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PubMed2003

Regioselectivity for condensation reactions of quinonoid models of tryptophan tryptophylquinone: a density functional theory study.

The model compounds of tryptophan tryptophylquinone (TTQ), o-benzoquinone (OBQ), 3-methyl-6,7-dihydro-1H-6,7-indoledione (MIQ), and 3-methyl-4-(3-methyl-1H-2-indolyl)-6,7-dihydro-1H-6,7-indoledione (IIQ), all of which are characteristic of o-quinone groups, have been studied with density functional theory. The dihedral angle of the two indole rings (chi) of IIQ is calculated to be 49.6 degrees for the global minimum. Another local minimum, 0.74 kcal/mol higher in energy, with a chi value of 123.5 degrees is also fully optimized. The transition state connecting the two minima, with a chi value of 97.9 degrees, has been located and the rotation barrier is 1.71 kcal/mol. A scan of the potential energy surface along this dihedral angle showed that the difference of the total energy was within 1.0 kcal/mol at a range of the dihedral angle from 30 degrees to 75 degrees. Hence, IIQ is flexible for the rotation of inter-indole rings. The origin of regioselectivity for the condensation reactions of the models MIQ and IIQ with NH(3) has been elucidated. It is shown that the energy difference between the two different types of carbinolamine intermediates (Delta E) and their corresponding transition structures (Delta E(++)) should be responsible for the regioselectivity. To assess the effect of the fused ring on regioselectivity of the condensation reaction, a series of models were designed. A good linear correlation has been found between the energy difference of the two different carbinolamine intermediates (Delta E) and that of the corresponding transition states (Delta E(++)), suggesting that the factors that stabilize the carbinolamine intermediate also favor the stability of the corresponding transition structure. The pair, 6-amino-6-hydroxy-8-methyl-6H-quinolin-5-one and 5-amino-5-hydroxy-8-methyl-5H-quinolin-6-one (7/8), deviates from the correlation and represents some anomalous behavior, which may be due to their structural particularity. It also has been shown that the tricyclic models, which consist of OBQ and two fused heterocyclic rings, represent more regioselectivity in contrast to the bicyclic systems. Moreover, the fused electron-donating pyrrole and the fused electron-withdrawing pyridine or pyrimidine show a somewhat synergistic effect on each other via the medial OBQ molecule. The barrier of the condensation reaction for pyrrolo[2,3-f]quinoline-4,5-dione is calculated to be ca. 22 kcal/mol. This is lower than that for MIQ (ca. 33 kcal/mol) and IIQ (ca. 32 kcal/mol) by as much as 10.0 kcal/mol, explaining reasonably the larger catalytic effect of pyrroloquinolinequinone (PQQ) relative to TTQ.

Chemical Phenomena↗

PubMed1999

Tuning Stereoselection in Tethered Biginelli Condensations. Synthesis of cis- or trans-1-Oxo- and 1-Iminohexahydropyrrolo[1,2-c]pyrimidines.

Stereoselection in tethered Biginelli condensations can be tuned to give either the cis or trans stereoisomer of 1-oxohexahydropyrrolo[1,2-c]pyrimidine and 1-iminohexahydropyrrolo[1,2-c]pyrimidine products (see eq 1). With substrates having urea and N-sulfonylguanidine functionality (Schemes 2 and 4), cis stereoselection (4-7:1) is observed when the condensation is accomplished under Knoevenagel conditions, while trans stereoselection (4-20:1) is observed when the condensation is carried out in the presence of polyphosphate ester (PPE). Under both conditions, steroselectivity is highest in the N-sulfonylguanidine series. With substrates bearing a basic guanidine unit, the trans product is formed exclusively under Knoevenagel conditions (Scheme 3).

Journal Article↗

PubMed2006

The kinetics of H2O vapor condensation and evaporation on different types of ice in the range 130-210 K.

