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PubMed1997

2'-Fluoro modified nucleic acids: polymerase-directed synthesis, properties and stability to analysis by matrix-assisted laser desorption/ionization mass spectrometry.

Fragmentation is a major factor limiting mass range and resolution in the analysis of DNA by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Protonation of the nucleobase leads to base loss and backbone cleavage by a mechanism similar to the depurination reactions employed in the chemical degradation method of DNA sequencing. In a previous study [Tang,W., Zhu,L. and Smith,L.M. (1997) Anal. Chem ., 69, 302-312], the stabilizing effect of substituting the 24 hydrogen with an electronegative group such as hydroxyl or fluorine was investigated. These 24 substitutions stabilized the N-glycosidic linkage, blocking base loss and subsequent backbone cleavage. For such chemical modifications to be of practical significance, it would be useful to be able to employ the corresponding 24-modified nucleoside triphosphates in the polymerase-directed synthesis of DNA. This would provide an avenue to the preparation of 24-modified PCR fragments and dideoxy sequencing ladders stabilized for MALDI analysis. In this paper methods are described for the polymerase-directed synthesis of 24-fluoro modified DNA, using commercially available 24-fluoronucleoside triphosphates. The ability of a number of DNA and RNA polymerases to incorporate the 24-fluoro analogs was tested. Four thermostable DNA polymerases [Pfu (exo-), Vent (exo-), Deep Vent (exo-) and UlTma] were found that were able to incorporate 24-fluoronucleotides with reasonable efficiency. In order to perform Sanger sequencing reactions, the enzymes' ability to incorporate dideoxy terminators in conjunction with the 24-fluoronucleotides was evaluated. UlTma DNA polymerase was found to be the best of the enzymes tested for this purpose. MALDI analysis of enzymatically produced 24-fluoro modified DNA using the matrix 2,5-dihydroxy benzoic acid showed no base loss or backbone fragmentation, in contrast to the extensive fragmentation evident with unmodified DNA of the same sequence.

DNA↗
PubMed2000

Solution structure of myosin-ADP-MgFn ternary complex by fluorescent probes and small-angle synchrotron X-ray scattering.

In the presence of excess amounts of fluorine, a physiological divalent cation, magnesium (Mg(2+)), forms a novel phosphate analogue, magnesium fluoride (MgFn). Park et al. [Biochim. Biophys. Acta 1430, 127-140 (1999)] previously demonstrated that MgADP. MgFn forms a complex with myosin subfragment-1 (S-1), and the S-1.ADP. MgFn ternary complex mimics a transient state in the activity cycle of ATPase. In the present study, localized conformations in the regions of highly reactive cysteine and lysine residues, Cys 707 (SH1), Cys 697 (SH2), and Lys 83 (RLR), which change their conformations markedly during ATP hydrolysis, were studied using fluorescent probes and chemical modification. The global shape of the complex was also studied using small angle X-ray solution scattering and compared it with other previously reported myosin.ADP. fluorometal ternary complexes. The results suggest that the overall conformation and localized functional regions of the complex are quite similar to those in the presence of ATP, indicating that the complex mimics the M(**).ADP.P(i) steady state.

Adenosine Diphosphate↗
PubMed2002

Low temperature studies in LiF:Mg,Cu,P.

Despite extensive investigations carried out in LiF:Mg,Cu,P, the nature of the emission centre is not clearly understood. Results of X ray excited emission in this material at room temperature and at 16 K are presented to obtain more data that can throw light on the emission characteristics of this material. At room temperature only a single emission peak is seen around 390 nm but at 16 K the emission appears to consist of more than one emission band. The emission spectra could be fitted with three bands with peaks at 332, 385 and 447 nm. The X ray excited emission at 16 K after annealing at 573 K for 5 min suppresses the 332 nm emission but enhances 447 nm emission. Moreover, annealing at 573 K greatly reduces the emission intensity, which signifies that the luminescent centres are also destroyed in this process. Temperature-dependent X ray excited fluorescence below room temperature provides evidence of the existence of shallow traps, which give rise to a thermoluminescence peak around 130 K. On the basis of the present investigations it is proposed that the complex nature of the emission may be related to the formation of donor-acceptor pairs in this material. The possible nature of this donor-acceptor complex is discussed.

