EPR-correlated dipolar spectroscopy by Q-band chirp SIFTER

Doll, Andrin; Jeschke, Gunnar

doi:10.1039/c6cp03067j

Cited by 34 publications

(38 citation statements)

References 54 publications

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“…All Q-band experiments have been performed on a homebuilt high-power ultra-wideband X/Q-band AWG spectrometer 28,36 . This spectrometer features versatile pulse sequence programming and synthesis by a 8 GSa/s AWG as well as acquisition of averaged echo transients with a short data transfer 125 time on the order of 1 ms. A home-built loop-gap resonator having a loaded quality factor around 120 and accepting 1.6 mm outer-diameter sample tubes was used.…”

Section: Experimental 120mentioning

confidence: 99%

“…In order to consider this compensation effect with two refocusing pulses, the potential interference brought by one single refocusing pulse is analyzed first. It is noted that dipolar evolution throughout a frequency-swept 540 non-selective refocusing pulse has already been considered in the analysis of the SIFTER experiment 28 . The basic effect is that, unlike a hard non-selective pulse, which inverts both spin partners simultaneously, the chirp pulse results in a non-zero delay δ 12 in-between the inversion of the two spin partners.…”

Section: 23mentioning

confidence: 99%

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Double electron–electron resonance with multiple non-selective chirp refocusing

Doll

Jeschke

2017

Phys. Chem. Chem. Phys.

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A new approach to double electron-electron resonance (DEER) for distance determination involving nitroxide spin labels at dilute concentrations is presented. In general, DEER pulse sequences rely on double resonance between pump and observer spins excited by selective pulses at two distinct microwave frequencies. In the new approach abbreviated as nDEER, non-selective chirp pulses that refocus all relevant spin pairs are combined with DEER. This non-selective refocusing results in suppression of unmodulated contributions, such as the constant contribution as well as the background curvature due to inter-molecular spin partners in ordinary DEER data. Due to this dipolar attenuation effect, primary nDEER data are closer to the dipolar modulation of primary interest than ordinary DEER data. Restrictions of nDEER are that secondary information related to these unmodulated contributions becomes difficult to retrieve. Accordingly, incomplete deconvolution of the inter-molecular background prevents the application of nDEER to rigid spin pairs at high concentrations. A key advantage of nDEER is the high fidelity of the chirp refocusing pulses, which is important for nDEER schemes that incorporate dynamical decoupling to access longer distances. In this context, nDEER with Carr-Purcell (CP) pulse trains having N = 2 and N = 4 refocusing pulses are demonstrated. These CP nDEER sequences require a total of N + 2 pulses, which is less than the 2N + 1 pulses required for CP DEER schemes. The pump pulse position is incremented throughout the refocusing pulses, which restricts the minimum time increment to 96 ns on our spectrometer and therefore complicates application to distances below 3 nm. At Q-band frequencies, unwanted modulations related to pulse imperfections contribute only 3.5% relative to the principal nDEER modulation. Accordingly, there is no need for dedicated data reconstruction methods as in CP DEER methods.

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Section: Experimental 120mentioning

confidence: 99%

Section: 23mentioning

confidence: 99%

Double electron–electron resonance with multiple non-selective chirp refocusing

Doll

Jeschke

2017

Phys. Chem. Chem. Phys.

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“…However, in the one-dimensional form, integrating over the echo signal has the effect of removing all orientation information [17]. Doll and Jeschke reported a frequency-correlated 2D-SIFTER recorded at Q-band frequencies (35 GHz) using broadband chirp pulses [18].…”

