Search PubMed⌕ Search
Home/Archive/PubMed

PubMed · 16289964

Multibore sample cell increases EPR sensitivity for aqueous samples.

Yuri E Nesmelov·David D Thomas
View original source ↗Export metadata ↓

Abstract

We have performed calculations, verified by experiment, to explain why the sensitivity of biological EPR can be dramatically increased by dividing the aqueous sample into separate compartments. In biological EPR, the major factor affecting sensitivity is the number of spins in the sample. For an aqueous sample at ambient temperature, this is limited by the requirement for a small volume, due to strong non-resonant absorption of microwaves by water. However, recent empirical studies have shown that this volume limitation can be greatly relieved by dividing the aqueous sample into separate volumes, which allows much more aqueous sample to be loaded into a resonant cavity without significant degradation of the cavity quality factor. Calculations, based on the Bruggeman mixing rule, show quantitatively that the composite aqueous sample has a permittivity much less than that of bulk water, depending on the aqueous volume fraction f. Analysis for X-band EPR spectroscopy shows that the optimal volume fraction of an aqueous composite sample, producing maximum sensitivity, is f=0.15, increasing the sensitivity by a factor of 8.7, compared with an aqueous sample in a single tube.

Explore related subjects

Electron Spin Resonance SpectroscopySensitivity and SpecificitySignal Processing, Computer-AssistedSpin LabelsWaterJournal ArticleResearch Support, N.I.H., Extramural

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Yuri E Nesmelov, David D Thomas. 2005-11-10. Multibore sample cell increases EPR sensitivity for aqueous samples.. https://doi.org/10.1016/j.jmr.2005.10.012

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed2007

The S1YZ* metalloradical EPR signal of photosystem II contains two distinct components that advance respectively to the multiline and g = 4.1 conformations of S2.

The S2 state of the oxygen-evolving complex (OEC) of photosystem II is heterogeneous, exhibiting two main EPR spectral forms, the multiline and the g = 4.1 signal. It is not clearly established whether this heterogeneity develops during the S1 to S2 transition or is already present in the precursor states. We have compared the spectra of the S1YZ* intermediate, obtained by visible light excitation (induction of charge separation) of the S1 state at liquid He temperatures, (S1YZ*)vis, or by near-infrared (NIR) light excitation of the S2 state (utilization of the unusual property of the Mn cluster to act as an oxidant of Yz when excited by NIR), (S1YZ*)NIR. The decay kinetics of the (S1YZ*)vis spectrum at 11 K was also studied by the application of rapid-scan EPR. The two spectra share in common a signal with a characteristic feature at g = 2.035, but the (S1YZ*)vis spectrum contains in addition a fast decaying component 26 G wide. The analysis of the surface of the rapid-scan spectra yielded 270 +/- 35 and 90 +/- 15 s for the respective half-times of the two components of the (S1YZ*)vis spectrum at 11 K. (S1YZ*)vis advances efficiently to S2 when annealed at 200 K; notably the g = 2.035 signal advances to the multiline while the 26 G component advances to the g = 4.1 conformation. The "26 G" component is absent or very small, respectively, in thermophilic cyanobacteria or glycerol-containing spinach samples, in correlation to vanishing or very small amounts of the g = 4.1 component in the S2 spectrum. The results validate the assignment of S1YZ* to a true S1 to S2 intermediate and imply that the heterogeneity observed in S2 is already present in S1. Tentative valences are assigned to the individual Mn ions of the OEC in the two heterogeneous conformations of S1.

Electron Spin Resonance Spectroscopy↗
PubMed2006

Modeling the effects of structure and dynamics of the nitroxide side chain on the ESR spectra of spin-labeled proteins.

In the companion paper (J. Phys. Chem. B 2006, 110, jp0629487), a study of the conformational dynamics of methanethiosulfonate spin probes linked at a surface-exposed alpha-helix has been presented. Here, on the basis of this analysis, X-band ESR spectra of these spin labels are simulated within the framework of the Stochastic Liouville equation (SLE) methodology. Slow reorientations of the whole protein are superimposed on fast chain motions, which have been identified with conformational jumps and fluctuations in the minima of the chain torsional potential. Fast chain motions are introduced in the SLE for the protein reorientations through partially averaged magnetic tensors and relaxation times calculated according to the motional narrowing theory. The 72R1 and 72R2 mutants of T4 lysozyme, which bear the spin label at a solvent-exposed helix site, have been taken as test systems. For the side chain of the R2 spin label, only a few noninterconverting conformers are possible, whose mobility is limited to torsional fluctuations, yielding almost identical spectra, typical of slightly mobile nitroxides. In the case of R1, more complex spectra result from the simultaneous presence of constrained and mobile chain conformers, with relative weights that can depend on the local environment. The model provides an explanation for the experimentally observed dependence of the spectral line shapes on temperature, solvent, and pattern of substituents in the pyrroline ring. The relatively simple methodology presented here allows the introduction of realistic features of the spin probe dynamics into the simulation of ESR spectra of spin-labeled proteins; moreover, it provides suggestions for a proper account of such dynamics in more sophisticated approaches.

Electron Spin Resonance Spectroscopy↗