Strategies for the Synthesis of Yardsticks and Abaci for Nanometre Distance Measurements by Pulsed EPR

Abstract

Pulsed electron paramagnetic resonance (EPR) techniques have been found to be efficient tools for the elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative orientations, structural flexibility or aggregation states. A wide variety of model systems for calibration and optimisation of pulsed experiments has been synthesised. Their design is based on mimicking biological systems or materials in specific properties such as the distances themselves and the distance distributions. Here, we review selected approaches to the synthesis of chemical systems bearing two or more spin centres, such as nitroxide or trityl radicals, metal ions or combinations thereof and outline their application in pulsed EPR distance measurements.

Orientation selection in high-field RIDME and PELDOR experiments involving low-spin CoII ions

“…Over the past two decades pulse dipolar electron paramagnetic resonance (PD-EPR) has become valuable in resolving biomolecular architectures 1,2 by allowing measurement of nanometre-range distances between intrinsic or chemically harboured unpaired electrons. Nitroxide (NO) labels and endogenous paramagnets are typically used in PD-EPR, 3 while other approaches involve the substitution of naturally occurring diamagnetic metal ions by paramagnets. 3 Pulsed electron-electron double resonance (PELDOR/DEER) [4][5][6] has been a cornerstone for measuring spin-spin distances (1-10 nm and beyond) 7 and relaxation induced dipolar modulation enhancement (RIDME) 8,9 has recently emerged as an attractive alternative.…”

“…Nitroxide (NO) labels and endogenous paramagnets are typically used in PD-EPR, 3 while other approaches involve the substitution of naturally occurring diamagnetic metal ions by paramagnets. 3 Pulsed electron-electron double resonance (PELDOR/DEER) [4][5][6] has been a cornerstone for measuring spin-spin distances (1-10 nm and beyond) 7 and relaxation induced dipolar modulation enhancement (RIDME) 8,9 has recently emerged as an attractive alternative. PELDOR and RIDME employ microwave (mw) pulses to observe one electron spin (A) whilst inverting a coupled electron spin (B).…”

Accurate Extraction of Nanometer Distances in Multimers by Pulse EPR

“…Towards this we have employed two tetraradical abaci19 to investigate this approach in more detail. Tetraradical 1 20 (Figure 1 a) is based on an adamantane core with six almost equal distances.…”

Spirocyclic Nitroxide Biradicals: Synthesis and Evaluation as Dynamic Nuclear Polarizing Agents

“…In the point-dipole approximation, the dipolar coupling between two electrons amounts to D = 52.2 MHz/r 3 nm À 3 , where r is the distance between the two electrons. [32] The dipolar couplings calculated with this formula are shown in Table 1 and range from 139 MHz for 6 (the compound with the longest interradical distance) to 500 MHz for 4b (the compound with the shortest distance). The expected dipolar coupling in all investigated bisnitroxides is hence markedly larger than in TEKPol, where this value amounts to around 30 MHz.…”