Optically Detected Magnetic Resonance Signals Research Articles

Optically detected magnetic resonance of tryptophan residues in Escherichia coli ssb gene product and E. coli plasmid-encoded single-stranded DNA-binding proteins and their complexes with poly(deoxythymidylic) acid.

Optically detected magnetic resonance (ODMR) spectroscopy has been applied to several single-stranded DNA-binding (SSB) proteins encoded by conjugative plasmids of enteric bacteria. Fluorimetric equilibrium binding isotherms confirm their preferential binding to single-stranded DNA and polynucleotides and reveal a limited protein solubility at low ionic strength. The plasmid SSB-like proteins show the highest affinity for polydeoxythymidylic acid; these complexes are the least sensitive to disruption by salt. ODMR data on these complexes suggest the existence of stacking interactions between tryptophan residue(s) and thymine bases, as evidenced by spectral red shifts of the tryptophan phosphorescence 0,0 band, reduction of the magnitude of D zero field splitting parameter, and a dramatic reversal of the polarity of the ODMR signals. Wavelength-selected ODMR results point to the existence of two distinct tryptophan sites in these complexes. The triplet state properties of the red-shifted site are drastically altered by its interaction with the thymine bases. The chromosomal Escherichia coli SSB protein-poly(dT) complex shows an additional tryptophan site with zero field splitting parameters similar to those of the free protein. This site can be attributed to Trp-135, which is missing in each of the other plasmid SSB proteins, suggesting that this particular residue is not involved in the interaction with polynucleotides.

Stacking interactions of tryptophan residues and nucleotide bases in complexes formed between Escherichia coli single-stranded DNA binding protein and heavy atom-modified poly(uridylic) acid. A study by optically detected magnetic resonance spectroscopy.

The complexes formed between Escherichia coli single-stranded DNA binding protein (SSBP) and the heavy atom-modified single-stranded polynucleotides poly(5-BrU) and poly(5-HgU) are investigated using optically detected magnetic resonance (ODMR) methods. In these complexes the triplet state properties of the tryptophan residues are subjected to the external heavy atom effect generated by bromine and mercury atoms and are characterized by a shortened triplet state lifetime and the appearance of the otherwise dark [D] + [E] slow passage ODMR signal. These features provide direct evidence for close range interactions between tryptophan residue(s) and the nucleotide bases in the complexes. The extent of the triplet state lifetime reduction in the case of the SSBP-poly(5-HgU) complex together with steric considerations of the complex structure is consistent only with a van der Waals contact between the perturbed molecule and the heavy atom perturber by means of a stacking interaction. Fast passage ODMR measurements show a lifetime for a sublevel of the perturbed tryptophan chromophore(s) in this complex on the order of 1 ms. The amplitude-modulated phosphorescence microwave double resonance technique captures selectively the broadened and red-shifted phosphorescence spectrum of the heavy atom-perturbed tryptophan residue(s). This work supports a model for the binding of SSBP to single-stranded polynucleotides in which the bases are inserted into hydrophobic regions of the protein, where they are likely to undergo stacking interactions with the indole moiety of buried tryptophan residues.

Phosphorescence and ODMR studies of isotopically mixed monoclinic and triclinic 1,4-dichlorobenzene

Phosphorescence and optically detected magnetic resonance (ODMR) studies were performed on the perprotonated triplet traps in isotopically mixed ( h 4 in d 4) 1,4-dichlorobenzene at 1.75 K. We find evidence for two distinct crystalline phases of the d 4 host crystal which are stable in temperature ranges near room temperature, each of which induces a distinct set of ODMR parameters on the trap species being monitored. Both cw and pulsed ODMR are used to probe for evidence of trap-to-trap triplet energy transfer at several trap concentrations. We find that at low concentrations ODMR signals are strong, and the excitations are definitely localized. At concentrations above 9%, both coherent and cw ODMR signals become hard to detect. The results are discussed in terms of likely mechanisms for energy exchange among the triplet traps.