Copper↗
PubMed2002

Microdosimetric interpretation of the photon energy response of LiF:Mg,Ti detectors.

Photon energy response of MTS-N (LiF:Mg,Ti) detectors (TLD Poland) and of MTS-N detectors sensitised with 200 Gy of 60Co gamma rays, followed by UV irradiation (sMTS-N), has been determined using X rays with narrow energy spectra, in the energy range from 20 to 300 keV. The over-response of LiF:Mg,Ti detectors for X rays (relative TL efficiency eta = 1.1) can be explained as an ionisation density effect. Low energy X rays produce short electron tracks, which locally deposit a high radiation dose and, consequently, lead to an enhanced (supralinear) response. This over-response has not been observed in sensitised MTS-N where supralinearity in the response after gamma ray doses above 1 Gy is not seen. Using the dose-response curves measured for MTS-N detectors after 137Cs gamma ray irradiation and local doses calculated using Monte Carlo generated electron tracks, it was possible to predict the relative TL effectiveness for different X ray energies. The calculation procedure can be applied to predict the photon energy response of LiF:Mg,Ti detectors in an arbitrary photon field.

Fluorides↗
PubMed2002

The composite structure of peak 5 in the glow curve of LiF:Mg,Ti (TLD-100): confirmation of peak 5a arising from a locally trapped electron-hole configuration.

The hypothesis that glow peak 5a arises from localised e-h capture is confirmed by the following experimental observations: (i) The high conversion efficiency (CE) (CE5a-->4 = 3 +/- 0.5) of peak 5a to peak 4 (a hole-only trap) deduced from detailed Im-Tstop optical bleaching studies at 310 nm compared to the much lower CE of peak 5 (an electron-only trap) (CE5-->4 = 0.0026+/-0.012). (ii) The lack of an increase in the sensitivity of glow peak 5a following 2.6 MeV and 6.8 MeV He ion irradiation in 'sensitised' material compared to the factor two increase in the sensitivity of peak 5; (S/S0)5a = 0.86+/-0.12, compared to (S/S0)5 = 2.0+/-0.2. (iii) The late entry into saturation of the 2.6 MeV and 6.8 MeV He ion TL-fluence response curves for peak 5a compared to peak 5 in sensitised and normal material resulting in the following values for the track radial saturation parameter: (r50)5a = 100+/-20) Angstroms compared to (r50)5 = 380+/-30 Angstroms. (iv) The low value of 0.1 for the 'track-escape' parameter of peak 5a deduced from the Extended Track Interaction Model analysis of He ion TL fluence response compared to order of magnitude greater values for peaks 5 and 5b.

Electrons↗
PubMed2002

Kinetic modelling of the optically stimulated conversion of peaks 5a and 5 to peak 4 in LiF:Mg,Ti (TLD-100).

The TC/LC conversion model for peaks 4, 5a and 5 in LiF:Mg,Ti (TLD-100) has been studied by solution of the coupled differential equations describing the charge carrier traffic following optical stimulation. Aspects of the model investigated were (i) the two-component exponential decay of the composite peak 5 TL intensity following the bleach, (ii) the role of retrapping during bleaching, (iii) the hole nature of peak 4 and (iv) the conversion of peak 5a traps to peak 4 traps. The high conversion efficiency is naturally explained due to the absence of conduction band competitive mechanisms in the optical ionisation of the electron in the e-h occupied structure corresponding to peak 5a and thereby leading to the hole-only occupied TC/LC leading to peak 4.

Fluorides↗
PubMed2002

Localised and delocalised optically induced conversion of composite glow peak 5 in LiF:Mg,Ti (TLD-100) to glow peak 4 as a function of postirradiation annealing temperature.

The composite structure of glow peak 5 in LiF:Mg,Ti (TLD-100) has been investigated using optical bleaching by 310 nm (4 eV) light. The glow peak conversion efficiency of peak 5a (Tm = 187 degrees C) to peak 4 traps is very high at a value of 3+/-0.5 (1 SD) whereas the glow peak conversion efficiency of peak 5 (Tm = 205 degrees C) to peak 4 traps is 0.0026+/-0.0012 (1 SD). The high conversion efficiency of peak 5a to peak 4 arises from direct optical ionisation of the electron in the electron-hole pair. leaving behind a singly-trapped hole (peak 4), a direct mechanism, relatively free of competitive mechanisms. Optical ionisation of the 'singly-trapped' electron (peak 5), however, can lead to peak 4 only via multi-stage mechanisms involving charge carrier transport in the valence and conduction bands, a mechanism subject to competitive processes. The conduction/valence band competitive processes lead to the factor of one thousand decrease in the conversion efficiency of peak 5 compared to peak 5a.

