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At least 451 records · Page 25Linked to original sources
PubMed2001

Mechanism of inner-sphere electron transfer via charge-transfer (precursor) complexes. Redox energetics of aromatic donors with the nitrosonium acceptor.

Spontaneous formation of colored (1:1) complexes of various aromatic donors (ArH) with the nitrosonium acceptor (NO+) is accompanied by the appearance of two new (charge-transfer) absorption bands in the UV-vis spectrum. IR spectral and X-ray crystallographic analyses of the [ArH,NO+] complexes reveal their inner-sphere character by the ArH/NO+ separation that is substantially less than the van der Waals contact and by the significant enlargement of the aromatic chromophore. The reversible interchange between such an inner-sphere complex [ArH,NO+] and the redox product (ArH+.+ NO.) is quantitatively assessed for the first time to establish it as the critical intermediate in the overall electron-transfer process. Theoretical formulation of the NO+ binding to ArH is examined by LCAO-MO methodology sufficient to allow the unambiguous assignment of the pair of diagnostic (UV-vis) spectral bands. The MO treatment also provides quantitative insight into the high degree of charge-transfer extant in these inner-sphere complexes as a function of the HOMO-LUMO gap for the donor/acceptor pair. The relative stabilization of [ArH,NO+] is traced directly to the variation in the electronic coupling element H(AB), which is found to be substantially larger than the reorganization energy (lambda/2). In Sutin's development of Marcus-Hush theory, this inequality characterizes a completely delocalized Class III complex (which occupies a single potential well) according to the Robin-Day classification. The mechanistic relevance of such an unusual (precursor) complex to the inner-sphere mechanism for organic electron transfer is discussed.

Journal Article↗
PubMed2002

Metal ion-catalyzed Diels-Alder and hydride transfer reactions. Catalysis of metal ions in the electron-transfer step.

Rates of Diels-Alder cycloaddition of anthracenes with p-benzoquinone and its derivatives as well as rates of hydride-transfer reactions from 10-methyl-9,10-dihydroacridine to the same series of p-benzoquinones are accelerated significantly in the presence of metal ions in acetonitrile. An extensive comparison of the catalytic effects of metal ions in electron transfer from one-electron reductants (cobalt tetraphenylporphyrin and decamethylferrocene) to p-benzoquinones with those in the Diels-Alder reactions of the quinones as well as the hydride-transfer reactions has revealed that the catalysis of metal ions in each case is ascribed to the 1:1 and 1:2 complexes formed between the corresponding semiquinone radical anions and metal ions. The transient absorption and ESR spectra of the semiquinone radical anion-metal ion complexes are detected directly in the electron-transfer reduction of p-benzoquinone derivatives in the presence of metal ions. The catalytic reactivities of a variety of metal ions in each reaction are well correlated with the energy splitting values of pi(g) levels because of the complex formation between O(2)(.-) and M(n+), which are derived from the g(zz) values of the ESR spectra of the O(2)(.-)-M(n+) complex.

Journal Article↗
PubMed2004

Repair of guanyl radicals in plasmid DNA by electron transfer is coupled to proton transfer.

By using gamma-irradiation in the presence of thiocyanate ions, we have generated guanyl radicals in plasmid DNA. These can be detected by using an Escherichia coli base excision repair endonuclease to convert their stable end products to strand breaks. The yield of enzyme-sensitive sites is strongly attenuated by the presence of micromolar concentrations of one of a series of singly substituted phenols, and it is possible to derive bimolecular rate constants for the reduction of DNA guanyl radicals by these phenols. More strongly reducing phenols were found to react more rapidly. This electron-transfer reaction also involves a proton transfer. By comparing the expected energetics of the reaction with the observed rate constants, the electron transfer is found to be mechanistically coupled with the proton transfer.

DNA↗
PubMed2006

Hydride transfer versus hydrogen radical transfer in thymidylate synthase.

