Anoxic Solution Research Articles

Exploring the role of crystal habit in the Ostwald rule of stages.

The crystallization of calcium carbonate is shown to be dictated by the Ostwald rule of stages (ORS), for high relative initial supersaturations (), under sweet (carbon dioxide saturated) and anoxic (oxygen depleted) solution conditions. Rhombohedral calcite crystals emerge after the sequential crystallization and dissolution of the metastable polymorphs: vaterite (snowflake-shaped) and aragonite (needle-shaped). However, the presence of certain cations, which can form trigonal carbonates (e.g. Fe2+ and Ni2+), in concentrations as low as 1.5 mM, triggers the emergence of calcite crystals, with a star-shaped crystal habit, first. These star-shaped crystals dissolve to yield needle-shaped aragonite crystals, which in turn dissolve to give the rhombohedral calcite crystals. The star-shaped crystals, formed at high SCaCO3, possess higher surface free energy (therefore higher apparent solubility) than their rhombohedral counterparts. This sequence of dissolution and recrystallization demonstrates that the ORS does not only drive the crystal towards its thermodynamically most stable polymorph but also towards its most stable crystal habit.

Insights into the Biogeochemical Cycling of Cobalt: Precipitation and Transformation of Cobalt Sulfide Nanoparticles under Low-Temperature Aqueous Conditions.

Cobalt sulfide precipitates, key phases in the natural biogeochemistry of cobalt and in relevant remediation and resource recovery processes, are poorly defined under low-temperature aqueous conditions. Here, we systematically studied Co (Fe) sulfides precipitated and aged in environmentally relevant solutions, defined by different combinations of pH, initial cobalt to iron ratios ([Co]aq/[Fe]aq), with/without S0, and the presence/absence of sulfate-reducing bacteria. The initial abiogenic precipitates were composed exclusively of amorphous Co sulfide nanoparticles (CoS·xH2O) that were stable in anoxic solution for 2 months, with estimated log K* values 1-5 orders of magnitude higher than that previously reported for Co sulfides. The addition of S0, in combination with acidic pH and elevated temperature (60 °C), resulted in recrystallization of the amorphous precipitates into nanocrystalline jaipurite (hexagonal CoS) within 1 month. In the presence of Fe(II)aq, the abiogenic precipitates were composed of more crystalline Co sulfides and/or Co-rich mackinawite, the exact phase being dependent on the [Co]aq/[Fe]aq value. The biogenic precipitates displayed higher crystallinity for Co sulfides (up to the formation of nanocrystalline cobalt pentlandite, Co9S8) and lower crystallinity for Co-rich mackinawite, suggestive of mineral-specific bacterial interaction. The revealed precipitation and transformation pathways of Co (Fe) sulfides in this study allows for a better constraint of Co biogeochemistry in various natural and engineered environments.

Structure, morphology and photocatalytic performance of BiVO4 nanoislands covered with ITO thin film

Epitaxial BiVO4 nanoislands and ITO–BiVO4 composite thin films were prepared on yttrium-stabilized zirconia (001) substrates by pulsed laser deposition. Structural characterization results indicated that the growth of ITO film followed the layer-by-layer mode and covered uniformly on the YSZ substrate and BiVO4 islands. The photocatalytic activities of ITO–BiVO4 composites were lower than bare BiVO4 islands, and decreased with the increase of thickness of deposited ITO films. Photocatalytic degradation experiments in anoxic solution showed that, photogenerated electrons in ITO–BiVO4 composites played a more important role in dye degradation than that in bare BiVO4. The complete coverage of ITO layer on BiVO4 nanoislands prevented the contact between photogenerated holes and RhB solution, leading to the accumulation of holes and the rapidly carrier recombination.

Bulk Dissolution Rates of Cadmium and Bismuth Tellurides As a Function of pH, Temperature and Dissolved Oxygen.

