ENDOR Study of Irradiated Histidine HCl

Box, Harold C.; Freund, Harold G.; Lilga, Kenneth T.

doi:10.1063/1.1841012

Cited by 40 publications

(8 citation statements)

References 8 publications

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“…21 Looking at the isotropic value of the a-hfc tensor, however, the spin density is estimated to be only about 0.59 from the McConnell relation 22 assuming a Q-value of about 80 MHz as suggested from a previous study on the 5-membered ring of histidine. 23 This situation is typical if the site of the unpaired spin density is bent, that is, the C 1 atom exhibits a somewhat pyramidal bonding structure. The lone electron orbital (LEO) will then acquire some 2s-orbital character that y An ENDOR line is superscripted with + if it belongs to the M s ¼ 1/ in turn will reduce the negative isotropic spin density at the a-proton.…”

Section: Results and Analysis Epr Resultsmentioning

confidence: 99%

EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

Hosseini

Lund

Sagstuen

2002

Phys. Chem. Chem. Phys.

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When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C 3 O 2 NH 5 ) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one a-coupling, and three b-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carboncentered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C 1 -H bond with respect to the N-C 1 -C 2 plane is estimated to be 13-15 (bending angle y % 7 ). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogenbonded proton hyperfine interactions.

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Section: Results and Analysis Epr Resultsmentioning

confidence: 99%

EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

Hosseini

Lund

Sagstuen

2002

Phys. Chem. Chem. Phys.

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“…A stable secondary histidine radical in histidine hydrochloride crystals irradiated at room temperature was formed by hydrogen addition to the imidazole ring resulting in two large proton couplings (isotropic value: 4.7 and 5.0 mT). 21 A deamination radical and carboxyl anion radicals have also been observed in X-irradiated single crystals of histidine dihydrochloride. 22 EPR and ENDOR data of X-irradiated single crystals of imidazole were published for a radical which is formed in a reducing step upon H-addition to the C2 position (for Cnumbering of the imidazole ring, see Figure 1).…”

Section: Introductionmentioning

confidence: 93%

“…The hyperfine coupling tensors for two ring α-protons of the cation radical (isotropic values: 1.06 and 1.21 mT) have been determined, but no hyperfine data were given for β-protons and imidazole nitrogens. A stable secondary histidine radical in histidine hydrochloride crystals irradiated at room temperature was formed by hydrogen addition to the imidazole ring resulting in two large proton couplings (isotropic value: 4.7 and 5.0 mT) …”

Section: Introductionmentioning

confidence: 99%

Electronic Structure of a Transient Histidine Radical in Liquid Aqueous Solution: EPR Continuous-Flow Studies and Density Functional Calculations

et al. 1999

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Transient histidine radicals formed in aqueous solutions by oxidation of histidine with a Ti3+/H2O2 Fenton system at pH 2.0 have been studied by EPR using a fast continuous-flow setup and a dielectric ring resonator equipped with a mixing chamber. A histidine peroxy radical with a single EPR line at g = 2.0151 and a histidine cation radical with a complex hyperfine structure and g = 2.0023 have been detected. The hyperfine structure of the latter radical was analyzed by investigating two selectively deuterated histidines and using an EPR simulation and fit program for analysis of the spectra. Isotropic hyperfine coupling constants of two β-protons, three ring protons, and two nitrogen nuclei have been determined in this way and assigned to a histidine−OH adduct cation radical. Density functional theory (DFT) calculations at the B3LYP and PWP86 levels have been performed on protonated cation radicals of 4-ethylimidazole (as histidine models), yielding isotropic hyperfine coupling constants for three different positions of OH addition. The C5 position for OH addition (a 5-oxohistidine cation radical) is clearly supported by the calculated hyperfine coupling constants. The agreement between DFT and EPR is further improved when hydrogen-bonding interactions to the N1 and C2 protons are introduced in the calculations.

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“…lB displays the ENDOR spectrum of a model histidine radical, which differs greatly from that observed for the tyrosine radical because of the presence of protons with extremely large hyperfine couplings. This radical, formed in x-irradiated histidine hydrochloride, has been assigned to a C2 H adduct of histidine (31). The Inset shows the ESE-EPR spectrum, with the arrow indicating the magnetic field position at which the ENDOR experiment was performed.…”

Section: Methodsmentioning

confidence: 99%

Proximity of the manganese cluster of photosystem II to the redox-active tyrosine YZ.

Gilchrist

Ball

Randall

et al. 1995

Proc. Natl. Acad. Sci. U.S.A.

266

309

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Electron spin echo electron-nuclear double resonance (ESE-ENDOR) experiments performed on a broad radical electron paramagnetic resonance (EPR) signal observed in photosystem II particles depleted of Ca2+ indicate that this signal arises from the redox-active tyrosine Yz. The tyrosine EPR signal width is increased relative to that observed in a manganese-depleted preparation due to a magnetic interaction between the photosystem II manganese cluster and the tyrosine radical. The manganese cluster is located asymmetrically with respect to the symmetry-related tyrosines Yz and YD. The distance between the Yz tyrosine and the manganese cluster is estimated to be approximately 4.5 A. Due to this close proximity of the Mn cluster and the redox-active tyrosine Yz, we propose that this tyrosine abstracts protons from substrate water bound to the Mn cluster.The photosystem II (PS II) component of the plant photosynthetic apparatus oxidizes water at an oxygen-evolving complex (OEC) that consists of a tetranuclear cluster of manganese ions along with essential cofactors calcium and chloride (1). The overall architecture of PS II shows homologies to the reaction centers of the purple non-oxygen-evolving bacteria, including the C2 symmetry found in these reaction centers (2-4). In PS II this symmetry appears to persist to the sites of two important tyrosine residues, Yz (tyrosine-161 of the Dl polypeptide, with residues designated for Synechocystis sp. 6803) and YD (tyrosine-160 of the D2 polypeptide) (5-7). The Yz tyrosine serves as an electron transfer intermediate between the chargeseparating chlorophyll moiety P680 and the manganese cluster of the OEC. The symmetry-related tyrosine YD, which is typically present as a dark-stable neutral radical (YD-), is bypassed in the fast electron transfer between P680 and the OEC. Whether the Mn cluster is located on the C2 symmetry axis has been the subject of much debate. Instead, the PS II symmetry may be broken at the OEC level, locating the Mn cluster closer to the active electron transfer intermediate Yz.Calcium depletion of PS II particles by NaCl/EGTA washing (8-10) or low-pH citrate treatment (11-13) eliminates oxygen-evolving activity. In such Ca2+-depleted PS II particles, illumination at a temperature of 273 K leads to the formation of a broad (130-180 G full width at half maximum) g = 2 EPR signal. Other treatments that block oxygen evolution such as acetate or fluoride incubation lead to similar signals upon such illumination, though the signal widths vary appreciably, depending on the details of the treatment and the resulting extrinsic polypeptide composition (14-18). The broad g = 2 signal is thought to arise from a radical center, with the large linewidth caused by a magnetic interaction with the Mn cluster (18,19). UV (19) and IR (20) absorption changes in PS II that are observed concomitantly with the formation of the radical have been interpreted to favor an oxidized histidine as the origin of the broad radical signal. Alternatively, on the basis of EPR s...

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ENDOR Study of Irradiated Histidine HCl

Cited by 40 publications

References 8 publications

EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

Electronic Structure of a Transient Histidine Radical in Liquid Aqueous Solution: EPR Continuous-Flow Studies and Density Functional Calculations

Proximity of the manganese cluster of photosystem II to the redox-active tyrosine YZ.

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