The nature of a H-transfer in the thymidylate synthase catalyzed reaction was investigated by comparison of the wild-type enzyme with the W80M mutant. The nature of the H-transfer was not affected, as indicated by intrinsic isotope effects and their temperature dependence. These findings support a single-step hydride transfer instead of a two-step radical transfer.

Hydrogen↗
PubMed2003

Kinetic isotope effect evidence for a concerted hydrogen transfer mechanism in transfer hydrogenations catalyzed by [p-(Me2CH)C6H4Me]Ru- (NHCHPhCHPhNSO2C6H4-p-CH3).

The isotope effects in the reaction of [p-(Me2CH)C6H4Me]Ru(NHCHPhCHPhNSO2C6H4-p-CH3) (1) with isopropyl alcohol were 1.79 for transfer of hydrogen from OH to N and 2.86 for transfer from CH to Ru. The isotope effect for transfer of deuterium from doubly labeled material (kCHOH/kCDOD = 4.88) was within experimental error of the product of the two individual isotope effects. These isotope effects provide convincing evidence for a mechanism involving concurrent transfer of hydrogen from oxygen to nitrogen and from carbon to ruthenium.

2-Propanol↗
PubMed2005

Variation of the resonant transfer rate when passing from nonadiabatic to adiabatic electron transfer.

Two competing theories are used for bridging the gap between the nonadiabatic and the deeply adiabatic electron transfer between symmetric parabolic wells. For the high friction limit, a simple analytic interpolation is proposed as a reasonable alternative to them, well-fitted to the results of numerical simulations. It provides a continuous description of the electron transfer rate in the whole range of variation of the nonadiabatic coupling between the diabatic states. For lower friction, the original theories are used for the same goal. With an increase in coupling, the cusped barrier transforms into the parabolic one. Correspondingly, the pre-exponent of the Arrhenius transfer rate first increases with coupling, then levels off approaching the "dynamic solvent effect" plateau but finally reduces reaching the limit of the adiabatic Kramers theory for the parabolic barrier. These changes proceeding with a reduction in the particle separation affect significantly the spatial dependence of the total transfer rate. When approaching the contact distance, the exact rate becomes smaller than in the theory of dynamical solvent effects and much smaller than predicted by perturbation theory (golden rule), conventionally used in photochemistry and electrochemistry.

Journal Article↗
PubMed2006

Ultrafast electron-transfer and solvent adiabaticity effects in viologen charge-transfer complexes.

We report ultrafast electron transfer (ET) in charge-transfer complexes that shows solvent relaxation effects consistent with adiabatic crossover models of nonadiabatic ET. The complexes of either dimethyl viologen (MV) or diheptyl viologen (HV) with 4,4'-biphenol (BP) (MVBP or HVBP complexes) have identical charge-transfer spectra and kinetics in ethylene glycol with approximately 900 fs ET decay. We assign this decay time as largely due to adiabatic control of a predicted approximately 40 fs nonadiabatic ET. The MVBP decay in methanol of 470 fs is reduced in mixtures having low (2-20%) concentrations of acetonitrile to as short as 330 fs; these effects are associated with faster relaxation time in methanol and its mixtures. In contrast, HVBP has much longer ET decay in methanol (730 fs) and mixture effects that only reduce its decay to 550 fs. We identify the heptyl substituent as creating major perturbations to solvent relaxation times in the methanol solvation shell of HVBP. These charge-transfer systems have reasonably well-defined geometry with weak electronic coupling where the electronic transitions are not dependent on intramolecular motions. We used a nonadiabatic ET model with several models for adiabatic crossover predictions to discuss the small variation of energy gap with solvent and the ET rates derived from adiabatic solvent control. A time correlation model of solvent relaxation was used to define the solvent relaxation times for this case of approximately zero-barrier ET.

Journal Article↗
PubMed2002

Photochemical properties of the triplet pi,pi* state, anion and ketyl radicals of 5,12-naphthacenequinone in solution studied by laser flash photolysis: electron transfer and phenolic H-atom transfer.