Section: Introductionmentioning

confidence: 99%

Orientation Selective 2D-SIFTER Experiments at X-Band Frequencies

Bowen

Erlenbach

et al. 2018

Appl Magn Reson

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Frequency-correlated 2D SIFTER with broadband pulses at X-band frequencies can be used to determine the inter-spin distance and relative orientation of nitroxide moieties in macromolecules when the flexibility of the spin-labels is restricted. At X-band frequencies the EPR spectrum of nitroxides is governed by the strongly anisotropic nitrogen hyperfine coupling. For rigid spin-labels, where the orientation of the inter-connecting vector R correlates to the relative orientations of the nitroxide labels, the dipolar oscillation frequency varies over the EPR spectral line shape. Broadband shaped pulses allow excitation of the complete nitroxide EPR spectra. In thi case, Fourier transform of the echo signal gives both fast and direct access to the orientation dependent dipolar coupling. This allows determination of not only the inter-spin distance R, but also their mutual orientation. Here we show the application of the frequency-correlated 2D SIFTER experiment with broadband pulses to a bis-nitroxide model compound and to a double stranded DNA sample. In both molecules, there is restricted internal mobility of the two spin-labels. The experimental results are compared to orientation selective PELDOR experiments and simulations based on a simple geometrical model or MD simulations describing the conformational flexibility of the molecules. Fourier transformation of the SIFTER echo signal yields orientation selective dipolar time traces over the complete EPR-spectral range. This leads to an improved frequency resolution and either to a reduced experimental measurement time or a larger span of frequency offsets measured compared to orientation selective PELDOR experiments. The experimental potential and limitations of the 2D SIFTER method for samples containing rigid spin-labels will be discussed.

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“…For distances below 3 nm, pump pulse durations on the order of a few tens of nanoseconds are common. In principle, even a short 64 ns pulse can exhibit remarkable inversion performance over several 100 MHz when centered around the main frequency of the microwave resonator, as demonstrated using high-power instru- 45 mentation at both X band [14] and Q band [15]. In a DEER experiment, however, it is not necessarily an optimum strategy to place a broadband pump pulse at the center of the resonator mode, since this pushes the observation frequency of the DEER spin echo to a position of weak detection efficiency.…”

Section: Introductionmentioning

confidence: 99%

“…Recently, frequency-swept pulses generated by fast arbitrary waveform generators (AWG) have been incorporated into DEER experiments. These chirp pulses have been well known for their enhanced excitation bandwidth [6,7], such that initial 15 DEER applications made use of frequency-swept pump pulses to enhance the dipolar modulation, i.e. the DEER modulation depth λ.…”

Section: Introductionmentioning

confidence: 99%

CIDME: Short distances measured with long chirp pulses

Doll

Godt

et al. 2016

Journal of Magnetic Resonance

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Frequency-swept pulses have recently been introduced as pump pulses into double electron-electron resonance (DEER) experiments. A limitation of this approach is that the pump pulses need to be short in comparison to dipolar evolution periods. The "chirp-induced dipolar modulation enhancement" (CIDME) pulse sequence introduced in this work circumvents this limitation by means of longitudinal storage during the application of one single or two consecutive pump pulses. The resulting six-pulse sequence is closely related to the five-pulse "relaxation-induced dipolar modulation enhancement" (RIDME) pulse sequence: While dipolar modulation in RIDME is due to stochastic spin flips during longitudinal storage, modulation in CIDME is due to the pump pulse during longitudinal storage. Experimentally, CIDME is examined for Gd-Gd and nitroxide-nitroxide distance determination using a high-power Q-band spectrometer. Since longitudinal storage results in a 50% signal loss, comparisons between DEER using short chirp pump pulses of 64 ns duration and CIDME using longer pump pulses are in favor of DEER. While the lower sensitivity restrains the applicability of CIDME for routine distance determination on high-power spectrometers, this result is not to be generalized to spectrometers having lower power and to specialized "non-routine" applications or different types of spin labels. In particular, the advantage of prolonged CIDME pump pulses is demonstrated for experiments at large frequency offset between the pumped and observed spins. At a frequency separation of 1 GHz, a Gd-Gd modulation depth larger than 10% is achieved, where broadening due to dipolar pseudo-secular contributions becomes largely suppressed. Moreover, a CIDME experiment at deliberately reduced power underlines the potential of the new technique for spectrometers with lower power, as often encountered at higher microwave frequencies. With longitudinal storage times T below 10 µs, however, CIDME appears rather susceptible to artifacts. For nitroxide-nitroxide experiments, these currently inhibit a faithful data analysis. To facilitate further developments, the artifacts are characterized experimentally. In addition, effects that are specific to the high spin of S = 7/2 Gd-centers are examined. Herein, population transfer within the observer spin's multiplet due to the pump pulse as well as excitation of dipolar harmonics are discussed.

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EPR-correlated dipolar spectroscopy by Q-band chirp SIFTER

Cited by 34 publications

References 54 publications

Double electron–electron resonance with multiple non-selective chirp refocusing

Double electron–electron resonance with multiple non-selective chirp refocusing

Orientation Selective 2D-SIFTER Experiments at X-Band Frequencies

CIDME: Short distances measured with long chirp pulses

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