The toxicity of Cd being well established and that of Te suspected, the bulk, surface-normalized steady-state dissolution rates of two industrially important binary tellurides-polycrystalline cadmium and bismuth tellurides- were studied over the pH range 3-11, at various temperatures (25-70 °C) and dissolved oxygen concentrations (0-100% O2 in the gas phase). The behavior of both tellurides is strikingly different. The dissolution rates of CdTe monotonically decreased with increasing pH, the trend becoming more pronounced with increasing temperature. Activation energies were of the order of magnitude associated with surface controlled processes; they decreased with decreasing acidity. At pH 7, the CdTe dissolution rate increased linearly with dissolved oxygen. In anoxic solution, CdTe dissolved at a finite rate. In contrast, the dissolution rate of Bi2Te3 passed through a minimum at pH 5.3. The activation energy had a maximum in the rate minimum at pH 5.3 and fell below the threshold for diffusion control at pH 11. No oxygen dependence was detected. Bi2Te3 dissolves much more slowly than CdTe; from one to more than 3.5 orders of magnitude in the Bi2Te3 rate minimum. Both will readily dissolve under long-term landfill deposition conditions but comparatively slowly.

IR Spectroscopic Behavior of Polaronic Trapped Electrons in TiO2 under Aqueous Photocatalytic Conditions

Attenuated total reflection infrared spectroscopy has been used to study a broad mid-IR absorption attributed to electron polarons that was exhibited during ultraviolet (UV) irradiation of anatase TiO2 particle films under aqueous photocatalytic conditions. The influence of formate hole scavenging and proton intercalation that accompanies electron trapping was investigated in anoxic solution conditions. The UV excitation and decay kinetics of the polaronic absorption (PA) varied with chemical conditions that influenced electron and hole scavenging and proton intercalation reactions. A striking near-linear decay of the PA under certain conditions indicated saturation of surface electron and oxygen sites, giving a constant reaction rate. The PA band shape varied with absorbance and between anatase samples. Water absorption losses concomitant with the PA have been ascribed to polaron-induced variation of sample refractive index. The PA likely arises from the photoionization of large polarons.

Does anoxic processing of dissolved organic matter affect organic–mineral interactions in paddy soils?

Adsorption of dissolved organic matter (DOM) in soil relates closely to hydrous Fe oxides, being one of the most important adsorbents. In paddy soils, reductive dissolution of hydrous Fe oxides may reduce the capacity of topsoils to retain organic matter. Dissolved organic matter produced and processed under anoxic conditions in the topsoils, thus, is leached into deeper soil horizons that still have considerable amounts of hydrous Fe oxides. Yet, the effect of the anoxic processing on adsorption of the leached DOM in oxic subsoil horizons is not known. We tested the adsorption of DOM from oxic and anoxic solution to minerals typical for paddy subsoils (goethite, ferrihydrite, ferruginous smectite). Adsorption of organic carbon (OC) to hydrous Fe oxides was up to five times larger for oxic than anoxic solutions. Ferruginous smectite adsorbed less OC than hydrous Fe oxides and differences between oxic and anoxic solutions were less pronounced than for the hydrous Fe oxides. Phosphate contained in the DOM test solution adsorbed strongly to the Fe oxide surfaces, while little to no phosphate was adsorbed by the smectite. Phosphate adsorption to hydrous Fe oxides was not affected by redox conditions. That finding is in line with the largely invariable equilibrium pH, suggesting no changes in number and reactivity of sorption sites at the hydrous oxides' surfaces. Also, we found no indications of reductive dissolution of minerals during the experiments. The reduction in DOM sorption, thus, seems to be due to compositional changes of DOM during anoxic processing.