The kinetics of condensation (kc) and the evaporation flux (J(ev)) of H2O on ice were studied in the range 130-210 K using pulsed-valve and steady-state techniques in a low-pressure flow reactor. The uptake coefficient gamma was measured for different types of ice, namely, condensed (C), bulk (B), single crystal (SC), snow (S), and cubic ice (K). The negative temperature dependence of gamma for C, B, SC, and S ice reveals a precursor-mediated adsorption/desorption process in agreement with the proposal of Davy and Somorjai.(1) The non-Arrhenius behavior of the rate of condensation, kc, manifests itself in a discontinuity in the range 170-190 K depending on the type of ice and is consistent with the precursor model. The average of the energy of sublimation DeltaH(S) degrees is (12.0 +/- 1.4) kcal/mol for C, B, S, and SC ice and is identical within experimental uncertainty between 136 and 210 K. The same is true for the entropy of sublimation DeltaS(S). In contrast, both gamma and the evaporative flux J(ev) are significantly different for different ices. In the range 130-210 K, J(ev) of H2O ice was significantly smaller than the maximum theoretically allowed value. This corroborates gamma values significantly smaller than unity in that T range. On the basis of the present kinetic parameters, the time to complete evaporation of a small ice particle of radius 1 mum is approximately a factor of 5 larger than that previously thought.

Journal Article↗

PubMed2006

Exploring the role of decoherence in condensed-phase nonadiabatic dynamics: a comparison of different mixed quantum/classical simulation algorithms for the excited hydrated electron.

Mixed quantum/classical (MQC) molecular dynamics simulation has become the method of choice for simulating the dynamics of quantum mechanical objects that interact with condensed-phase systems. There are many MQC algorithms available, however, and in cases where nonadiabatic coupling is important, different algorithms may lead to different results. Thus, it has been difficult to reach definitive conclusions about relaxation dynamics using nonadiabatic MQC methods because one is never certain whether any given algorithm includes enough of the necessary physics. In this paper, we explore the physics underlying different nonadiabatic MQC algorithms by comparing and contrasting the excited-state relaxation dynamics of the prototypical condensed-phase MQC system, the hydrated electron, calculated using different algorithms, including: fewest-switches surface hopping, stationary-phase surface hopping, and mean-field dynamics with surface hopping. We also describe in detail how a new nonadiabatic algorithm, mean-field dynamics with stochastic decoherence (MF-SD), is to be implemented for condensed-phase problems, and we apply MF-SD to the excited-state relaxation of the hydrated electron. Our discussion emphasizes the different ways quantum decoherence is treated in each algorithm and the resulting implications for hydrated-electron relaxation dynamics. We find that for three MQC methods that use Tully's fewest-switches criterion to determine surface hopping probabilities, the excited-state lifetime of the electron is the same. Moreover, the nonequilibrium solvent response function of the excited hydrated electron is the same with all of the nonadiabatic MQC algorithms discussed here, so that all of the algorithms would produce similar agreement with experiment. Despite the identical solvent response predicted by each MQC algorithm, we find that MF-SD allows much more mixing of multiple basis states into the quantum wave function than do other methods. This leads to an excited-state lifetime that is longer with MF-SD than with any method that incorporates nonadiabatic effects with the fewest-switches surface hopping criterion.

Journal Article↗

PubMed2006

Relation of the number of cross-links and mechanical properties of multi-walled carbon nanotube films formed by a dehydration condensation reaction.

Multi-walled carbon nanotube (MWCNT) films were prepared by employing a condensation reaction utilizing 1,3-dicyclohexylcarbodiimide (DCC) to cross-link each MWCNT with carboxylic acid and hydroxyl groups. Morphological changes in the resultant MWCNT films were monitored using scanning electron microscopy and showed that the MWCNTs were randomly intertwined in the films. The prepared MWCNT films were 17 mm in diameter and 20 microm in thickness, and the apparent density was 0.59 g/cm(3). Fourier transform-infrared spectroscopy confirmed that each MWCNT modified with carboxylic acid and hydroxyl groups was cross-linked through the ester bond. It was found that the ratio of the number of ester cross-links and carbon atoms of the nanotubes per unit apparent volume (cm(3)) of condensed-MWCNT films was 5.27 x 10(-3) using thermogravimetric analysis (TGA). The tensile strength and Vickers hardness of condensed-MWCNT films achieved an average of 15 and 9.2 MPa, respectively, and were greater than those of free-standing MWCNT films without ester bond.

