Electron transfer in chemically and genetically modified myoglobins

The temperature dependences of the reduction potentials (E^o') of wildtype human myoglobin (Mb) and three site-directed mutants have been measured by using thin-layer spectroelectrochemistry. Residue Val68, which is in van der Waals contact with the heme in Mb, has been replaced by Glu, Asp, and Asn. At pH 7.0, reduction of the heme iron (III) in the former two proteins is accompanied by uptake of a proton by the protein. The changes in E^o', and the standard entropy (ΔS^o') and enthalpy (ΔH^o') of reduction in the mutant proteins were determined relative to values for wild-type; the change in E^o' at 25°C was about -200 millivolts for the Glu and Asp mutants, and about -80 millivolts for the Asn mutant. Reduction of Fe(III) to Fe(II) in the Glu and Asp mutants is accompanied by uptake of a proton. These studies demonstrate that Mb can tolerate substitution of a buried hydrophobic group by potentially charged and polar residues, and that such amino acid replacements can lead to substantial changes in the redox thermodynamics of the protein.

Through analysis of the temperature dependence and shapes of NMR dispersion signals, it is determined that a water molecule is bound to the sixth coordination site of the ferric heme in the Val68Asp and in the Val68Asn recombinant proteins while the carboxyl group of the sidechain of Glu68 occupies this position in Val68Glu. The relative rhombic distortions in the ESR spectra of these mutant proteins combined with H₂¹⁷O and spin interconversion experiments performed on them confirm the conclusions of the NMRD study.

The rates of intramolecular electron transfer (ET) of (NH₃)₅Ru-His48 (Val68Asp, His81GIn, Cys110AIa)Mb and (NH₃)₅Ru-His48 (Val68GIu,His81GIn,Cys110Ala)Mb were measured to be .85(3)s^-1 and .30(2)s^-1, respectively. This data supports the hypothesis that entropy of 111 reduction and reorganization energy of ET are inversely related. The rates of forward and reverse ET for (NH₃)₅ Ru-His48 (Val68GIu, His81 GIn, Cys110AIa)ZnMb -7.2(5)•10⁴s^-1and 1.4(2)•10⁵s^-1, respectively- demonstrate that the placement of a highly polar residue nearby does not significantly change the reorganization energy of the photoactive Zn porphyrin.

The distal histidine imidazoles of (NH₃)₄isnRu-His48 SWMb and (NH₃)₅Ru-His48 SWMb were cyanated with BrCN. The intramolecular ET rates of these BrCN-modified Mb derivatives are 5.5(6)s^-1 and 3.2(5)s^-1, respectively. These respective rates are 20 and 10 times faster than those of their noncyanated counterparts after the differences in ET rate from driving force are scaled according to the Marcus equation. This increase in ET rate of the cyanated Mb derivatives is attributed to lower reorganization energy since the cyanated Mb heme is pentacoordinate in both oxidation states; whereas, the native Mb heme loses a water molecule upon reduction so that it changes from six to five coordinate. The reorganization energy from Fe-OH₂ dissociation is estimated to be .2eV. This conclusion is used to reconcile data from previous experiments in our lab. ET in photoactive porphyrin-substituted myoglobins proceed faster than predicted by Marcus Theory when it is assumed that the only difference in ET parameters between photoactive porphyrins and native heme systems is driving force. However, the data can be consistently fit to Marcus Theory if one corrects for the smaller reorganization in the photoactive porphyrin systems since they do not undergo a coordination change upon ET.

Finally, the intramolecular ET rate of (NH₃)₄isnRu-His48 SWMb was measured to be 3.0(4)s^-1. This rate is within experimental error of that for (NH₃)₄pyrRu-His48 SWMb even though the former has 80mV more driving force. One likely possibility for this observation is that the tetraamminepyridineruthenium group undergoes less reorganization upon ET than the tetraammineisonicotinamideruthenium group. Moreover, analysis of the (NH₃)₄isnRu-His48 SWMb experimental system gives a likely explanation of why ET was not observed previously in (NH₃)₄isnRu-Cytochrome C.

