Radiochemical measurements of99Tc: Sources and environmental levels

A method of⁹⁹Tc determination has been developed based on sample decomposition followed by purification from interfering elements by hydroxide and oxide precipitations. Technetium is extracted by TBP and electrodeposited onto stainless steel discs from sodium hydroxide solution.⁹⁹Tc^m is used as yield monitor. The method has been applied to study releases from different sources and contents in different samples of atmospheric, terrestrial and marine origin. Results show enhanced⁹⁹Tc/¹³⁷Cs activity ratios for samples collected in recent years which are only expected to be contaminated from fallout from nuclear tests. Integrated fallout measurements did not reflect these enhanced ratios.     

Improved ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons in sediments

The aim of this work was to optimize an ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons (PAHs) in sediments and to compare it with the reflux procedure using methanolic potassium hydroxide. Sample extracts were purified with a miniaturized silica gel chromatographic column and analyzed by gas chromatography-mass spectrometry (GC-MS). Ultrasonication using n-hexane-acetone (1:1, v/v) solvent mixture on dried homogenized marine sediment gave better precision (smaller relative standard deviation (RSD) values) and comparable quantities of individual PAH's compared to the reflux procedure. Ultrasonication with the n-hexane-acetone (1:1, v/v) mixture, utilizing four 15 min extraction cycles, was found to be sufficient for extracting PAHs from wet sediments. The optimized ultrasonic extraction procedure extracted aliphatic and aromatic hydrocarbons from the National Institute of Standards and Technology SRM 1941a with recoveries greater than 90%. The major advantages of ultrasonication compared to the reflux method are the lower extraction times, simplicity of the apparatus and extraction procedure. The optimized ultrasonication procedure has been used in our laboratory to extract hydrocarbons from naturally wet sediments from rivers, and coastal and marine areas.     

Donor states in tellurium-doped silicon

Hall effect and conductivity measurements are performed on Te-doped silicon in the temperature range 30KT800K. A Hall equipment suited for high temperatures up to 800 K has been constructed. The temperature dependence of the free electron concentration is analyzed for Te-doped silicon including one double-donor and several monovalent donor species. A deep level with an electrical activation energy of 200 meV is determined from the saturation of the free electron concentration at temperatures above 400 K. This level represents the first ionization stage of the Te double-donor. The second ionization stage is estimated to have an activation energy of 440 meV. The maximum electrically active Te concentration obtained is 5×10¹⁶cm^–3. Three different shallow donor states are resolved in the low-temperature range. The concentrations of these shallow donors are partially sensitive to a subsequent heat-treatment.     

Photohydration and photosolvolysis of biphenyl alkenes and alcohols via biphenyl quinone methide-type intermediates and diarylmethyl carbocations

Evidence is presented for the photochemical generation of novel biphenyl quinone methide (BQM)-type intermediates on photolysis of hydroxybiphenyl alkenes 7 and 8 and hydroxybiphenyl alcohols 9 and 10. Mechanistic investigations utilizing product, fluorescence, and nanosecond laser flash photolysis (LFP) studies indicate two distinct pathways for the formation of these BQMs depending upon the functional groups of the progenitor. Formal excited-state intramolecular proton transfer (ESIPT) between the phenol and the alkene led to BQMs upon irradiation of the hydroxybiphenyl alkenes 7 and 8, while excited-state proton transfer (ESPT) to solvent followed by dehydroxylation was responsible for BQM formation from the hydroxybiphenyl alcohols 9 and 10. Photolysis of 7 and 8 in aqueous CH(3)CN gave photohydration products via attack of water on the respective BQMs, while photolysis of the analogous methyl ethers (of the phenolic moiety) gave only carbocation intermediates. Hydroxybiphenyl alcohols 9 and 10 yielded the corresponding photomethanolysis products in aqueous methanol, through attack of CH(3)OH on the respective BQMs. Although no evidence was found for BQM formation in LFP studies of 8 and 10, due to its suspected short lifetime, the respective diaryl carbocation (lambda(max) 420 nm, tau = 8.5 micros) has been observed upon irradiation of 8 in 2,2,2-trifluoroethanol. A BQM (lambda(max) 580 nm) was observed for 9 but not for 10, the latter having more complex chemistry on laser excitation, resulting in a transient that appears to mask any BQM absorption. Significant quenching of fluorescence from the hydroxybiphenyl alkenes at low water content implies that H(2)O is directly involved in reaction from the singlet excited state. The decrease in fluorescence intensity of 8 was found to depend on [H(2)O](3); however, the distance required for ESIPT in these systems is too large to be bridged by a water trimer. The nonlinear quenching has been attributed to deprotonation of the phenol by two water molecules, with concerted protonation at the alkene by another molecule of water. Fluorescence quenching of the hydroxybiphenyl alcohols required much higher water content, implying a different mechanism of reaction, consistent with the proposal of ESPT (to solvent water) followed by dehydroxylation.     