Photochemical properties of the lowest triplet pi,pi* state of 5,12-naphthacenequinone (5,12-NQ) have been investigated in solution by means of nanosecond laser flash photolysis at 295 K. The transient absorption spectrum of triplet 5,12-NQ was measured in CCl4 and the molar absorption coefficient was determined. In CCl4, triplet 5,12-NQ is shown to be quenched by the ground-state 5,12-NQ. In acetonitrile, triplet 5,12-NQ is quenched by 1,2,4,5-tetramethoxybenzene (TMB) via electron transfer to produce the TMB cation and 5,12-NQ anion radicals. The absorption spectrum and the molar absorption coefficient of the 5,12-NQ anion radical were determined based on those of the TMB cation radical. Although triplet 5,12-NQ does not abstract H-atom of ethanol or cyclohexane, H-atom transfer occurs from phenol and p-phenylphenol to triplet 5,12-NQ, producing the 5,12-NQ ketyl radical. The absorption spectrum and the molar absorption coefficient of the 5,12-NQ ketyl radical were determined based on those of the p-phenylphenoxyl radical. It is shown that the quenching rate constants by phenol for triplet 5,12-NQ and other triplet paraquinones previously reported obey a Rehm-Weller relationship. The mechanism for the phenolic H-atom transfer to triplet paraquinones is interpreted in terms of coupled electron and proton transfer.

Anions↗
PubMed2000

Comparison of the transfer precision of three different impression materials in combination with transfer caps for the Frialit-2 system.

Transfer of the precise position of implants to a master cast is a prerequisite for accurate and passive fit of the superstructure. Implants lack the mobility of natural teeth and, therefore, inaccurate frameworks result in stress within the mechanical parts and the implant-bone interface. Various impression methods have been described to achieve accurate reproduction of the intra-oral relation of the implants. The aim of this experimental study was to compare three different impression materials (polyether, polyvinyl siloxane, hydrocolloid) with the Frialit(R)-2 system and with the indirect technique. In addition, the use of transfer caps (TCs) to improve transfer precision was tested with all three materials. All measurements were performed using a three-dimensional (3D) co-ordinate measuring machine that is capable of locating points in space and calculating the relative distortions as angles of tilt (rot-XY, rot-XZ, rot-YZ) and 3D displacement. The results suggest that addition-silicone (a-silicone) and polyether are the materials of choice for implant transfer procedures. The use of TCs resulted in a significantly reduced rotation in the XY-plane but did not improve the absolute 3D displacement. A-silicone with the use of TCs proved to be most precise. Comparison between polyether and polyvinyl siloxane showed significant differences in the XY-rotation and the 3D displacement in favour of the silicone. Because the mean distortions between the original model and the master casts were about 100 microm, absolutely precise fit may be unattainable owing to the physical properties of the materials. Further studies will have to evaluate the amount of tolerable stress at the implant-bone interface.

Alginates↗
PubMed2003

The single breath transfer factor (Tl,co) and the transfer coefficient (Kco): a window onto the pulmonary microcirculation.

The transfer factor, Tl,co (with the transfer coefficient, Kco, also known as the transfer factor per unit alveolar volume, [Tl/Va]), is one of the most useful clinical tests of pulmonary function, the only one which specifically focuses on pulmonary microcirculation. It was originally devised in 1909 as a physiological tool to assess the diffusive capacity of the lung as a gas exchanger. It was subsequently developed as a clinical tool, but cumbersome analytical techniques delayed its introduction into clinical medicine until 1950s. The physiology of the carbon monoxide transfer factor (also called the diffusing capacity Dl,co) is based on the Roughton-Forster equation which partitions Dl,co, a conductance, into membrane (Dm) and red cell (thetaVc) diffusion conductances. Recent work (1987-2001) suggests that 70-80% of the resistance to CO (and O2) diffusion may reside in the red cell fraction. The clinical implication is that Tl,co and Kco are 'windows' onto the pulmonary microcirculation. As regards reference values for clinical use, Tl,co depends on age, height and gender. Kco, which is actually a rate constant, is independent of gender, and is affected principally by age. A schema is presented for the clinical interpretation of Tl,co. As Tl,co is derived from the product of Kco and the accessible alveolar volume (Va), examination of these two components (Kco and Va) will usually suggest a specific pathophysiological mechanism as the explanation for a reduction in Tl,co.