Resistência à flexão e microestrutura de vidro-cerâmicas feldspáticas processadas por injeção em moldes à alta temperatura

Enquanto os métodos tradicionais de processamento de restaurações cerâmicas tornaram-se notórios por sua complexidade, as técnicas mais modernas vêm privilegiando a simplicidade de execução e a automação. Dentre estas, destaca-se a injeção em moldes, que recentemente, foi associada a métodos CAD-CAM. Estudos anteriores demonstraram a viabilidade de utilização de um vidro feldspático de baixa expansão térmica, Alpha (Vita Zahnfabrik), para injeção, porém, faltam informações quanto às propriedades mecânicas e a microestrutura deste material quando submetido à injeção. Os objetivos deste estudo são: produzir pastilhas vidrocerâmicas para injeção a quente a partir de Alpha e da mistura deste vidro com partículas de alumina e zircônia; avaliar a resistência à flexão dos materiais processados, e compará-la a um material compatível existente no mercado (PM9 - Vita Zahnfabrik); estudar a estrutura microscópica dos materiais e correlacioná-la com suas propriedades mecânicas; identificar por meio de difração de raios X a formação de fases cristalinas durante as diferentes etapas de processamento. A injeção aumentou a resistência do vidro Alpha devido à redução na quantidade e tamanho dos defeitos internos, principalmente porosidades. Apesar de ter sido observada nucleação de cristais nos dois materiais, durante o processamento, não foi possível determinar de que forma este fenômeno afetou as propriedades mecânicas dos materiais. Não foi detectada alteração no padrão de distribuição das fases cristalinas observadas em microscópio eletrônico de varredura antes e depois da injeção. Não foi verificada diferença estatística significante entre a resistência à flexão de Alpha injetado e PM9. A adição de partículas de alumina e zirconia ao vidro Alpha provocou redução da resistência, devido à formação de aglomerados durante a confecção das pastilhas e a incapacidade da injeção em dispersá-los. Tais aglomerados funcionaram como concentradores de tensões, enfraquecendo o material.

A simple method for counting bacteria with active electron transport system in water and sediment samples [Translation from: Kiel. Meeresforsch. 31(2) 83-86, 1975]

Description of a simple method for counting bacteria with active electron transport systems in water and sediment samples. Sodium succinate, NADH and NADPH served as electron donors. It is possible to see several sites of electron transport in the larger cells. Especially impressive are the plankton-algae, protozoa, and small metazoa. This is a partial translation of the ”method” section only.

Compact parallel optical modified-signed-digit arithmetic-logic array processor with electron-trapping device

A compact two-step modified-signed-digit arithmetic-logic array processor is proposed. When the reference digits are programmed, both addition and subtraction can be performed by the same binary logic operations regardless of the sign of the input digits. The optical implementation and experimental demonstration with an electron-trapping device are shown. Each digit is encoded by a single pixel, and no polarization is included. Any combinational logic can be easily performed without optoelectronic and electro-optic conversions of the intermediate results. The system is compact, general purpose, simple to align, and has a high signal-to-noise ratio. (C) 1999 Optical Society of America.

High resolution electron microscope studies of chromosomal fibers

Techniques are described for mounting and visualizing biological macromolecules for high resolution electron microscopy. Standard techniques are included in a discussion of new methods designed to provide the highest structural resolution. Methods are also discussed for handling samples on the grid, for making accurate size measurements at the 20 Å level, and for photographically enhancing image contrast.

The application of these techniques to the study of the binding of DNA polymerase to DNA is described. It is shown that the electron micrographs of this material are in agreement with the model proposed by Dr. Arthur Kornberg. A model is described which locates several active sites on the enzyme.

The chromosomal material of the protozoan tetrahymena has been isolated and characterized by biochemical techniques and by electron microscopy. This material is shown to be typical of chromatin of higher creatures.

Comparison with other chromatins discloses that the genome of tetrahymena is highly template active and has a relatively simple genetic construction.

High resolution electron microscope procedures developed in this work have been combined with standard biochemical techniques to give a comprehensive picture of the structure of interphase chromosome fibers. The distribution of the chromosomal proteins along its DNA is discussed.

A theory of strong and weak scintillations with applications to astrophysics

The propagation of waves in an extended, irregular medium is studied under the "quasi-optics" and the "Markov random process" approximations. Under these assumptions, a Fokker-Planck equation satisfied by the characteristic functional of the random wave field is derived. A complete set of the moment equations with different transverse coordinates and different wavenumbers is then obtained from the characteristic functional. The derivation does not require Gaussian statistics of the random medium and the result can be applied to the time-dependent problem. We then solve the moment equations for the phase correlation function, angular broadening, temporal pulse smearing, intensity correlation function, and the probability distribution of the random waves. The necessary and sufficient conditions for strong scintillation are also given.