Substitution and oxidation reactions of bis(dithiolene)tungsten complexes of potential relevance to enzyme sites

Structurally characterized tungstoenzymes contain mononuclear active sites in which tungsten is coordinated by two pterin-dithiolene ligands and one or two additional ligands that have not been identified. In this and prior investigations (Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275; J. Am. Chem. Soc. 2001, 123, 1931), stable coordination units of bis(dithiolene)tungsten(IV,V,VI) complexes potentially related to enzyme sites have been sought by exploratory synthesis. In this work, additional members of the sets [WL(S2C2Me2)2](2-,-) and [WLL'(S2C2Me2)2](2-,-) have been prepared and structurally characterized. Tungsten(IV) complexes obtained by substitution are carbonyl displacement products of [W(CO)2(S2C2Me2)2] and include those with the groups W(IV)S (4), W(IV)(O2CPh) (5), and W(IV)(2-AdQ)(CO) (Q = S (6), Se (7); Ad = adamantyl). Those obtained by oxidation reactions contain the groups W(V)O (9), W(V)(QPh)2 (Q = S (10), Se (11)), W(VI)S(OPh) (12), and W(VI)O2 (14). The latter two complexes were obtained from W(IV) precursors using sulfur and oxygen atom transfer reactions, respectively. Complexes 4 and 9 are square pyramidal; 6, 7, 10, and 11 are distorted trigonal prismatic with cis ligands LL'; and 12 and 14 are distorted octahedral. Complexes 4, 10, and 11 support three-membered electron transfer series. Attempts to oxidize 4 to the W(V)S complex results in the formation of binuclear [W2(mu2-S)2(S2C2Me2)4](2-) having distorted octahedral coordination. The 21 known functional groups WL and WLL' in mononuclear bis(dithiolene) complexes prepared in this and prior investigations are tabulated. Of those with physiological-type ligands, it remains to be seen which (if any) of these ligation modes are displayed by enzyme sites.     

Reactivity of [Fe4S4(SR)4]2-,3- clusters with sulfonium cations: analogue reaction systems for the initial step in biotin synthase catalysis