Cross-Linking Reagents↗

PubMed2005

Role of the spacer of cationic gemini amphiphiles in the condensation of DNA.

The condensation of calf thymus DNA into the cholesteric-like psi-phase was observed by circular dichroism in liposome suspensions formulated with specific cationic gemini surfactants. The stereochemistry of the gemini spacer, the presence of specific functional groups, and the covalent link between the headgroups are fundamental issues in the condensation of DNA. Transmission electron microscopy images showed a multilamellar morphology, which corresponds with condensation.

Animals↗

PubMed1998

Formation of a fluorescent adduct in the reaction of 2'-deoxyadenosine with a malonaldehyde-acetaldehyde condensation product.

Malonaldehyde (malondialdehyde, MDA) was reacted with 2'-deoxyadenosine in buffered aqueous solution. HPLC analyses of the reaction mixtures showed that, besides the two previously characterized N6-propenal (M1dA) and N6-oxazocinyl (M3dA) adenine adducts, a third compound eluting at longer retention time was formed. The compound generated a strong peak in the chromatogram recorded by a fluorescence detector. The new compound was isolated by preparative C18 chromatography, and its structure was characterized by UV absorbance, fluorescence emission, 1H and 13C NMR spectroscopy, and mass spectrometry. The product was identified as 9-(2'-deoxyribosyl)-6-(3,5-diformyl-4-methyl-1, 4-dihydro-1-pyridyl)purine (M2AA-dA). The yield of the product was 0.8% following 7 days of reaction at 37 degreesC and pH 4.6. Lower yields were obtained at higher pH conditions. By the addition of acetaldehyde, the yield increased about 10-fold at all studied pH conditions. The adduct was most likely formed by an initial condensation of two molecules of malonaldehyde with one molecule of acetaldehyde followed by reaction of the condensation product with the exocyclic amino group of 2'-deoxyadenosine. The identification of this adduct shows that acetaldehyde may react with DNA bases also through an initially formed malonaldehyde-acetaldehyde condensation product.

Acetaldehyde↗

PubMed2000

The effect of oral administration of polyethylene glycol on faecal helminth egg counts in pregnant goats grazed on browse containing condensed tannins.

Thirty yearling F1 Anglo-Nubian x Mubende goats, averaging 21 +/- 0.45 kg, kept on free-range feeding in the Ankole range land, Uganda, were screened for health and nutritional status, effectively treated against helminth parasites, mated, and randomly divided into two equal groups during a 3-month preparatory phase. During the 6 months that followed, the goats in one group received a daily oral dose (50 g/goat) of poly(ethylene glycol) (PEG), while the other group acted as the control (no PEG). The goats were monitored for faecal nematode egg counts and body weight gains, along with the quality of their diet, nematode contamination of the pasture, and the prevailing climatic factors in the area. Goats treated with PEG had significantly (p < 0.05) higher faecal helminth egg loads. The mean nematode eggs per gram of faeces (epg) of the PEG group (290 epg) was more than double that of the control group (129 epg). All the PEG-treated goats exhibited moderate to severe infections at the end of the experiment. The gain in body weight during gestation was lower (p < 0.05) in the PEG group (70.4 g per goat per day) than in the control group (91.8 g per goat per day). The PEG group lost 2.3 g per goat per day in the fifth month. PEG deactivates condensed tannins, and it was concluded that condensed tannins play a significant role in reducing the negative effects of gastrointestinal helminth burdens in the natural free-range feeding system of the Ankole range land in Uganda. Selective feeding on such range lands might expose goats to optimal concentrations of dietary condensed tannins with resultant beneficial effects.

Administration, Oral↗

PubMed1997

Assessment of errors when expiratory condensate PCO2 is used as a proxy for mixed expired PCO2 during mechanical ventilation.