Humans↗
PubMed2002

The arterial system of the first intermetatarsal space and its influence in toe-to-hand transfer: a report of 53 long-pedicle transfers.

A study of the arterial system in the first intermetatarsal space was performed during 53 toe-to-hand or vascularized tissue of the toe transfers with long arterial pedicles. The first dorsal metatarsal artery was used as a single pedicle in 25 transfers and the first plantar metatarsal artery in 21: both arteries were taken simultaneously in seven transfers. In 11 transfers, the dorsalis pedis artery and the perforating branch were absent, although this did not exclude the presence of a first dorsal metatarsal artery which arose from the plantar system by means of an ascending vascular branch in seven cases.

Adolescent↗
PubMed2002

Carotenoid to chlorophyll energy transfer in the peridinin-chlorophyll-a-protein complex involves an intramolecular charge transfer state.

Carotenoids are, along with chlorophylls, crucial pigments involved in light-harvesting processes in photosynthetic organisms. Details of carotenoid to chlorophyll energy transfer mechanisms and their dependence on structural variability of carotenoids are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to reveal energy transfer pathways in the peridinin-chlorophyll-a-protein (PCP) complex containing the highly substituted carotenoid peridinin, which includes an intramolecular charge transfer (ICT) state in its excited state manifold. Extending the transient absorption spectra toward near-infrared region (600-1800 nm) allowed us to separate contributions from different low-lying excited states of peridinin. The results demonstrate a special light-harvesting strategy in the PCP complex that uses the ICT state of peridinin to enhance energy transfer efficiency.

Journal Article↗
PubMed1977

Serial transfer of a human gene to rodent cells by sequential chromosome-mediated gene transfer.

The human hypoxanthine phosphoribosyl-transferase (IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) gene (hprt) has been serially transferred to mouse cells and then to Chinese hamster fibroblasts by two cycles of metaphase chromosome isolation and incubation with recipient cells. Human metaphase chromosomes were incubated with mouse A9 cells deficient in hypoxanthine phosphoribosyltransferase, and independent colonies expressing the human species form of this gene were isolated in a selective medium. Metaphase chromosomes isolated from two of these clonal lines were incubated with Chinese hamster fibroblasts deficient in hypoxanthine phosphoribosyltransferase; five resulting independent colonies again expressed the human species of this gene. The transfer frequencies in the two cycles of chromosome-mediated gene transfer were similar (about 10(-7)). These results indicate that the transferred human chromosome fragment is closely associated with the chromosomes of the mouse A9 cells and it is probably integrated into the chromosomal DNA of the recipient cell.

Animals↗
PubMed1989

Conjugative transfer of promiscuous IncP plasmids: interaction of plasmid-encoded products with the transfer origin.