We also consider the problem of diffraction of waves by a random, phase-changing screen. The intensity correlation function is solved in the whole Fresnel diffraction region and the temporal pulse broadening function is derived rigorously from the wave equation.

The method of smooth perturbations is applied to interplanetary scintillations. We formulate and calculate the effects of the solar-wind velocity fluctuations on the observed intensity power spectrum and on the ratio of the observed "pattern" velocity and the true velocity of the solar wind in the three-dimensional spherical model. The r.m.s. solar-wind velocity fluctuations are found to be ~200 km/sec in the region about 20 solar radii from the Sun.

We then interpret the observed interstellar scintillation data using the theories derived under the Markov approximation, which are also valid for the strong scintillation. We find that the Kolmogorov power-law spectrum with an outer scale of 10 to 100 pc fits the scintillation data and that the ambient averaged electron density in the interstellar medium is about 0.025 cm^-3. It is also found that there exists a region of strong electron density fluctuation with thickness ~10 pc and mean electron density ~7 cm^-3 between the PSR 0833-45 pulsar and the earth.

The photorefractive sensitivity and the effective electron transport length in two-centre holographic recording

In this paper the photorefractive sensitivity defined for single-centre holographic recording is modified to adapt two-centre holographic recording. Based on the time analytic solution of Kukhtarev equations for doubly doped crystals, the analytical expression of photorefractive sensitivity is given. For comparison with single-centre holographic recording and summing the electron competition effects between the deeper and shallower traps, an effective electron transport length is proposed, which varies with the intensity ratios of recording light to sensitive light. According to analyses in this paper, the lower photorefractive sensitivity in two-centre holographic recording is mainly due to the lower concentration of unionized dopants in the shallower centre and the lower effective electron transport length.

Study of oscillatory behavior of recording process arising from electron-hole competition in strongly oxidized LiNbO3 : Ce : Cu

We have investigated ultraviolet (UV) photorefractive effect of lithium niobate doubly doped with Ce and Cu. It is found the diffraction efficiency shows oscillating behavior Under UV-1ight-recording. A model in which electrons and holes can be excited from impurity centers in the UV region is proposed to study the oscillatory behavior of the diffraction efficiency. Oil the basis of the material equations and the coupled-wave equations, we found that the oscillatory behavior is due to the oscillation of the relative spatial phase shift Phi. And the electron-hole competition may cause the oscillation of the relative spatial phase shift. A switch point from electron grating to hole grating is chosen to realize nonvolatile readout by a red light with high sensitivity (0.4 cm/J). (c) 2005 Elsevier GmbH. All rights reserved.

Polaron hopping in olivine phosphates studied by nuclear resonant scattering

Valence fluctuations of Fe²⁺ and Fe³⁺ were studied in a solid solution of Li_xFePO₄ by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamil- tonian. These frequencies were analyzed to obtain activation energies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an anomalously large activation volume. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons, and Li⁺ ions that alter the dynamics of both.

In a parallel study of Na_xFePO₄, the interplay between sodium ordering and electron mobility was investigated using a combination of synchrotron x-ray diffraction and nuclear resonant scattering. Conventional Mossbauer spectra were collected while the sample was heated in a resistive furnace. An analysis of the temperature evolution of the spectral shapes was used to identify the onset of fast electron hopping and determine the polaron hopping rate. Synchrotron x-ray diffraction measurements were carried out in the same temperature range. Reitveld analysis of the diffraction patterns was used to determine the temperature of sodium redistribution on the lattice. The diffraction analysis also provides new information about the phase stability of the system. The temperature evolution of the iron site occupancies from the Mossbauer measurements, combined with the synchrotron diffraction results give strong evidence for a relationship between the onset of fast electron dynamics and the redistribution of sodium in the lattice.

Measurements of activation barriers for polaron hopping gave fundamental insights about the correlation between electronic carriers and mobile ions. This work established that polaron-ion interactions can alter the local dynamics of electron and ion transport. These types of coupled processes may be common in many materials used for battery electrodes, and new details concerning the influence of polaron-ion interactions on the charge dynamics are relevant to optimizing their electrochemical performance.