OBJECTIVES: We designed a series of experiments to determine whether expiratory water condensate (PconCO2) can be used as a proxy for mixed expired gas collection. METHODS: In 18 adult mechanically ventilated patients with ARDS (40 samples), simultaneous collections of arterial blood, expiratory water trap condensate, mixed expired gas, and minute ventilation were used to calculate VCO2 and VD/VT. To assess the effect of temperature, a constant gas flow (PCO2 10-30 mm Hg) was bubbled through water at temperatures of 19.5-37 degrees C. Gas and water samples were collected, immediately analyzed for PCO2, and a temperature correction factor was calculated. A lung model was constructed using a 5 L anesthesia bag connected to a mechanical ventilator with a heated humidifier. Temperature at the Y-piece was set to approximately 37 degrees C and CO2 was injected into the bag to establish an end-tidal PCO2 of 20-70 mm Hg. After equilibration, condensate was collected, PCO2 was measured, and the temperature-corrected PCO2 was compared to PECO2. The capnogram at points along the expiratory limb circuit was used to evaluate gas mixing. RESULTS: There was an over-estimation of PECO2 by PconCO2 (p < 0.001) for the patient data, resulting in an underestimation of VD/VT (p < 0.001) and an overestimation of VCO2 (p < 0.001). The temperature correction factor for PCO2 in water was -0.010 (about half of the factor used for whole blood). The bias between temperature-corrected PconCO2 and PECO2 was 0.3 +/- 3.2 mm Hg in the lung model. Mixing in the expiratory limb was poor, as evaluated by the capnogram. CONCLUSIONS: Even with temperature correction, we failed to precisely predict PECO2 from PconCO2. For measurement of VD/VT and VCO2, we do not recommend methods that use PconCO2.

Adult↗

PubMed2003

Effects of Acacia condensed tannins on urinary parameters, body mass, and diet choice of an Acacia specialist rodent, Thallomys nigricauda.

The aim of this study was to investigate the dietary and physiological effects of condensed tannin ingestion on foregut fermenters, using Thallomys nigricauda, a folivorous rodent, as a model. We initially investigated the variability in physiological parameters, such as daily body mass (DMb), daily feed intake, daily fecal energy loss (FE), daily energy intake (DEI), daily urine pH, and daily urinary ammonia and urea concentrations, in response to different diets with low condensed tannin levels. This experiment was conducted to identify which physiological variables showed the least variation in the absence of tannin. In a second experiment, we investigated the response of the same dietary and physiological parameters to the effects of high dietary condensed tannin ingestion in T. nigricauda. We hypothesized that DMb, daily feed intake, FE, and DEI of T. nigricauda would be adversely affected by high dietary tannin content. We predicted that detoxification activity by T. nigricauda would increase at higher tannin levels. Ingestion of tannins affected the nutritional status of T. nigricauda, as shown by a decrease in body mass at high tannin levels. We also found that fewer ammonium ions were excreted in the urine by T. nigricauda, as would be expected if this were a means of regulating metabolic acidosis. The urine produced was more alkaline. This result indicates that T. nigricauda is not metabolizing these allelochemicals. Urea production was initially reduced, indicating conservation of bicarbonate ions that will neutralize blood acidity if there is detoxification. A diet choice experiment showed that tree rats avoid high tannin diets, even to the extent that they lose body mass on an alternative diet. This last-mentioned result is noteworthy because previous studies of the effects of tannins on herbivorous mammals have shown that there is physiological control rather than behavioral avoidance of the negative effects of tannin ingestion.

Acacia↗

PubMed1988

Translational control of InsP3-induced chromatin condensation during the early cell cycles of sea urchin embryos.