The first step in catalysis by a class of iron-sulfur enzymes that includes biotin synthase is the one-electron reductive cleavage of the obligatory cofactor S-adenosylmethionine by an [Fe(4)S(4)](+) cluster to afford methionine and the deoxyadenosyl radical (DOA*). To provide detailed information about the reactions of sulfonium ions with [Fe(4)S(4)](2+,+) clusters, the analogue reaction systems [Fe(4)S(4)(SR')(4)](2)(-)(,3)(-)/[PhMeSCH(2)R](+) (R' = Et (4, 6), Ph (5, 7); R = H (8), COPh (9), p-C(6)H(4)CN (10)) were examined by (1)H NMR spectroscopy. Sulfonium ions 8-10 react completely with oxidized clusters 4 and 5 to afford PhSMe and R'SCH(2)R in equimolar amounts as a result of electrophilic attack by the sulfonium ion on cluster thiolate ligands. Reactions are also complete with reduced clusters 6 and 7 but afford, depending on the substrate, the additional products RCH(3) (R = PhCO, p-C(6)H(4)CN) and the ylid PhMeS=CHR or (p-NCC(6)H(4)CH(2))(2). Redox potentials of 9 and 10 allow electron transfer from 6 or 7. The reaction systems 6/9,10 and 7/9,10 exhibit two reaction pathways, reductive cleavage and electrophilic attack, in an ca. 4:1 ratio inferred from product distribution. Cleavage is a two-electron process and, for example in the system 6/9, is described by the overall reaction 2[Fe(4)S(4)(SR')(4)](3)(-) + 2[PhMeSCH(2)R](+) --> 2[Fe(4)S(4)(SR')(4)](2)(-) + PhSMe + RCH(3) + PhMeS=CHR. This and other reactions may be summarized as [PhMeSCH(2)R](+) + 2e(-) + H(+) --> PhSMe + RCH(3); proposed reaction sequences parallel those for electrochemical reduction of sulfonium ions. This work demonstrates the intrinsic ability of [Fe(4)S(4)](+) clusters with appropriate redox potentials to reductively cleave sulfonium substrates in overall two-electron reactions. The analogue systems differ from the enzymes in that DOA* is generated in a one-electron reduction and is sufficiently stabilized within the protein matrix to abstract a hydrogen atom from substrate or an amino acid residue in a succeeding step. In the present systems, the radical produced in the initial step of the reaction sequence, [Fe(4)S(4)(SR')(4)](3)(-) + [PhMeSCH(2)R](+) --> [Fe(4)S(4)(SR')(4)](2)(-) + PhSMe + RCH(2)*, is not stabilized and is quenched by reduction and protonation.     

X-ray spectroscopy of enzyme active site analogues and related molecules: bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands

The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L2,3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [MoIVL(S2C2Me2)2]- (L = O2-, R3SiO-, RO-, RS-, RSe-) and [WIV(OR)(S2C2Me2)2]-, distorted trigonal prismatic [MoIV(CO)(SeR)(S2C2Me2)2]- and [WIV(CO)L(S2C2Me2)2]- (L = RS-, RSe-), and distorted octahedral [WVIO(OR)(S2C2Me2)2]-. The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to < or = 0.02 A as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [MoIV(QR)(S2C2Me2)2]- simulate protein ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)2]- and six-coordinate [M(CO)L(S2C2Me2)2]- are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Soc. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.     

Charge-field formulation of quantum electrodynamics (QEMED)

By expressing classical electron theory in terms of charge-field functional structures, it is shown that a finite formulation of the classical electrodynamics of point charges emerges in a simple and elegant fashion. The classical charge-field form of microscopic electron theory plays the role of a covering theory for renormalized classical electron theory, with the distinct advantage that this is accomplished by adynamic subtraction mechanism, built into the theory. We then generalize this formalism into a hole-theoretic, second-quantized Dirac formulation, in order to construct a charge-field quantum electrodynamic theory, and discuss its basic properties. We find, in addition to the possibility that the finiteness of the classical theory may be propagated into the quantum field theory, that interacting photon states are generated as a secondary manifestation of electron-positron quantization, and do not require the usual free canonical quantization scheme. We discuss the possibility that this approach may lead to a better formulation of quantum electrodynamics in the Heisenberg picture and suggest a crucial experimental test to distinguish this new charge-field quantum electrodynamics QEMED from the standard QED formulation. Specifically QEMED predicts that the Einstein principle of separability should be found to be valid for correlated photon polarization measurements, in which the polarizers are changed more rapidly than a characteristic photon travel time. Such an experiment (Aspect, 1976) can distinguish between QEMED and QED in a complete and clear-cut fashion.