To characterize protein-DNA interactions involved in the initiation of conjugative transfer replication, we isolated and sequenced the transfer origins (oriT) of the promiscuous IncP plasmids RP4 and R751. The central initiating event at the transfer origin of a conjugative plasmid is the cleavage at a unique site (nic) of the strand to be transferred to a recipient cell. This process can be triggered after the assembly of "relaxosomes" (plasmid DNA-protein relaxation complexes), requiring plasmid-encoded gene products. We analyzed the nicking reaction for plasmid RP4 and demonstrated that one of the plasmid strands is specifically cleaved within oriT. The fully functional oriT of RP4 represents an intergenic DNA region of approximately 350 base pairs. Dissection of oriT revealed that a portion carrying nic and symmetric sequence repeats determines oriT specificity. This part of oriT is contiguous to a region that is essential for efficient mobilization of oriT plasmids. In addition, oriT contains potential promoter sites allowing divergent transcription of two operons flanking oriT. We over-produced gene products and, from analyzing the products of defined deletion mutants, deduced the gene arrangements. Formation of RP4 relaxosomes is likely to depend on the presence of at least two plasmid-encoded components, which act in trans. Corresponding genes map on one side of oriT. Purification of the traJ product revealed it to be an 11-kDa polypeptide that binds to oriT DNA in vitro. The protein recognizes the part of oriT that is responsible for oriT specificity.

Base Sequence↗
PubMed2004

Extensive domain motion and electron transfer in the human electron transferring flavoprotein.medium chain Acyl-CoA dehydrogenase complex.

The crystal structure of the human electron transferring flavoprotein (ETF).medium chain acyl-CoA dehydrogenase (MCAD) complex reveals a dual mode of protein-protein interaction, imparting both specificity and promiscuity in the interaction of ETF with a range of structurally distinct primary dehydrogenases. ETF partitions the functions of partner binding and electron transfer between (i) the recognition loop, which acts as a static anchor at the ETF.MCAD interface, and (ii) the highly mobile redox active FAD domain. Together, these enable the FAD domain of ETF to sample a range of conformations, some compatible with fast interprotein electron transfer. Disorders in amino acid or fatty acid catabolism can be attributed to mutations at the protein-protein interface. Crucially, complex formation triggers mobility of the FAD domain, an induced disorder that contrasts with general models of protein-protein interaction by induced fit mechanisms. The subsequent interfacial motion in the MCAD.ETF complex is the basis for the interaction of ETF with structurally diverse protein partners. Solution studies using ETF and MCAD with mutations at the protein-protein interface support this dynamic model and indicate ionic interactions between MCAD Glu(212) and ETF Arg alpha(249) are likely to transiently stabilize productive conformations of the FAD domain leading to enhanced electron transfer rates between both partners.

Acyl-CoA Dehydrogenase↗
PubMed2007

Transfer of chemicals from feed to animal products: The use of transfer factors in risk assessment.

The human risk assessment of feed contaminants has often been hampered by a lack of knowledge concerning their behaviour when consumed by livestock. To gain a better understanding of the transfer of contaminants from animal feed to animal products, a meta-analysis of public literature was made. Data concerning feed contaminant concentrations, feeding periods, residue levels in animal products, and other parameters were gathered and recorded. For each case a 'transfer factor', which was defined as the ratio of the concentration of a chemical in an animal product to the concentration of the chemical in animal feed, was calculated. Scientifically founded transfer factors were calculated and analysed for groups of chemicals based on their contaminant classes or physicochemical properties. These database-derived transfer factors enable a more accurate risk assessment in the case of a feed contamination, and enable rapid risk management decision-making and/or intervention.

Animal Feed↗
PubMed2002

The role of parameter variability on retention, parameter transfer, and effector transfer.

The purpose of this study was to examine the effects of parameter variability on the learning of generalized motor programs (GMP) and movement parameterization. Participants attempted to exert a force pattern that resembled in force and time a waveform displayed on a computer monitor. The analysis suggested that relative timing (a measure of the GMP) performance remained remarkably stable across retention and transfer tests, whereby the structure of the movement remained intact, although the parameter or muscle group (effector) changed during transfer The results also indicated that variable parameter (time) practice did not enhance GMP learning but did degrade the learning parameter that was not varied (force). In addition, parameter specification was substantially less stable than the GMP, with time and force parameter performance deteriorating from the retention to transfer tests. These findings suggest that parameter specification, and not the GMP, is the primary cause of poorer performance in parameter and effector transfer.

Adolescent↗