The cycles of DNA synthesis and chromatin condensation in dividing cells are controlled by signals from the cytoplasm. Changes in the concentration of free calcium (Cai) in the cytoplasm control a variety of cellular functions and it has thus been suggested that observed variations in Cai during the cell cycle may be the cytoplasmic signal that co-ordinates nuclear and cytoplasmic division. We show here that increases in Cai induced by the calcium-releasing second messenger inositol 1,4,5-triphosphate (Ins(1,4,5)P3), or by calcium buffers, cause premature chromatin condensation and breakdown of the nuclear envelope in sea urchin (Lytechinus pictus) early embryos. Both natural and induced chromatin condensation are prevented by calcium chelators. The nucleus becomes sensitive to the Cai signal 45 min after fertilization, but remains insensitive if protein synthesis is prevented. Our experiments demonstrate that Cai regulates the behaviour of the nucleus during the cell cycle, suggest that Ins(1,4,5)P3 is a cell cycle messenger and indicate that there is an interaction between the protein and ionic signals that control the state of chromatin during the cell cycle.

Animals↗

PubMed2001

Many-particle entanglement with Bose-Einstein condensates.

The possibility of creating and manipulating entangled states of systems of many particles is of significant interest for quantum information processing; such a capability could lead to new applications that rely on the basic principles of quantum mechanics. So far, up to four atoms have been entangled in a controlled way. A crucial requirement for the production of entangled states is that they can be considered pure at the single-particle level. Bose-Einstein condensates fulfil this requirement; hence it is natural to investigate whether they can also be used in some applications of quantum information. Here we propose a method to achieve substantial entanglement of a large number of atoms in a Bose-Einstein condensate. A single resonant laser pulse is applied to all the atoms in the condensate, which is then allowed to evolve freely; in this latter stage, collisional interactions produce entanglement between the atoms. The technique should be realizable with present technology.

Journal Article↗

PubMed2002

Bose-Einstein condensation of atomic gases.

The early experiments on Bose-Einstein condensation in dilute atomic gases accomplished three long-standing goals. First, cooling of neutral atoms into their motional ground state, thus subjecting them to ultimate control, limited only by Heisenberg's uncertainty relation. Second, creation of a coherent sample of atoms, in which all occupy the same quantum state, and the realization of atom lasers - devices that output coherent matter waves. And third, creation of a gaseous quantum fluid, with properties that are different from the quantum liquids helium-3 and helium-4. The field of Bose-Einstein condensation of atomic gases has continued to progress rapidly, driven by the combination of new experimental techniques and theoretical advances. The family of quantum-degenerate gases has grown, and now includes metastable and fermionic atoms. Condensates have become an ultralow-temperature laboratory for atom optics, collisional physics and many-body physics, encompassing phonons, superfluidity, quantized vortices, Josephson junctions and quantum phase transitions.

Journal Article↗

PubMed2002

Towards Bose-Einstein condensation of excitons in potential traps.

An exciton is an electron-hole bound pair in a semiconductor. In the low-density limit, it is a composite Bose quasi-particle, akin to the hydrogen atom. Just as in dilute atomic gases, reducing the temperature or increasing the exciton density increases the occupation numbers of the low-energy states leading to quantum degeneracy and eventually to Bose-Einstein condensation (BEC). Because the exciton mass is small--even smaller than the free electron mass--exciton BEC should occur at temperatures of about 1 K, many orders of magnitude higher than for atoms. However, it is in practice difficult to reach BEC conditions, as the temperature of excitons can considerably exceed that of the semiconductor lattice. The search for exciton BEC has concentrated on long-lived excitons: the exciton lifetime against electron-hole recombination therefore should exceed the characteristic timescale for the cooling of initially hot photo-generated excitons. Until now, all experiments on atom condensation were performed on atomic gases confined in the potential traps. Inspired by these experiments, and using specially designed semiconductor nanostructures, we have collected quasi-two-dimensional excitons in an in-plane potential trap. Our photoluminescence measurements show that the quasi-two-dimensional excitons indeed condense at the bottom of the traps, giving rise to a statistically degenerate Bose gas.

Journal Article↗

PubMed2002

Atom-molecule coherence in a Bose-Einstein condensate.

