Echo envelope modulation with quadrupole interaction

Rowan, L. G.

doi:10.1016/0022-2364(87)90340-4

Cited by 4 publications

(5 citation statements)

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“…The observation that echo modulation depth decreases with larger quadrupole interaction was discussed by Rowan, who showed that when the quadrupole interaction is much stronger than the electron nuclear hyperfine interaction, the nuclear spin quantum axis is determined by the direction of the quadrupole interaction and is not changed appreciably by electron spin flips induced by the microwave pulses. Therefore there is little nuclear spin state coherent superposition induced by the microwave pulse and therefore little echo envelope modulation results.…”

Section: Discussionmentioning

confidence: 98%

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using ⁸⁷Sr ESEEM Spectroscopy

et al. 2004

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The proximity of the calcium/strontium binding site of the oxygen evolving complex (OEC) of photosystem II (PSII) to the paramagnetic Mn cluster is explored with (87)Sr three-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy. CW-EPR spectra of Sr(2+)-substituted Ca(2+)-depleted PSII membranes show the modified g = 2 multiline EPR signal as previously reported. We performed three-pulse ESEEM on this modified multiline signal of the Mn cluster using natural abundance Sr and (87)Sr, respectively. Three-pulse ESEEM of the natural abundance Sr sample exhibits no detectable modulation by the 7% abundance (87)Sr. On the other hand, that of the (87)Sr enriched (93%) sample clearly reveals modulation arising from the I = (9)/(2) (87)Sr nucleus weakly magnetically coupled to the Mn cluster. Using a simple point dipole approximation for the electron spin, analysis of the (87)Sr ESEEM modulation depth via an analytic expression suggests a Mn-Ca (Sr) distance of 4.5 A. Simulation of three-pulse ESEEM with a numerical matrix diagonalization procedure gave good agreement with this analytical result. A more appropriate tetranuclear magnetic/structural model for the Mn cluster converts the 4.5 A point dipole distance to a 3.8-5.0 A range of distances. DFT calculations of (43)Ca and (87)Sr quadrupolar interactions on Ca (and Sr substituted) binding sites in various proteins suggest that the lack of the nuclear quadrupole induced splitting in the ESEEM spectrum of (87)Sr enriched PSII samples is related to a very high degree of symmetry of the ligands surrounding the Sr(2+) ion in the substituted Ca site. Numerical simulations show that moderate (87)Sr quadrupolar couplings decrease the envelope modulation relative to the zero quadrupole case, and therefore we consider that the 3.8-5.0 A range obtained without quadrupolar coupling included in the simulation represents an upper limit to the actual manganese-calcium distance. This (87)Sr pulsed EPR spectroscopy provides independent direct evidence that the calcium/strontium binding site is close to the Mn cluster in the OEC of PSII.

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Section: Discussionmentioning

confidence: 98%

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using ⁸⁷Sr ESEEM Spectroscopy

et al. 2004

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“…42 Simulation of ESEEM recordings was performed using the density operator (DO) formalism as described previously. 43 The general strategy consisted of evaluation of the DO matrix, F ˆ, which is initially equal to that of the thermal equilibrium DO, F ˆ0, and then evolves during the subsequent periods of pulse action and the pulse-free evolution, terminating at the final instant of echo detection…”

Section: Methodsmentioning

confidence: 99%

“…They were programmed and executed using the EasySpin toolbox for the Matlab program package . Simulation of ESEEM recordings was performed using the density operator (DO) formalism as described previously . The general strategy consisted of evaluation of the DO matrix, ρ̂, which is initially equal to that of the thermal equilibrium DO, ρ̂ 0 , and then evolves during the subsequent periods of pulse action and the pulse-free evolution, terminating at the final instant of echo detection ρ ^ SSE = U ^ SSE · ρ ^ 0 · U ^ SSE − 1 Here ρ̂ 0 = Z ·exp(− h ℋ̂/ kT ) with Z −1 = Tr{exp(− h ℋ̂/ kT )}.…”

