Saturation-Recovery EPR with Nitroxyl Radical–Dy(III) Spin Pairs: Distances and Orientations

Electron paramagnetic resonance of lanthanides

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd³⁺. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides. Magnetic resonance imaging contrast agents use the long relaxation time of the S-state Gd³⁺ ion, and NMR shift reagents use the fast relaxation of selected other lanthanides. Other than Gd³⁺ and the isoelectronic Eu²⁺ ion, spin relaxation of the lanthanides is so fast that their EPR spectra can be observed only in the liquid helium temperature range. In this chapter the EPR properties of each of the lanthanides is briefly summarized, with an emphasis on electron spin relaxation.

Improving the accuracy of Cu(ii)–nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)–nitroxide rulers

Detailed analysis of artefacts in the Cu(ii)–nitroxide RIDME experiments, related to orientation averaging, echo-crossing, ESEEM and background-correction is presented.

Isoindoline nitroxide-labeled porphyrins as potential fluorescence-suppressed spin probes

A series of isoindoline nitroxide-labeled porphyrins were synthesized by the reaction of 5-phenyldipyrromethane and 5-(4'-carboethoxy-methyleneoxyphenyl)dipyrromethane with 5-formyl-1,1,3,3-tetramethylisoindolin-2-yloxyl (FTMIO) using the Lindsey method. The corresponding water-soluble spin-labeled porphyrins were also prepared. Subsequently, these compounds were characterized and their in vitro properties were evaluated. The electrochemical assay demonstrated that these isoindoline nitroxide-labeled porphyrins had similar electrochemical and redox properties to 5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl (CTMIO). The electron paramagnetic resonance test showed that these porphyrins exhibited hyperfine splittings and characteristic spectra of CTMIO with typical nitroxide g-values and nitrogen isotropic hyperfine coupling constants. The in vitro cytotoxicity assay indicated that these porphyrins possessed low cytotoxicity to human renal tubular epithelial 293T cells (normal cells) and human hepatoma HepG2 cells (tumor cells). Fluorescence spectroscopy revealed that free base isoindoline nitroxide-labeled porphyrins exhibited fluorescence suppression characteristic of nitroxide-fluorophore systems. In vitro fluorescene imaging demonstrated that the reduced isoindoline nitroxide-labeled porphyrins eliminated fluorescence suppression and displayed strong red fluorescence imaging in HepG2 cells. Thus these isoindoline nitroxide-labeled porphyrins may be considered potentially as biological spin probes for fluorescence imaging and EPR spectroscopy.

Long-range distance measurements in proteins at physiological temperatures using saturation recovery EPR spectroscopy

Site-directed spin labeling in combination with EPR is a powerful method for providing distances on the nm scale in biological systems. The most popular strategy, double electron-electron resonance (DEER), is carried out at cryogenic temperatures (50-80 K) to increase the short spin-spin relaxation time (T2) upon which the technique relies. A challenge is to measure long-range distances (20-60 Å) in proteins near physiological temperatures. Toward this goal we are investigating an alternative approach based on the distance-dependent enhancement of spin-lattice relaxation rate (T1(-1)) of a nitroxide spin label by a paramagnetic metal. With a commonly used nitroxide side chain (R1) and Cu(2+), it has been found that interspin distances ≤25 Å can be determined in this way (Jun et al. Biochemistry 2006, 45, 11666). Here, the upper limit of the accessible distance is extended to ≈40 Å using spin labels with long T1, a high-affinity 5-residue Cu(2+) binding loop inserted into the protein sequence, and pulsed saturation recovery to measure relaxation enhancement. Time-domain Cu(2+) electron paramagnetic resonance, quantum mechanical calculations, and molecular dynamics simulations provide information on the structure and geometry of the Cu(2+) loop and indicate that the metal ion is well-localized in the protein. An important aspect of these studies is that both Cu(2+)/nitroxide DEER at cryogenic temperatures and T1 relaxation measurements at room temperature can be carried out on the same sample, allowing both validation of the relaxation method and assessment of the effect of freezing on protein structure.