Publisher's Note: Radiation Induced Corrosion of Copper in Anoxic Aqueous Solution [Electrochem. Solid-State Lett., 15, C5 (2012)

Erratum to : Radiation Induced Corrosion of Copper in Anoxic Aqueous Solution (vol 15, pg C5, 2012)

Oxygen-Sensing of L-type Calcium Channels in Rat Cardiomyocytes: The Possible Role of Hemoxygenase

Hemoxygenases (HO)-1 and −2 are enzymes that metabolize the heme group, generating CO, biliverdin and Fe+2. HO-2 is believed to be an O2 sensor in the carotid body controlling the release of dopamine as it needs O2 for its function. HO-2 is necessary for the anoxic activation of nociceptive neuron and binds to calmodulin in neurons, and the complex is modulated by Ca2+. Previously, we have found that L-type Ca2+ channel is suppressed by 22-43% with acute anoxia prior to metabolic impairment or run down of Ica . This property appeared to require the calmodulin-binding IQ motif on the cytoplasmic carboxy-tail of the channel. Here we probe whether HO mediates this rapid Ica-suppressant effect of anoxia thus serving as the O2 sensor of the heart. Freshly isolated rat cardiomyocites were maintained in Tyrode containing (in mM): 135NaCl, 5.4KCl, 10Hepes, 10glucose, 1MgCl2 and 2CaCl2). Internal solution used was (in mM): 15NaCl, 118CsCl, 5.5Glucose, 14EGTA, 3.92CaCl2, 10Hepes, 3MgATP, 0.5MgCl2. Solutions bubbled with 100% O2 or 100% N2 were rapidly (50ms) applied. IBa was measured replacing CaCl2 for BaCl2. HO inhibitors ZnPP-IX 100nM or SnPP-IX 100nM were added to either 100% O2 or 100% N2 solutions. Both the O2 removal and the inhibition of HO blocked about 30% of the ICa and IBa. Administration of anoxic solution did not further inhibit ICa in cells previously exposed to ZnPP-IX or SnPP-IX. We conclude that both anoxia and HO suppress ICa and IBa with the same intensity and kinetics and O2 sensing effect becomes negligible in cells exposed to HO inhibitors.

Mobility of major-, minor- and trace elements in solutions of a planosolic soil: Distribution and controlling factors

Subsurface waters circulating in an unpolluted soil of a planosolic horizon (Massif Central, France) were studied in order to determine their physico-chemical characteristics. Three water sampling sites were chosen along a toposequence. For each site, two piezometers were placed above and in the gravelly and concretion-rich horizon (Fe- and Mn- oxyhydroxides). Concentrations of major-, minor- (cations, anions, Fe, Mn, P and Si) and trace elements (Al, Ba, Cd, Co, Cr, Cu, Ni, Pb, Rb, Sr, Zn and U) were monitored on bulk and filtered water (0.45 μm) to study both the particulate and the dissolved components, from 2004 to 2006, during the soil saturation period (i.e., from November to May). Chemical characteristics of soil solutions provide evidence for various chemical water compositions and for temporal variations of water quality, revealing that the hydrodynamic and chemical reactivity in the solution is different for the three sites. Calculations of pe values indicate a range of redox state of the soil solutions. The pe ranges are different for each piezometer but correspond to anoxic solution. For all piezometers, distribution between the dissolved and the particulate fraction and correlations between the various elements in the soil solutions indicate that: (i) Al and Fe show similar behaviour, (ii) Al is mainly present as oxyhydroxides and (iii) some trace metals are mainly associated with particles which have a mixed nature. The impact of a concretion-rich horizon is noticed both on the nature of particles and on the speciation of trace metals and could be explained by the hydrodynamic and chemical reactivity of the circulating solution. Very few correlations exist between elements in the dissolved phase.