Recent advances in the precise control of ultracold atomic systems have led to the realisation of Bose Einstein condensates (BECs) and degenerate Fermi gases. An important challenge is to extend this level of control to more complicated molecular systems. One route for producing ultracold molecules is to form them from the atoms in a BEC. For example, a two-photon stimulated Raman transition in a (87)Rb BEC has been used to produce (87)Rb(2) molecules in a single rotational-vibrational state, and ultracold molecules have also been formed through photoassociation of a sodium BEC. Although the coherence properties of such systems have not hitherto been probed, the prospect of creating a superposition of atomic and molecular condensates has initiated much theoretical work. Here we make use of a time-varying magnetic field near a Feshbach resonance to produce coherent coupling between atoms and molecules in a (85)Rb BEC. A mixture of atomic and molecular states is created and probed by sudden changes in the magnetic field, which lead to oscillations in the number of atoms that remain in the condensate. The oscillation frequency, measured over a large range of magnetic fields, is in excellent agreement with the theoretical molecular binding energy, indicating that we have created a quantum superposition of atoms and diatomic molecules two chemically different species.

Journal Article↗

PubMed1978

Squamous lesions in lungs of rats exposed to tobacco-smoke-condensate fractions by repeated intratracheal instillation.

Twice-weekly intratracheal instillations in rats of up to 24 mg of Fraction (R + P)G suspended in either infusine (I) or buffered saline/gelatine (BS/G) gave rise to foci of squamous metaplasia of alveolar epithelium (SqM) and squamous neoplasms (SqN). Fraction (R + P)G, which is a fraction of cigarette-smoke condensate almost as tumorigenic for mouse skin as the nearly 30 × larger mass of condensate from which it is derived, could be given in this way for up to 40 weeks without excessive mortality or any marked effect on the rate of body-weight gain. By contrast, similar treatment with Fraction N(QG), a fraction having very low tumorigenic activity for mouse skin, induced no SqN and barely any excess of SqM over that induced by either vehicle alone.The effects of Fraction (R + P)G on the incidence of SqM and SqN were both time and dose related, the effect on SqM incidence being already evident after 10 weeks of treatment. No SqN seen were unequivocally malignant, though, due to the design of the experiment, only 5 rats exposed to Fraction (R + P)G were observed more than 60 weeks after the start of the experiment.Other changes in the lung, including aggregates of alveolar macrophages laden with golden-brown pigment (GBM) and foci of cuboidal/columnar metaplasia of alveolar epithelium (CCM), were frequently seen in response to both fractions. Fraction (R + P)G administered in I was more effective in causing SqM and SqN than the same fraction administered in BS/G. The implications of the findings are discussed, particularly the possibility that the intratracheal/instillation technique might be useful as a rapid bioassay for comparing the tumorigenicity of different cigarette-smoke condensates.

Animals↗

PubMed1978

Effect of glycerol on local and systemic carcinogenicity of topically applied tobacco condensate.

When glycerol was added to tobacco smoke condensate in acetone solvent, the topical carcinogenicity and the ability to produce epithelial hyperplasia in mice was reduced. Two doses of condensate were applied, combined with 2 concentrations of added glycerol. Age-standardized results show that glycerol reduced the incidence of tumours and malignant tumours and of hyperplasia in animals not developing skin tumours. The relative incidences of malignant tumours, benign tumours, hyperplasia and unaffected skin suggest that there is a sequential relationship (i.e. normal skin to hyperplasia to benign neoplasia to malignant neoplasia) which is impeded by glycerol. There was no systemic effect attributable to the condensate.

Administration, Topical↗

PubMed2004

Bose-Einstein condensation of excitons in bilayer electron systems.

An exciton is the particle-like entity that forms when an electron is bound to a positively charged 'hole'. An ordered electronic state in which excitons condense into a single quantum state was proposed as a theoretical possibility many years ago. We review recent studies of semiconductor bilayer systems that provide clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime, where these experiments were performed, is as likely to occur in electron-electron bilayers as in electron-hole bilayers. In current quantum Hall excitonic condensates, disorder induces mobile vortices that flow in response to a supercurrent and limit the extremely large bilayer counterflow conductivity.

Journal Article↗