Electronic structure of the YD tyrosyl radical in photosystem II: A high-frequency electron paramagnetic resonance spectroscopic and density functional theoretical study

Christian T. Farrar, Gary J. Gerfen, Robert G. Griffin, Dee Ann Force, R. David Britt

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Abstract

We report the high-frequency (139.5 GHz) electron paramagnetic resonance (EPR) spectrum of the Y. D tyrosyl radical of photosystem II. A rhombic powder pattern with principal g values g1 = 2.007 82, g2 = 2.004 50, and g3 = 2.002 32 is observed. The well-defined turning points and the value of the largest principal g value are indicative of ordered hydrogen bonding to the tyrosyl phenyl oxygen. Hyperfine structure is resolved on all three turning points. Proton hyperfine couplings obtained from the simulation of the 139.5 GHz EPR spectrum are in good agreement with X-band electron spin echo-electron nuclear double resonance studies. The high-frequency EPR spectrum was acquired under conditions of saturation in which the dispersion signal is detected. Proper replication of the high-frequency EPR spectral features is only achieved in simulations which account for the line shapes characteristic of saturated dispersion signals. Comparison of the Y. D spectrum with spectra of non-hydrogen bonded tyrosyl radicals indicates that the largest principal g value (g1), oriented along the C-O bond, is sensitive to hydrogen bonding at the phenyl oxygen. Density functional calculations indicate that the decreased downfield shift in g1 from the free electron g value with increasing hydrogen bond strength arises from both a decreased spin density on the phenyl oxygen-5-30% over a range of reasonable hydrogen bond distances (2.0-1.1 Å) - and an increased splitting between ground state and excited state singly occupied molecular orbitals.

Original languageEnglish (US)
Pages (from-to)6634-6641
Number of pages8
JournalJournal of Physical Chemistry B
Volume101
Issue number33
StatePublished - Aug 14 1997

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Photosystem II Protein Complex
Electronic structure
Paramagnetic resonance
electron paramagnetic resonance
Hydrogen bonds
electronic structure
Oxygen
Electrons
oxygen
hydrogen bonds
Molecular orbitals
Excited states
Powders
Ground state
Density functional theory
Protons
hydrogen
superhigh frequencies
hyperfine structure
electron spin

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Engineering(all)

Cite this

Electronic structure of the YD tyrosyl radical in photosystem II : A high-frequency electron paramagnetic resonance spectroscopic and density functional theoretical study. / Farrar, Christian T.; Gerfen, Gary J.; Griffin, Robert G.; Force, Dee Ann; Britt, R. David.

In: Journal of Physical Chemistry B, Vol. 101, No. 33, 14.08.1997, p. 6634-6641.

Research output: Contribution to journalArticle

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abstract = "We report the high-frequency (139.5 GHz) electron paramagnetic resonance (EPR) spectrum of the Y. D tyrosyl radical of photosystem II. A rhombic powder pattern with principal g values g1 = 2.007 82, g2 = 2.004 50, and g3 = 2.002 32 is observed. The well-defined turning points and the value of the largest principal g value are indicative of ordered hydrogen bonding to the tyrosyl phenyl oxygen. Hyperfine structure is resolved on all three turning points. Proton hyperfine couplings obtained from the simulation of the 139.5 GHz EPR spectrum are in good agreement with X-band electron spin echo-electron nuclear double resonance studies. The high-frequency EPR spectrum was acquired under conditions of saturation in which the dispersion signal is detected. Proper replication of the high-frequency EPR spectral features is only achieved in simulations which account for the line shapes characteristic of saturated dispersion signals. Comparison of the Y. D spectrum with spectra of non-hydrogen bonded tyrosyl radicals indicates that the largest principal g value (g1), oriented along the C-O bond, is sensitive to hydrogen bonding at the phenyl oxygen. Density functional calculations indicate that the decreased downfield shift in g1 from the free electron g value with increasing hydrogen bond strength arises from both a decreased spin density on the phenyl oxygen-5-30{\%} over a range of reasonable hydrogen bond distances (2.0-1.1 {\AA}) - and an increased splitting between ground state and excited state singly occupied molecular orbitals.",
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N2 - We report the high-frequency (139.5 GHz) electron paramagnetic resonance (EPR) spectrum of the Y. D tyrosyl radical of photosystem II. A rhombic powder pattern with principal g values g1 = 2.007 82, g2 = 2.004 50, and g3 = 2.002 32 is observed. The well-defined turning points and the value of the largest principal g value are indicative of ordered hydrogen bonding to the tyrosyl phenyl oxygen. Hyperfine structure is resolved on all three turning points. Proton hyperfine couplings obtained from the simulation of the 139.5 GHz EPR spectrum are in good agreement with X-band electron spin echo-electron nuclear double resonance studies. The high-frequency EPR spectrum was acquired under conditions of saturation in which the dispersion signal is detected. Proper replication of the high-frequency EPR spectral features is only achieved in simulations which account for the line shapes characteristic of saturated dispersion signals. Comparison of the Y. D spectrum with spectra of non-hydrogen bonded tyrosyl radicals indicates that the largest principal g value (g1), oriented along the C-O bond, is sensitive to hydrogen bonding at the phenyl oxygen. Density functional calculations indicate that the decreased downfield shift in g1 from the free electron g value with increasing hydrogen bond strength arises from both a decreased spin density on the phenyl oxygen-5-30% over a range of reasonable hydrogen bond distances (2.0-1.1 Å) - and an increased splitting between ground state and excited state singly occupied molecular orbitals.

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