Redundant Synthesis of Cysteinyl-tRNACys in Methanosarcina mazei

A subset of methanogenic archaea synthesize the cysteinyl-tRNA(Cys) (Cys-tRNA(Cys)) needed for protein synthesis using both a canonical cysteinyl-tRNA synthetase (CysRS) as well as a set of two enzymes that operate via a separate indirect pathway. In the indirect route, phosphoseryl-tRNA(Cys) (Sep-tRNA(Cys)) is first synthesized by phosphoseryl-tRNA synthetase (SepRS), and this misacylated intermediate is then converted to Cys-tRNA(Cys) by Sep-tRNA:Cys-tRNA synthase (SepCysS) via a pyridoxal phosphate-dependent mechanism. Here, we explore the function of all three enzymes in the mesophilic methanogen Methanosarcina mazei. The genome of M. mazei also features three distinct tRNA(Cys) isoacceptors, further indicating the unusual and complex nature of Cys-tRNA(Cys) synthesis in this organism. Comparative aminoacylation kinetics by M. mazei CysRS and SepRS reveals that each enzyme prefers a distinct tRNA(Cys) isoacceptor or pair of isoacceptors. Recognition determinants distinguishing the tRNAs are shown to reside in the globular core of the molecule. Both enzymes also require the S-adenosylmethione-dependent formation of (m1)G37 in the anticodon loop for efficient aminoacylation. We further report a new, highly sensitive assay to measure the activity of SepCysS under anaerobic conditions. With this approach, we demonstrate that SepCysS functions as a multiple-turnover catalyst with kinetic behavior similar to bacterial selenocysteine synthase and the archaeal/eukaryotic SepSecS enzyme. Together, these data suggest that both metabolic routes and all three tRNA(Cys) species in M. mazei play important roles in the cellular physiology of the organism.

Hydrosulfide Oxidation Pathways in Oxic Solutions Containing Iron(III) Chelates

The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-; C(HS-)0 = 50-150 micromol/L) into polysulfides (S(n)2-; n = 2-9), colloidal sulfur, and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta; C(Fe(III)0 = 50-300 micromol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +/- 1 degree C. At higher pH, oxygen was seen to slow down the hydrosulfide conversion rate. For instance, the HS- half-life was 24.8 min in a DO2-saturated iron(III)-cdta solution compared to 11.3 min in the corresponding anoxic solution (pH 10.2, C(HS-)0 = 80 micromol/L, C(Fe(III))0 = 200 micromol/L). In anoxia, HS- oligomerizes into chain-like polysulfides which behave as autocatalysts on the HS- conversion rates. The presence of DO2 disrupts the HS- oligomerization process by generating thiosulfate precursors from polysulfides, a pathway that impedes the HS- uptake. At lower alkaline pH where the hydroxide-free Fe(3+)cdta(4-) is the prevailing iron(III)-cdta species, the "iron(II)-cdta + DO2" oxidative reaction becomes crucial. Oxidative regeneration of iron(III) as Fe(3+)cdta(4-) (being more reactive than Fe(3+)OH(-)cdta(4-)) offsets to some extent the restrictive role of oxygen on the accumulation of polysulfides. Thiosulfate and sulfate were the main end-products for the current experimental conditions to the detriment of colloidal sulfur, which did not form in DO2-saturated solutions.

Insulin improves cardiomyocyte contractile function through enhancement of SERCA2a activity in simulated ischemia/reperfusion1

Insulin exerts anti-apoptotic effects in both cardiomyocytes and coronary endothelial cells following ischemia/reperfusion (I/R) via the Akt-endothelial nitric oxide synthase survival signal pathway. This important insulin signaling might further contribute to the improvement of cardiac function after reperfusion. In this study, we tested the hypothesis that sarcoplasmic reticulum calcium-ATPase (SERCA2a) is involved in the insulin-induced improvement of cardiac contractile function following I/R. Ventricular myocytes were enzymatically isolated from adult SD rats. Simulated I/R was induced by perfusing cells with chemical anoxic solution for 15 min followed by reperfusion with Tyrode's solution with or without insulin for 30 min. Myocyte shortening and intracellular calcium transients were assessed and underlying mechanisms were investigated. Reperfusion with insulin (10(-7) mol/L) significantly improved the recovery of contractile function (n=15-20 myocytes from 6-8 hearts, P<0.05), and increased calcium transients, as evidenced by the increased calcium [Ca2+] fluorescence ratio, shortened time to peak Ca2+ and time to 50% diastolic Ca2+, compared with those in cells reperfused with vehicle (P<0.05). In addition, Akt phosphorylation and SERCA2a activity were both increased in insulin-treated I/R cardiomyocytes, which were markedly inhibited by pretreatment of cells with a specific Akt inhibitor. Moreover, inhibition of Akt activity abolished insulin-induced positive contractile and calcium transients responses in I/R cardiomyocytes. These data demonstrated for the first time that insulin improves the recovery of contractile function in simulated I/R cardiomyocytes in an Akt-dependent and SERCA2a-mediated fashion.

