High-field ELDOR-detected NMR study of a nitroxide radical in disordered solids: Towards characterization of heterogeneity of microenvironments in spin-labeled systems

• Published in: Journal of magnetic resonance (1997) 2014-05, Vol.242, p.203-213
• Main Authors: Nalepa, Anna, Möbius, Klaus, Lubitz, Wolfgang, Savitsky, Anton
• Format: Article
• Language: English
• Subjects: Algorithms; Ecosystem; ELDOR-detected NMR; ENDOR; Heterogeneity; High-field EPR; Hyperfine interaction; Magnetic Resonance Spectroscopy - methods; Mathematical analysis; Matrix effects; Nitrogen Oxides - analysis; Nitrogen Oxides - chemistry; Nitroxide radical; Nuclear magnetic resonance; Polarity; Proteins; Quadrupoles; Radicals; Spin Labels; Tensors
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1016/j.jmr.2014.02.026

[Display omitted] •We have applied W-band ELDOR-detected NMR to a nitroxide radical.•EDNMR method is shown to be capable of characterizing heterogeneous environments.•Superior sensitivity of EDNMR as compared to ENDOR method is demonstrated. The combination of high-field EPR with site-directed spin-labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in probing the polarity and proticity characteristics of protein/matrix systems. This information is concluded from the principal components of the nitroxide Zeeman (g), nitrogen hyperfine (A) and quadrupole (P) tensors of the spin labels attached to specific sites. Recent multi-frequency high-field EPR studies underlined the complexity of the problem to treat the nitroxide microenvironment in proteins adequately due to inherent heterogeneities which result in several principal x-components of the nitroxide g-tensor. Concomitant, but distinctly different nitrogen hyperfine components could, however, not be determined from high-field cw EPR experiments owing to the large intrinsic EPR linewidth in fully protonated guest/host systems. It is shown in this work that, using the W-band (95GHz) ELDOR- (electron–electron double resonance) detected NMR (EDNMR) method, different principal nitrogen hyperfine, Azz, and quadrupole, Pzz, tensor values of a nitroxide radical in glassy 2-propanol matrix can be measured with high accuracy. They belong to nitroxides with different hydrogen-bond situations. The satisfactory resolution and superior sensitivity of EDNMR as compared to the standard ENDOR (electron–nuclear double resonance) method are demonstrated.