Fluorides↗
PubMed2002

Thermoluminescent response of LiF:Mg,Ti to 20 keV electrons.

The thermoluminescence response of LiF:Mg,Ti (TLD-100) to 20 keV electrons from a scanning electron microscope has been measured. Radiochromic dye films previously calibrated were used to determine the fluence incident on TLD-100 chips. The procedure for irradiation and glow curve deconvolution was adhered to the protocols previously determined in our laboratory for gamma rays and heavy charged particles. The response at electron fluences higher than 4 x 10(10) cm(-2) is supralinear, due to the increasingly relevant contribution of the high temperature peaks. The relative contribution of the high temperature peaks to the TL signal is abnormally small, about half that observed in gamma irradiation and four times smaller than what has been measured in low-energy X ray exposure.

Electrons↗
PubMed2002

New evidence for spatially correlated TC-LC mechanisms in thermoluminescence of LiF:Mg,Ti.

New experimental investigations, consistent with the spatial coupling/perturbation between the trapping centres (TC) and luminescent centres (LC) giving rise to the peaks 5 and 4 thermoluminescence (TL) of LiF:Mg,Ti are discussed: (1) The glow peak widths of peaks 2 and 3 are constant as a function of Ti concentration, while the width of peak 4 decreases with a correlated increase in the width of peak 5. The dependence of glow peak shape on Ti concentration indicates that peaks 4 and 5 arise from a TC/LC spatially correlated extended defect. (2) The increased TL intensity of peak 5a following high ionisation density HCP irradiation compared to low ionisation density gamma irradiation strongly suggests that peak 5a arises from the geminate recombination of a locally trapped e-h pair. (3) The observation that the optical bleaching conversion efficiency (CE) of peak 5a to peak 4 is measured to be 3+/-0.5 and is three orders of magnitude greater than the CE of peak 5 to peak 4, is strong evidence for the unique structure of peak 5a--trap, i.e. peak 5a is due to a TC/LC structure which has captured an e-h pair, optical ionisation of the electron leads directly to the peak 4 TC.

Crystallization↗
PubMed2002

High resolution emission spectra of TL materials.

High resolution emission spectra of several TL materials, that are commonly used in dosimetry, were measured using a low cost fibre optic spectrometer containing a 2048 pixel CCD array. The spectra were taken from 177 to 890 nm with a wavelength resolution of 1.3 nm. This allowed for line width discrimination and the resolution of transitions that have not been seen so far. For rare earth doped materials like CaF2:Tm (TLD-300) and CaF2:Dy (TLD-200) the spectral lines were compared to the energy levels measured by Dieke and Crosswhite leading to the identification of most of the transition lines.

Aluminum Oxide↗
PubMed2002

Optically and thermally stimulated luminescence of KMgF3:Ce3 and NaMgF3:Ce3+.

The potentialities of optically stimulated luminescence (OSL) for personal dosimetry of ionising radiation have stimulated the search for new synthetic materials with good dosimetric properties. The sensitivity of two new OSL materials KMgF3 and NaMgF3 doped with Ce3+ ions has been evaluated and found to be of the same order of magnitude as that of Al2O3:C. Several other characteristics have also been investigated. Promising results for KMgF3:Ce are the high sensitivity and the low fading. However, this material suffers from a high self-dose due to the presence of 40K. NaMgF5:Ce is sensitive as well but shows strong fading. Interesting information on the mechanism has been obtained by correlating the signals of OSL and TL. Furthermore, the different bleachabilities under blue LED illumination of the strongly overlapping glow peaks allowed the extraction of one single peak for KMgF3:Ce3+. The results demonstrate new possibilities offered by the combination of TL and OSL.

Cerium↗
PubMed2002

Characterisation of thermoluminescent LiF:Mg,Ti (MTS-N) detectors in the 85-550 K temperature range.

