Determination of High-Spin Iron(III)–Nitroxyl Distances in Spin-Labeled Porphyrins by Time-Domain EPR

✍ Scribed by Margaret H. Rakowsky; Ana Zecevic; Gareth R. Eaton; Sandra S. Eaton

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 297 KB
• Volume: 131
• Category: Article
• ISSN: 1090-7807
• DOI: 10.1006/jmre.1997.1338

✦ Synopsis

Continuous wave EPR spectra of the nitroxyl signals for four

Continuous wave (CW) electron paramagnetic resonance spin-labeled high-spin (h.s.) Fe(III) porphyrins showed partially (EPR) has been used to obtain electron-electron interspin resolved splittings at temperatures near 4 K. Axial ligands were distances primarily via analysis of lineshape changes due to fluoride, chloride, or bromide. As temperature was increased to spin-spin splitting (5) or by measurement of the relative 20 to 30 K the iron-nitroxyl splitting collapsed due to increasing intensity of the half-field transition (6). The intensity of the rates of iron relaxation. Electron spin-echo (ESE) spectroscopy half-field transition falls off as r 06 so it is useful primarily for showed that above about 6 K collapse of the iron-nitroxyl spinrelatively short interspin distances. Dipolar splittings have an spin splitting caused a dramatic increase in the nitroxyl phase r 03 dependence. The longest distances for which resolved memory relaxation rates. Electron spin relaxation rates were deterlineshape changes can be observed depends upon linewidths, mined for Fe(tetratolylporphyrin) X, X Å F, Cl, Br, in toluene but 12-15 A ˚is a generous upper limit, except for systems solution by ESE or inversion recovery at 4.5 to 6 K and by analysis of the temperature-dependent contributions to the continuous with very narrow lines. To observe resolved spin-spin splitwave EPR linewidths between about 10 and 120 K. Above about tings the relaxation rate of both spins must be slow relative 10 K iron relaxation rates increase in the order X Å F õ Cl õ to the splitting, which limits the application of these methods Br, which is the order of increasing zero-field splitting. Saturation to slowly relaxing metals. Electron spin relaxation times are recovery data for two spin-labeled h.s. iron(III) porphyrins beexpected to be sensitive to interaction with a rapidly relaxing tween about 15 and 120 K and for two additional spin-labeled h.s. spin over longer distances than can be detected by changes iron(III) porphyrins between about 85 and 120 K demonstrated in CW lineshapes.

that interaction with the h.s. iron enhanced the electron spin relax-Impact of spin-spin interaction on nitroxyl T 1 . Equation ation rate of the spin label. The saturation recovery curves for the nitroxyl were analyzed to determine interspin distances using a , which is based on the NMR work of Bloembergen and modified version of the Bloembergen equation and independently colleagues (7-9), and is sometimes referred to as the determined iron relaxation rates. Interspin distances were between Bloembergen equation, describes the effect of a rapidly re-11.6 and 15.0 A ˚, were independent of axial ligand, and were in laxing electron spin on the relaxation rate for a more slowly good agreement with values obtained previously for low-spin relaxing spin due to dipolar coupling in rigid media. It has Fe(III) and Cu(II) analogs. ᭧ 1998 Academic Press been applied to mixtures of radicals with paramagnetic metal ions and to pairwise interaction between paramagnetic centers (13)(14)(15)(17)(18)(19)(21)(23). One early study (11) made 97