Cross-polarisation edited ENDOR

Abstract

Electron-nuclear double resonance (ENDOR) is a fundamental technique in electron paramagnetic resonance (EPR) spectroscopy that directly detects hyperfine transitions of nuclei coupled to a paramagnetic centre. Despite its wide use, spinsensitivity and restricted spectral resolution in powder samples pose limitations of this technique in modern application fields of EPR. In this contribution, we examine the performance of an ENDOR pulse sequence that utilises a preparation scheme different from conventional Davies ENDOR. The scheme is based on electron-nuclear cross-polarisation (eNCP), which requires concomitant microwave (MW) and radio-frequency (RF) irradiation satisfying specific matching conditions between the MW and RF offsets and the hyperfine coupling. Changes in nuclear polarisation generated during eNCP can be detected via a conventional ENDOR read-out sequence consisting of an RF π -pulse followed by EPR-spin echo detection. Using 1 H-BDPA as a standard sample, we first examine the CP matching conditions by monitoring the depolarisation of the electron spin magnetisation. Subsequently, so-called CP-edited ENDOR spectra for different matching conditions are reported and analysed based on the provided theoretical description of the time evolution of the spin density matrix during the experiment. The results demonstrate that CP-edited ENDOR provides additional information with respect to the sign of the hyperfine couplings. Furthermore, the sequence is less sensitive to nuclear saturation effects encountered in conventional ENDOR.

Cross‐Polarization Electron‐Nuclear Double Resonance Spectroscopy

“…On the other hand, excitation on EPR line β gives peaks at Δ ω n /2π=±26 MHz, which satisfy Equation (2) for |( ω α ‐ ω β )|/2 (matching conditions [1–2] and [3–4]). Prominent resonances at Δ ω n /2π≈±6 MHz match the mw field strength ω 1e and arise from polarization transfer to matrix 1 H . These results provide an important insight that is not obtainable from previous experiments on powder samples.…”

“…Only recently has the realization of electron‐nuclear CP (called eNCP) been proposed, based on modified, or more generalized, Hartmann–Hahn conditions . Following this idea, we adopted the concept to address sensitivity issues in ENDOR spectroscopy . In the present paper, we first demonstrate the foundation of eNCP and CP‐edited ENDOR on a model single crystal and we subsequently illustrate the performance on a representative protein study, that is, with the tyrosyl radical in E.coli RNR.…”

“…Polarization transfer is illustrated when states |2′> and |3′> become degenerate (matching condition [2–3]) during rf irradiation with a frequency offset Δ ω n CP [2–3] The resulting ENDOR spectrum is predicted to display one absorptive and one emissive line with intensity ratio 3:1 (see the Supporting Information). R is a constant related to the Boltzmann factor …”

“…Irradiation of the radio frequency ω rf concomitant to the mw spin lock at an offset Δ ω n = ω n ‐ ω rf ( ω n is the nuclear Larmor frequency [Eq. ]) tunes the energy of the states in the tilted frame (Figure ) and induces polarization transfer if either one of the four conditions is fulfilled (see SI1 in the Supporting Information) …”

Topical review introduction

“…Polarisation transfer between electron spins [4][5][6] or from electron to nuclear spins [7,8] is a much discussed topic in Molecular Physics. In spin chemistry, further complexity arises from coupling between spin dynamics and chemical reaction dynamics [9][10][11].…”