Miniature LiF:Mg,Ti (MTS-N) pellets, specially designed for dosimetry in proton radiotherapy, were studied in the 85-550 K temperature range. TL glow curves for differently Mg- or Ti-doped detectors were recorded, showing the preponderant influence of Mg concentration on the intensity of peak 5. The presence of a three-peak complex was also confirmed near 140 K; spectral emission of this peak consists of two bands whose maxima occur at 290 and 420 nm. TSEE glow curves above 300 K similar to those of TL were observed. The behaviour of TSEE and TL peak 5 following an X ray exposure from 33 to 500 Gy was studied: its intensity is a linear function of air kerma in both cases.

Fluorides↗
PubMed2002

Thermoluminescent spectra of rare earth doped MgB4O7 dosemeters.

This paper presents X ray excited TL spectra of magnesium borate doped with either single rare earth ions Dy or Tm, or co-doped with Dy/Tm, Tm/Mn or Dy/Tb. Intrinsic emission from the host material is in the UV/blue region at approximately 375 nm, with a tail extending to 200 nm. The main dosimetric peak is detected at approximately 180 degrees C but slight differences are noted between the glow peak maxima from the different rare earth ions and there were changes following thermal treatments. The results are discussed according to the interaction between trapping and rare earth sites.

Borates↗
PubMed2002

New advances in LiF:Mg,Cu,P TLDs (GR-200A).

In order to volume-produce LiF:Mg,Cu,P TLDs, increase the repeatability of preparation technology and further improve the features of TLDs, especially the moisture resistant property and the accuracy in ultra-low dose, the prescription and preparation techniques have been investigated further in recent years. The main features of GR-200A have been improved dramatically by adding a chemical additive and adopting a particular preparation technology. Recent improvements are presented including prescription, preparation technology, TL sensitivity, signal-to-noise rate, detection threshold, residual signal, the moisture resistant property and the uncertainty in low dose measurements. The sensitivity of GR-200A has been increased to 65 times as high as TLD-100 from the original 29 times. The residual signal has been decreased to less than 1% from the original of about 2.5% following 240 degrees C readout. The detection threshold decreases and the signal-to-noise ratio increases. The uncertainty in low dose measurements is reduced significantly. The resistance to humidity has been improved remarkably. The preparation technology to volume-produce GR-200A is stable.

Copper↗
PubMed2002

On the correct measurement of relative heavy charged particles to gamma thermoluminescent efficiencies.

In order to better understand the most important experimental aspects for performing correct measurements of relative thermoluminescent (TL) efficiencies, an investigation has been carried out to quantify the effect of using different experimental procedures in the evaluation of 3 MeV proton-to-gamma relative efficiency (etap,gamma) of LiF:Mg,Ti. Variations in batch, presentation, annealing and reader have been studied. When the same protocol is used to measure proton and gamma TL response, efficiency values obtained range from 0.36 to 0.59 for peak 5 and from 0.44 to 0.79 for the total signal. The use of different annealings and different batches leads to 20% and 10% differences in etap,gamma respectively. Large differences (40%) are found between efficiency values measured with TLD-100 chips and those obtained using TLD-100 microcubes. When 'mixed' procedures are used to measure the proton and the gamma response, differences in etap,gamma may increase even more. The main conclusion of this work is to stress the importance of measuring an entire series of experiments in the same laboratory with a carefully defined protocol and using dosemeters from the same batch to obtain heavy charged particle TL response and gamma TL response with identical annealing and readout procedures.

Fluorides↗
PubMed2002

The TL fluence response to heavy charged particles using the track interaction model and track structure information.

The extended track interaction model, ETIM, has recently being proposed to explain the TLD-100 fluence response of peak 5 to heavy ions. This model includes the track structure information through the use of the luminescent centre occupation probability obtained from radial dose distributions produced by the ions as they travel through the dosemeter. In this work an implementation of ETIM using Monte Carlo techniques is presented. The simulation was applied to calculate the response of peak 5 of both sensitised and normal TLD-100 crystals to 2.6 and 6.8 MeV 4He ions. The simulation shows that the TL-fluence response has a strong dependence on ion energy, in disagreement with experimental observations. In spite of this, good agreement between the simulated TL-fluence response calculated for the 6.8 MeV 4He radial distributions and the experimental data for the two energies was achieved.

Fluorides↗