Section: Methodsmentioning

confidence: 99%

High-Field EPR and ESEEM Investigation of the Nitrogen Quadrupole Interaction of Nitroxide Spin Labels in Disordered Solids: Toward Differentiation between Polarity and Proticity Matrix Effects on Protein Function

et al. 2008

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The combination of high-field electron paramagnetic resonance (EPR) with site-directed spin labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in revealing subtle changes of the polarity and proticity profiles in proteins enbedded in membranes. This information can be obtained by orientation-selective high-field EPR resolving principal components of the nitroxide Zeeman (g) and hyperfine ( A) tensors of the spin labels attached to specific molecular sites. In contrast to the g- and A-tensors, the (14)N ( I = 1) quadrupole interaction tensor of the nitroxide spin label has not been exploited in EPR for probing effects of the microenvironment of functional protein sites. In this work it is shown that the W-band (95 GHz) high-field electron spin echo envelope modulation (ESEEM) method is well suited for determining with high accuracy the (14)N quadrupole tensor principal components of a nitroxide spin label in disordered frozen solution. By W-band ESEEM the quadrupole components of a five-ring pyrroline-type nitroxide radical in glassy ortho-terphenyl and glycerol solutions have been determined. This radical is the headgroup of the MTS spin label widely used in SDSL protein studies. By DFT calulations and W-band ESEEM experiments it is demonstrated that the Q(yy) value is especially sensitive to the proticity and polarity of the nitroxide environment in H-bonding and nonbonding situations. The quadrupole tensor is shown to be rather insensitive to structural variations of the nitroxide label itself. When using Q(yy) as a testing probe of the environment, its ruggedness toward temperature changes represents an important advantage over the g xx and A(zz) parameters which are usually employed for probing matrix effects on the spin labeled molecular site. Thus, beyond measurenments of g xx and A(zz) of spin labeled protein sites in disordered solids, W-band high-field ESEEM studies of (14)N quadrupole interactions open a new avenue to reliably probe subtle environmental effects on the electronic structure. This is a significant step forward on the way to differentiate between effects from matrix polarity and hydrogen-bond formation.

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“…A 14 N eeqQ value of 2.0 MHz (η ) 0) was chosen on the basis of an azide solution NMR study (44,45). For the modest isotropic coupling employed here (lower than the "exact cancellation" matching condition that gives favorable 14 N modulation) (46), the effect of the nonzero nqi is to broaden and decrease the amplitude of the 14 N peaks (47). Peaks were observed in the undamped simulation (not shown), but after the exponential damping function was applied, the simulated frequency-domain spectrum was quite featureless, matching the experimental observation.…”

Section: Resultsmentioning

confidence: 99%

Evidence That Azide Occupies the Chloride Binding Site near the Manganese Cluster in Photosystem II

Aznar

et al. 2005

Biochemistry

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The effect of adding azide to photosystem II (PS II) membrane samples (BBY preparation), with or without chloride, has been investigated using continuous wave (CW) and pulsed EPR spectroscopy. In the BBY samples with 25 mM chloride, we observed that the inhibition induced by azide is partly recovered by the addition of bicarbonate. Electron spin-echo envelope modulation (ESEEM) was used to search for spin transitions of 15N nuclei magnetically coupled to the S2 state Mn cluster (multiline EPR signal form) in 15N (single terminal label) azide-treated samples with negative results. However, an 15N ESEEM peak was observed in parallel chloride-depleted PS II samples when the 15N-labeled azide is added. However, this peak is absent in chloride-depleted samples incubated in buffer containing both chloride and [15N]azide. Thus these results demonstrate an azide binding site in the immediate vicinity of the Mn cluster, and since this site appears to be competitive with chloride, these results provide further evidence that chloride is bound proximal to the Mn cluster as well. Discussion on the possible interplay between azide, chloride, and bicarbonate is provided.

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Echo envelope modulation with quadrupole interaction

Cited by 4 publications

References 4 publications

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using ⁸⁷Sr ESEEM Spectroscopy

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using ⁸⁷Sr ESEEM Spectroscopy

High-Field EPR and ESEEM Investigation of the Nitrogen Quadrupole Interaction of Nitroxide Spin Labels in Disordered Solids: Toward Differentiation between Polarity and Proticity Matrix Effects on Protein Function

Evidence That Azide Occupies the Chloride Binding Site near the Manganese Cluster in Photosystem II

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Echo envelope modulation with quadrupole interaction

Cited by 4 publications

References 4 publications

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using 87Sr ESEEM Spectroscopy

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using 87Sr ESEEM Spectroscopy

High-Field EPR and ESEEM Investigation of the Nitrogen Quadrupole Interaction of Nitroxide Spin Labels in Disordered Solids: Toward Differentiation between Polarity and Proticity Matrix Effects on Protein Function

Evidence That Azide Occupies the Chloride Binding Site near the Manganese Cluster in Photosystem II

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Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using ⁸⁷Sr ESEEM Spectroscopy

Investigation of the Calcium-Binding Site of the Oxygen Evolving Complex of Photosystem II Using ⁸⁷Sr ESEEM Spectroscopy