Purification and Initial Biochemical Characterization of ATP:Cob(I)alamin Adenosyltransferase (EutT) Enzyme of Salmonella enterica

ATP:cob(I)alamin adenosyltransferase (EutT) of Salmonella enterica was overproduced and enriched to approximately 70% homogeneity, and its basic kinetic parameters were determined. Abundant amounts of EutT protein were produced, but all of it remained insoluble. Soluble active EutT protein (approximately 70% homogeneous) was obtained after treatment with detergent. Under conditions in which cobalamin (Cbl) was saturating, Km(ATP) = 10 microm, kcat = 0.03 s(-1), and Vmax = 54.5 nm min(-1). Similarly, under conditions in which MgATP was saturating, Km(Cbl) = 4.1 microm, kcat = 0.06 s(-1), and Vmax = 105 nm min(-1). Unlike other ATP:co(I)rrinoid adenosyltransferases in the cell (i.e. CobA and PduO), EutT activity was > or =50-fold higher with ATP versus GTP, and EutT retained 80% of its activity with ADP substituted for ATP and was completely inactive with AMP as substrate, indicating that the enzyme requires the beta-phosphate group of the nucleotide substrate. The data suggest that the amino group of adenine might play a role in nucleotide recognition and/or binding. Unlike the housekeeping CobA enzyme, EutT was not inhibited by inorganic tripolyphosphate (PPPi). Results from 31P NMR spectroscopy studies identified PPi and Pi as by-products of the EutT reaction. In the absence of Cbl, EutT cleaved ATP into adenosine and PPPi, suggesting that PPPi is broken down into PPi and Pi. Electron transfer protein partners for EutT were not encoded by the eut operon. EutT-dependent activity was detected in cell-free extracts of cobA strains enriched for EutT when FMN and NADH were used to reduce cob(III)alamin to cob(I)alamin.

Insulin improves cardiac myocytes contractile function recovery in simulated ischemia-reperfusion: Key role of Akt

The present study examined cardiac myocyte contractile and Ca2+ transient responses to insulin during simulated ischemia/reperfusion (I/R) and further investigated the role of protein kinase B (Akt) in the insulininduced inotropic effect. Ventricular myocytes were enzymatically isolated from adult Sprague-Dawley rats and perfused with Tyrode solution while electrically field-stimulated. Simulated I/R was induced by perfusing the cells with chemical anoxic solution including sodium cyanide-sodium lactate for 15 min followed by reperfusion with normal oxygenated Tyrode solution with or without insulin. It is found that insulin only at concentration as high as 10 IU/L could increase cell shortening (16 ± 5%,P< 0.05) in normal myocytes, whereas it concentration-dependently (0.01–10 IU/L) increased the contraction, the velocity of shortening/relengthening and Ca72+ transient in I/R myocytes. In addition, insulin treatment (1 IU/L) increased Akt phosphorylation of I/R cardiomyocytes by 2.4-fold compared with that of the control (P < 0.01). Most importantly, pretreatment with LY 294002, a specific inhibitor of phosphatidylinositol 3′-kinase (PI3-kinase), significantly inhibited both Akt phosphorylation and the positive inotropic response to insulin in the I/R cardiomyocytes. These results suggest that insulin exerts direct positive inotropic effect by increasing Ca2+ transient of cardiomyocytes, which is enhanced in the pathological condition of I/R. Akt activation plays an important role in the insulin-induced improvement of myocyte contractile function following I/R.

