DEER are broadening artifacts that appear at distances below ≈3 nm if the offset between the observer and pump microwave frequency is in the range of only a few 100 MHz. − One technique with which both of these disadvantages for Gd III -based PDS can be overcome is relaxation induced dipolar modulation enhancement (RIDME). , In RIDME, the spin flips of the pump spins take place as stochastic events due to relaxation and are thus not limited by the excitation bandwidth of the microwave pulses. , Hence, modulation depths higher than 50% are possible with Gd III –Gd III RIDME . However, because spin flips with Δ| m S | > 1 are also possible, Gd III –Gd III RIDME suffers from the excitation of overtones .…”

“… 23 , 26 Another drawback of Gd III –Gd III DEER are broadening artifacts that appear at distances below ≈3 nm if the offset between the observer and pump microwave frequency is in the range of only a few 100 MHz. 29 − 32 One technique with which both of these disadvantages for Gd III -based PDS can be overcome is relaxation induced dipolar modulation enhancement (RIDME). 33 , 34 In RIDME, the spin flips of the pump spins take place as stochastic events due to relaxation and are thus not limited by the excitation bandwidth of the microwave pulses.…”