ODMR in glow-discharge deposited silicon nitride

We report optically detected magnetic resonance (ODMR) measurements on glow-discharge deposited a-SiN x:H in the range 0 < x < 0.95. We observe one quenching and two enhancing resonances close to g=2. The ODMR signals in silicon nitride are analogous to those in a-Si:H, showing the same dependence on experimental parameters (microwave power and chopping frequency, light intensity), although the widths are larger as a result of hyperfine interactions. The magnitude of the quenching resonance is also larger in the nitrides owing to the higher density of dangling bonds, which participate in non-radiative recombination. The results are in agreement with the Robertson-Powell model in which the principal defect in the energy gap of a-SiN x:H is the silicon dangling bond.

Optically detected magnetic resonance study of SiC:Ti.

We describe optically detected magnetic resonance (ODMR) experiments carried out on the green luminescence band in 4H, 6H, and 15R polytypes of SiC:Ti. It is found that the luminescence is due to radiative transitions from excited spin triplet (S=1) to ground singlet (S=0) states. Unusually large isotope shifts in the zero-field splittings demonstrate that the light-emitting centers involve a single Ti and six equivalent neighboring silicon atoms. Spectral dependence studies of the ODMR signals allow us to discuss the microscopic nature of the inequivalent lattice sites for the Ti atom as well as to correctly assign the zero-phonon line and the accompanying characteristic phonons associated with each Ti site. We interpret our results in terms of the model proposed by Patrick and Choyke of Ti substituting on the silicon sublattice, the excitonic triplet state being formed from an electron tightly bound in a normally unoccupied ${d}_{e}$ orbital of the Ti atom plus a Coulombically bound hole. The ODMR spectra are extremely sensitive to subtle (second- and third-nearest-neighbor) differences between the inequivalent lattice sites. This, plus a sizable Franck-Condon shift observed in the luminescence and the unusual isotope effects, are interpreted as possible evidence for vibronic properties of the bound hole, implying intermediate to strong localization for it also.

ODMR detected via the electroluminescence in p+-i-n+a-Si:H cells

Optically detected magnetic resonance (ODMR) has been observed via electroluminescence for the first time. AC electroluminescence was excited by both forward and reverse bias and a quenching signal was observed at resonance. The results are compared with photoluminescence ODMR and PDMR (photovoltaic detected magnetic resonance) signals taken under the same conditions.

Comment on "Recombination ina-Si: H: Spin-dependent effects"

Street concludes from his recent time-resolved optically detected magnetic resonance (ODMR) data that the enhancing ODMR signals in $a$-Si: H are consistent with a model involving geminate pair recombination at electron and hole tail states, with one spin thermalized, and one unthermalized. We show, firstly, that this model is incompatible with the steady-state ODMR signals in low-defect-density $a$-Si: H; secondly, we note that qualitatively different time-resolved ODMR data have been obtained from two independent ODMR studies, neither of which supports the geminate model; thirdly, we show that the steady-state and time-resolved ODMR data are entirely consistent with the alternative model of nongeminate, distant pair recombination.

Determination of the g values of the ODMR signals in a-Si : H

Abstract Recent studies of optically detected magnetic resonance (ODMR) in a-Si : H have provided conflicting interpretations of the principal recombination processes in this material. In all cases the identification of the ODMR signals has rested on a comparison of the ODMR signal g values and those from the reported ESR and light-induced ESR (LESR) measurements in the literature. The present controversy arises because of the diverse g values reported for the ODMR signals. The problem of measuring the g values associated with the ODMR signals in a-Si : H is discussed, and it is shown how most of the present discrepancies among the various reports can be resolved by experimental considerations. Finally, the implications of this study for the various recombination models deduced from the ODMR results are mentioned.

Spin dependent carrier capture processes observed by ODMR on the 0.84 eV luminescence in SI-GaAs:Cr

Optically detected magnetic resonance (ODMR) observed on the 0.84eV Cr-related emission in semi-insulating GaAs:Cr implies that the excited state associated with this internal transition of the axial Cr 2+-X centre is formed by the spin dependent free carrier capture process: Cr 3+-X + e CB a ̊ Cr 2+-X ∗ The results imply that the luminescence is not due to the presence of Cr 2+-X ground state centres in the material as previously assumed. A competing charge transfer process, possibly involving Cr 2+-X , is observed as a quenching ODMR signal on the 0.84eV luminescence.

