Pulsed EPR Dipolar Spectroscopy on Spin Pairs with one Highly Anisotropic Spin Center: The Low‐Spin Fe^III Case

Abstract

Pulsed electron paramagnetic resonance (EPR) dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling constantsa nd thus the distance between electron spin centers. Up to now,P DS measurements have been mostly applied to spin centers whose g-anisotropies are moderatea nd therefore have an egligible effect on the dipolar coupling constants. In contrast, spin centers with large g-anisotropy yield dipolarc oupling constants that depend on the g-values.I nt his case, the usual methods of extracting distances from the raw PDSd ata cannot be applied. Here, the effect of the g-anisotropyo nP DS data is studied in detail on the example of the low-spin Fe 3 + ion. First, this effect is described theoretically,u sing the work of Bedilo andM aryasov (Appl. Magn. Reson. 2006,3 0, 683-702) as ab asis. Then, two knownF e 3 + /nitroxide compounds and one new Fe 3 + /trityl compound were synthesized and PDS measurements were carried out on them using am ethod called relaxation inducedd ipolarm odulation enhancement (RIDME). Based on the theoretical results, aR IDME data analysis procedure was developed, which facilitated the extraction of the inter-spin distance and the orientation of the inter-spin vector relative to the Fe 3 + g-tensor frame from the RIDME data. The accuracyo ft he determined distances and orientationsw as confirmed by comparison with MD simulations. This methodc an thus be appliedt ot he highly relevant class of metalloproteins with, for example, low-spin Fe 3 + ions.[a] Dr.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.org/10.

Pulsed Dipolar EPR Spectroscopy and Metal Ions: Methodology and Biological Applications

“…Compared to Cu 2+ , low‐spin Fe 3+ (ls Fe 3+ ) ions have an even larger g ‐anisotropy and shorter relaxation times . This imposes further challenges on the PDS measurements and the corresponding data analysis.…”

“…However, the determination of these angles from the experimental data was done only in a semi‐quantitative way, because the RIDME time traces had a significant dead time and a quite low SNR. Recently, a quantitative analysis of high‐quality RIDME data acquired on Fe 3+ /nitroxide and Fe 3+ /trityl model compounds was reported . This analysis was performed using the program DipFit, which was build based on Equation (10).…”

“…Recently, a quantitative analysis of high‐quality RIDME data acquired on Fe 3+ /nitroxide and Fe 3+ /trityl model compounds was reported . This analysis was performed using the program DipFit, which was build based on Equation (10). In this program, the distributions of all variables, namely r , ξ and φ , were assumed to have a Gaussian distribution and the correlation between individual distributions was neglected.…”

“…Compared to Cu 2 + , low-spin Fe 3 + (ls Fe 3 + ) ions have an even larger g-anisotropy and shorter relaxation times. [82] This imposes further challenges on the PDS measurements and the corresponding data analysis. Due to the significant g-anisotropy, the spectral width of ls Fe 3 + largely exceeds the bandwidths of typical rectangular pulses and even the bandwidths of ultrawideband shaped pulses at typical microwave frequencies.…”

“…Colored bars depict the positions of artificial distances that are obtained for the actual distance < r > and three principal g-values of the ls Fe 3 + center when the g-anisotropy of the Fe 3 + center is neglected in the RIDME data analysis. Reproduced from Ref [82]. with permission from Wiley-VCH.…”