Mössbauer studies of photsynthetic reaction centers from Rhodopseudomonas sphaeroides R-26
Boso, Brian
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https://hdl.handle.net/2142/25419
Description
Title
Mössbauer studies of photsynthetic reaction centers from Rhodopseudomonas sphaeroides R-26
Author(s)
Boso, Brian
Issue Date
1982
Doctoral Committee Chair(s)
Debrunner, Peter G.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Mossbauer
photosynthetic reaction centers
ubiquinone
high-spin ferrous iron
o-phenanthroline
Language
en
Abstract
"Zero field Mossbauer measurements on 57Fe-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 6EQ, center shift 0, 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 ~ B < 6.15T) variable temperature (4.2K ~ T < 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 Nand 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 Nand Rare similir, 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, Vzz ' is positive, the asymmetry parameter, n , 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 components of the internal field differ widely in magnitude and temperature dependence.Spi'1 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 6E, and
DeltaEq
indicates considerable ""unquenched"" . Both the zero field and magnetic measurements suggest the ground state is primarily an orbital and together they suggest that the symmetry at the iron site is lower than rhombic."
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