Effects of docosahexaenoic acid on calcium pathway in adult rat cardiomyocytes

In this study we examined the effect of polyunsaturated fatty acids (PUFAs), in particular of docosahexaenoic acid (DHA), on calcium homeostasis in isolated adult rat cardiomyocytes exposed to KCl, ET-1 and anoxia. Free [Ca 2+] i in rat cardiomyocytes was 135.7 ± 0.5 nM. Exposure to 50 mM KCl or 100 nM ET-1 resulted in a rise in free [Ca 2+] i in freshly isolated cells (465.4 ± 15.6 nM and 311.3 ± 12.6 nM, respectively) and in cultured cells (450.8 ± 14.8 nM and 323.5 ± 14.8 nM respectively). An acute treatment (20 minutes) with 10 μM DHA significantly reduced the KCl- and ET-1-induced [Ca 2+] i increase (300.9 ± 18.1 nM and 232.08 ± 11.8 nM, respectively). This reduction was greater after chronic treatment with DHA (72 h; 257.7 ± 13.08 nM and 192.18 ± 9.8 nM, respectively). Rat cardiomyocytes exposed to a 20 minute superfusion with anoxic solution, obtained by replacing O 2 with N 2 in gas mixture, showed a massive increase in cytosolic calcium (1200.2 ± 50.2 nM). Longer exposure to anoxia induced hypercontraction and later death of rat cardiomyocytes. Preincubation with DHA reduced the anoxic effect on [Ca 2+] i (498.4 ± 7.3 nM in acute and 200.2 ± 12.2 nM in chronic treatment). In anoxic conditions 50 mM KCl and 100 nM ET-1 produced extreme and unmeasurable increases of [Ca 2+] i. Preincubation for 20 minutes with DHA reduced this phenomenon (856.1 ± 20.3 nM and 782.3 ± 7.6 nM, respectively). This reduction is more evident after a chronic treatment with DHA (257.7 ± 10.6 nM and 232.2 ± 12.5 nM, respectively). We conclude that in rat cardiomyocytes KCl, ET-1 and anoxia interfered with intracellular calcium concentrations by either modifying calcium levels or impairing calcium homeostasis. Acute, and especially chronic, DHA administration markedly reduced the damage induced by calcium overload in those cells.

Accurate measurement of near-micromolar oxygen concentrations in aqueous solutions based on enzymatic extradiol cleavage of 4-chlorocatechol: applications to improved low-oxygen experimental systems and quantitative assessment of back diffusion of oxygen from the atmosphere.

An enzymatic method for measuring the O(2) concentrations of aqueous solutions was developed by involving 4-chlorocatechol and catechol 2,3-dioxygenase from Pseudomonas putida. With this system, the amount of O(2) in a sample solution can be measured as the amount of 5-chloro-2-hydroxymuconate semialdehyde formed through the enzyme reaction. The product was stable and its anion exhibited strong absorption around 380 nm (molar absorption coefficient of 4.3 x 10(4) M(-1) cm(-1), pK value of 5.4). A sensitive HPLC method involving a BioAssist Q column was developed to individually quantify the products derived from 4-chlorocatechol and catechol. When the O(2) concentration in a sample solution sealed in a vial was lowered from the air-saturation level by means of the amount enzymatically reacted with a known amount of catechol, the concentration of remaining O(2) could be successfully measured by the HPLC method. We developed devices through which reagents could be added to solutions sealed in cuvettes or the vessel of an oxygen electrode system under a flow of argon. By applying these devices, the submicromolar O(2) concentration of an anoxic solution and the back diffusion of O(2) from the atmosphere could be directly determined for the first time. The K(m) values of the dioxygenase and an ascorbate oxidase for oxygen were also determined to be 7.2 (at pH 7.5) and 114 microM (at pH 6.5), respectively, at 25 degrees C.

Stereoelectronic effect on one-electron reductive release of 5-fluorouracil from 5-fluoro-1-(2-oxocycloalkyl)uracils as a new class of radiation-activated antitumor prodrugs.

