Multifield Saturation Magnetization and Multifrequency EPRMeasurements of Deoxyhemerythrin Azide. A Unified Picture

The magnetic properties of deoxyhemerythrin azide have been measured by combining multifield saturation magnetization data and multifrequency EPR spectra. The exchange coupling [J = -3.4 (1 ) cm-l; JSI-S2], average zero-field splitting parameters [lD1l = 1D21 = 12 ( I ) cm-I and E l / D I = E 2 / D 2 = 0.31, and average gvalues [gl = g2 = 2.23 ( 5 ) ] have been determined assuming identical iron sites with parallel zero-field splitting axes for the ferromagnetically coupled high-spin diferrous center. The observed ratio of the exchange coupling to the zero-field splitting ( J / D i = 0.28) lies in the weak exchange coupling regime of a perturbation treatment of the full 25 X 25 spin Hamiltonian of the coupled diiron center. Perturbation expressions required for the description of the integer-spin EPR signals of two S = 2 metals in either the strong (J/Di > 1 ) or weak ( J / D i < 1/3) exchange coupling regimes are presented. For deoxyhemerythrin azide the EPR signal arises from the lowest pair of energy levels of the coupled system and can be interpreted in either coupling regime. The saturation magnetization family of curves, on the other hand, are a thermal average of all levels populated from 2 to 200 K and can resolve this ambiguity in the interpretation of the EPR data. An excellent fit to the entire family of multifield saturation magnetization curves has determined both the exchange coupling and zero-field splitting of deoxyhemerythrin azide, which lies in the weak coupling regime. In addition, quantitative agreement between saturation magnetization, integer-spin EPR, and optical data has been obtained. Hemerythrin is a well-characterized oxygen-storage protein isolated from several marine organisms.lS2 For deoxyhemerythrin the active site consists of two high-spin iron atoms that are bridged by a hydroxo g r o ~ p ~ , ~ and two carboxolato groups, one each from an aspartate and a glutamate r e s i d ~ e . ~ Five of the remaining coordination sites of the diiron center are histidine residues, leaving a single coordination site available for exogenous ligands. In deoxyhemerythrin the two high-spin ferrous ions a re antiferro*Address correspondence to this author at the Department of Physics, 'Gray Freshwater Biological Institute, University of Minnesota. 'Chemistry Department, University of New Hampshire. Emory University, Atlanta, GA 30322. Department of Chemistry, University of Minnesota. 0002-7863/91/1513-3039$02.50/0 magnetically coupled with J estimated to be between 20 and 70 cm-I (JsI.S2).1923496-7 Reem and Solomon showed, in studies combining magnetic circular dichroism (MCD) and EPR, that upon addition of azide to deoxyhemerythrin the exchange coupling (1) Klotz, I. M.; Kurtz, D. M., Jr. Ace. Chem. Res. 1984, 17, 16-22. (2) Wilkins, P. C.; Wilkins, R. G. Coord. Chem. Reu. 1987, 79, 195-214. (3) Reem, R. C.; Solomon, E. I. J. Am. Chem. Soc. 1984,106,8323-8325. (4) Zhang, K.; Stern, E. A.; Ellis, T.; Sanders-Lcehr, J.; Shiemke, A. K. ( 5 ) Stenkamp, R. E.; Sieker, L. C.; Jensen, L. H.; McCullum, J. D.; (6) Maroney, M. J.; Kurtz, D. M.; Nocek, J. M.; Pearce, L. L.; Que, L. (7) Reem, R.C.;Solomon, E. I. J . Am. Chem.Soc. 1987,109,1216-1226. Biochemistry 1988, 27, 747Q-7479. Sanders-Loehr, J. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 713-716. J . Am. Chem. SOC. 1986, 108, 6871-6879.