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|Title:||Moessbauer Studies of Photosynthetic Reaction Centers From Rhodopseudomonas Sphaeroides R-26|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Zero field Mossbauer measurements on ('57)Fe-enriched reaction centers (RCs) differing in ubiquinone content, detergent, oxidation state, or the presence of o-phenanthroline all show a single quadrupole doublet of similar splitting (DELTA)E(,Q), center shift (delta), and temperature dependence. The results are indicative of high-spin ferrous iron (Fe('+2), S = 2) with an approximately invariant first coordination sphere consisting most likely of 6 (or 5) oxygens and nitrogens. The spectra show no evidence for either of the quinones or o-phenanthroline binding directly to the iron.
High field (1.0T (LESSTHEQ) B (LESSTHEQ) 6.15T) variable temperature (4.2K (LESSTHEQ) T (LESSTHEQ) 211K) Mossbauer measurements on native (N) and dithionite reduced (R) RCs yield details concerning the electronic state of the iron. At low temperatures and fields the spectra of N and R differ; the former consists of a sharp quadrupole doublet, while the lines in the latter are magnetically broadened via the spin-spin interaction with the semiquinone free radical. In high field the spectra of N and R are similar, showing broad unresolved doublets at 4.2K, which resolve at higher temperatures. The width of the lower energy line has a minimum near 25K.
Analysis of the magnetic data using an empirical model of the internal magnetic field shows that the largest component of the quadrupole tensor, V(,zz), is positive, the asymmetry parameter, (eta), is 0.3 at 4.2K and 0.6 at 211K, and the quadrupole tensor is rotated with respect to the internal magnetic field. The compounds of the internal field differ widely in magnitude and temperature dependence. Spin Hamiltonian simulations of the data suggest an unusually small A-tensor at 4.2K, (-9.41T,+1.72T,-10.3T), however, good simulations at other temperatures and fields are obtained if the A-tensor components are allowed to vary.
Analysis using a crystal field model with strong electron delocalization can account for the temperature dependence of (DELTA)E(,Q), and indicates considerable "unquenched"(' ). Both the zero field and magnetic measurements suggest the ground state is primarily an (VBAR)xy> orbital and together they suggest that the symmetry at the iron site is lower than rhombic.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.
|Date Available in IDEALS:||2015-05-13|