A series of 5-fluoro-1-(2'-oxocycloalkyl)uracils (3-11) that are potentially novel radiation-activated prodrugs for the radiotherapy of hypoxic tumor cells have been synthesized to evaluate a relationship between the molecular structure and the reactivity of one-electron reductive release of antitumor 5-fluorouracil (1) in anoxic aqueous solution. All the compounds 3-11 bearing the 2'-oxo group were one-electron reduced by hydrated electrons (eaq-) and thereby underwent C(1')-N(1) bond dissociation to release 5-fluorouracil 1 in 47-96% yields upon radiolysis of anoxic aqueous solution, while control compounds (12, 13) without the 2'-oxo substituent had no reactivity toward such a reductive C(1')-N(1) bond dissociation. The decomposition of 2-oxo compounds in the radiolytic one-electron reduction was more enhanced, as the one-electron reduction potential measured by cyclic voltammetry in N,N-dimethylformamide became more positive. The efficiency of 5-fluorouracil release was strongly dependent on the structural flexibility of 2-oxo compounds. X-ray crystallographic studies of representative compounds revealed that the C(1')-N(1) bond possesses normal geometry and bond length in the ground state. MO calculations by the AM1 method demonstrated that the LUMO is primarily localized at the pi* orbital of C(5)-C(6) double bond of the 5-fluorouracil moiety, and that the LUMO + 1 is delocalized between the pi* orbital of 2'-oxo substituent and the sigma* orbital of adjacent C(1')-N(1) bond. The one-electron reductive release of 5-fluorouracil 1 in anoxic aqueous solution was presumed to occur from the LUMO + 1 of radical anion intermediates possessing a partial mixing of the antibonding C(2')=O pi* and C(1')-N(1) sigma* MO's, that may be facilitated by a dynamic conformational change to achieve higher degree of (pi* + sigma*) MO mixing.

Room Temperature Phosphorescence Study of Refolding of Disulfide Reduced RNase T1

Refolding kinetics of disulfide reduced ribonuclease T 1 (RNase T 1 ) was studied by the recovery of room temperature phosphorescence emitted from a tryptophan residue (Trp 59) of the protein. When unfolded in a deoxygenated aqueous solution, the enzyme does not emit phosphorescence at room temperature, but as soon as the protein was transferred into an anoxic refolding buffer solution, it began emitting phosphorescence within the mixing dead time (∼30 s) of our apparatus. The initial quick recovery of RNase T 1 phosphorescence was followed by a gradual increase in the phosphorescence lifetime lasting over several tens of minutes and then slight decrease to a final value comparable to that of native RNase T 1 . The initial quick recovery of phosphorescence suggests that the tryptophan residue, being exposed to surrounding water molecules when the protein is unfolded, is quickly isolated from the external water when the enzyme starts refolding, and the gradual change in the phosphorescence lifetime implies...

Isolation and characterization of the products of anoxic irradiation of d(CpGpTpA)

An improved method for separating the products of DNA oligomers irradiated in aqueous solution has been devised. Altogether 39 products were isolated from the tetramer d(CpGpTpA) X-irradiated in dilute anoxic solution. Of these, 16, including most of the major products, were identified through the use of proton nmr spectroscopy. The identified products fall into four categories: (1) base products, (2) strand scission products, (3) base modifications and (4) tandem lesion. A tandem lesion in which the methyl carbon atom of thymine is covalently linked to the guanine C8 carbon atom was produced in larger yield than any of the simple base modifications.

Single-Strand Breaks in Double-Stranded DNA Irradiated in Anoxic Solution: Contribution of tert-Butanol Radicals

Yields of single-strand breaks induced by 60Co gamma or pulse irradiation in double-stranded calf thymus DNA have been measured in N2O-saturated aqueous solution as a function of the concentration of tert-butanol. The yields were found to be dependent on dose rate. The experimental data were analyzed using a theoretical model based on non-homogeneous scavenging kinetics. It is concluded from this analysis that after 60Co gamma irradiation in the absence of oxygen, aside from breaks caused by hydroxyl radicals, additional breaks occur which are initiated by hydrogen atoms and secondary radicals of tert-butanol. The efficiency of hydrogen atoms in causing single-strand breaks in double-stranded calf thymus DNA was determined to be 2.3%, while the rate constant for the reaction of tert-butanol radicals with DNA and their efficiency in causing single-strand breaks was determined to be 4.1 x 10(3) dm3 mol-1 s-1 and 2%, respectively.