ODMR observation of a quenching centre for the copper-green and other visible emission bands in zinc sulphide

Aspects of the copper-green emission in zinc sulphide crystals have been studied by means of optically detected magnetic resonance (ODMR). The observation of ODMR signals that correspond to decreases in emission intensity at resonance is taken as evidence for the presence of recombination paths that compete with the copper-green and other emission processes. One centre that can provide such a path has a g value of 2.0038+or-0.0007, which corresponds to that reported for the F+ centre in ZnS.

Spin-Dependent Radiative and Nonradiative Recombinations in Hydrogenated Amorphous Silicon: Optically Detected Magnetic Resonance

Spin-dependent radiative and nonradiative recombinations in hydrogenated amorphous silicon have been discussed on the basis of the rate equation model. The experimental results on dependences of optically detected magnetic resonance (ODMR) signals on microwave power and excitation intensity are reasonably interpreted in terms of this model. A brief discussion is given on the nature of recombination centres responsible for ODMR signals.

Triplet state of tryptophan in proteins. 2. Differentiation between tryptophan residues 62 and 108 in lysozyme

We have used optically detected magnetic resonance (ODMR) to characterize the degree of solvent availability of the tryptophan residues in lysozyme that are likely to be responsible for the observed phosphorescence. From the phosphorescence spectra, ODMR zero-field splittings (zfs), and ODMR line widths, we concur with the X-ray structure [Blake, C. C., Mair, G. A., North, A. C. T., Phillips, D. C., & Sarma, V. R. (1967) Proc. R. Soc. London, ser. B 167, 365-377] that Trp-62 behaves as an exposed residue and Trp-108 is buried. In addition, we present evidence that ODMR can be used in conjunction with conventional phosphorescence to evaluate the degree of order in the microenvironments of tryptophan in a protein containing several tryptophans. By the specific modification of residues Trp-62 and Trp-108, we have identified those portions of the ODMR lines in the native enzyme that are due to those specific residues. Barring major enzyme conformational changes in the vicinity of unmodified tryptophan residues when Trp-62 or Trp-108 are selectively modified, we find that Trp-108 dominates both the phosphorescence and the ODMR signals in native lysozyme. The results are discussed in view of previous fluorescence findings.

Optical detection of triplet exciton resonance in GaSe

Optically detected magnetic resonance (ODMR) of triplet excitons in GaSe has been observed by monitoring microwave-induced changes in the bound exciton emissions at 2 K. The g-value of triplet excitons along the c-axis is g ex‖ =1.85 ± 0.03 and the zero field splitting parameter D = + 0.110 ± 0.004 cm −1. ODMR signals due to free electrons and free holes have also been observed; g e‖ =1.13 ± 0.01 and g h‖ =1.72 ± 0.02.

Study of the triplet state properties of tyrosines and tryptophan in azurins using optically detected magnetic resonance

Optically detected magnetic resonance (ODMR) signals and phosphorescence spectra were seen of tyrosine in the P. aeruginosa and tryptophanless P. fluorescens azurins and of tryptophan in the former. This confirmed a conclusion from other experiments that the tryptophan of P. aeruginosa cannot effectively quench the singlet energy of both tyrosines. The ODMR signals were all very narrow, additional evidence that the chromophores are buried in the interior of the protein. Accurate values of the zero-field coupling constants D and E lead to a tentative correlation of D values with the red shift of the 0 leads to 0 peak of the phosphorescence spectrum. The environment of tryptophan in P. aeruginosa is the most hydrocarbon like of any tryptophan so far observed. The experiments raise a number of unanswered questions concerning rate processes. The intensities of the 2E transition of tyrosine and the phosphorescence of both tyrosine and tryptophan are substantially reduced when the copper is oxidized. Nevertheless the phsphorescence lifetimes are unaffected. A hole cannot be burned in the ODMR resonances. The homogeneously broadened lines may conceivably be a result of low-temperature proton tunnelling.