Metrologia de instrumentos endodônticos por microscopia eletrônica de varredura e análise digital de imagens: uma proposta metodológica

A padronização para a fabricação de instrumentos endodônticos em aço inoxidável contribuiu para o desenvolvimento de novos aspectos geométricos. Surgiram propostas de alterações no desenho da haste helicoidal, da seção reta transversal, da ponta, da conicidade e do diâmetro na extremidade (D0). Concomitantemente, o emprego de ligas em Níquel-Titânio possibilitou a produção de instrumentos acionados a motor, largamente empregados hoje. A cada ano a indústria lança instrumentos com diversas modificações, sem, contudo, disponibilizar informações suficientes quanto às implicações clínicas destas modificações. Existe um crescente interesse no estudo dos diferentes aspectos geométricos e sua precisa metrologia. Tradicionalmente, a aferição de aspectos geométricos de instrumentos endodônticos é realizada visualmente através de microscopia ótica. Entretanto, esse procedimento visual é lento e subjetivo. Este trabalho propõe um novo método para a metrologia de instrumentos endodônticos baseado no microscópio eletrônico de varredura e na análise digital das imagens. A profundidade de campo do MEV permite obter a imagem de todo o relevo do instrumento endodôntico a uma distância de trabalho constante. Além disso, as imagens obtidas pelo detector de elétrons retro-espalhados possuem menos artefatos e sombras, tornando a obtenção e análise das imagens mais fáceis. Adicionalmente a análise das imagens permite formas de mensuração mais eficientes, com maior velocidade e qualidade. Um porta-amostras específico foi adaptado para obtenção das imagens dos instrumentos endodônticos. Ele é composto de um conector elétrico múltiplo com terminais parafusados de 12 pólos com 4 mm de diâmetro, numa base de alumínio coberta por discos de ouro. Os nichos do conector (terminais fêmeas) têm diâmetro apropriado (2,5 mm) para o encaixe dos instrumentos endodônticos. Outrossim, o posicionamento ordenado dos referidos instrumentos no conector elétrico permite a aquisição automatizada das imagens no MEV. Os alvos de ouro produzem, nas imagens de elétrons retro-espalhados, melhor contraste de número atômico entre o fundo em ouro e os instrumentos. No porta-amostras desenvolvido, os discos que compõem o fundo em ouro são na verdade, alvos do aparelho metalizador, comumente encontrados em laboratórios de MEV. Para cada instrumento, imagens de quatro a seis campos adjacentes de 100X de aumento são automaticamente obtidas para cobrir todo o comprimento do instrumento com a magnificação e resolução requeridas (3,12 m/pixel). As imagens obtidas são processadas e analisadas pelos programas Axiovision e KS400. Primeiro elas são dispostas num campo único estendido de cada instrumento por um procedimento de alinhamento semi-automático baseado na inter-relação com o Axiovision. Então a imagem de cada instrumento passa por uma rotina automatizada de análise de imagens no KS400. A rotina segue uma sequência padrão: pré-processamento, segmentação, pós-processamento e mensuração dos aspectos geométricos.

Photochemical electron transfer at fixed distance : a synthetic model of the photosynthetic primary process

A series of meso-phenyloctamethylporphyrins covalently bonded at the 4'phenyl position to quinones via rigid bicyclo[2.2.2]octane spacers were synthesized for the study of the dependence of electron transfer reaction rate on solvent, distance, temperature, and energy gap. A general and convergent synthesis was developed based on the condensation of ac-biladienes with masked quinonespacer-benzaldehydes. From picosecond fluorescence spectroscopy emission lifetimes were measured in seven solvents of varying polarity. Rate constants were determined to vary from 5.0x10⁹sec^-1 in N,N-dimethylformamide to 1.15x10¹⁰ Sec^-1 in benzene, and were observed to rise at most by about a factor of three with decreasing solvent polarity. Experiments at low temperature in 2-MTHF glass (77K) revealed fast, nearly temperature-independent electron transfer characterized by non-exponential fluorescence decays, in contrast to monophasic behavior in fluid solution at 298K. This example evidently represents the first photosynthetic model system not based on proteins to display nearly temperature-independent electron transfer at high temperatures (nuclear tunneling). Low temperatures appear to freeze out the rotational motion of the chromophores, and the observed nonexponential fluorescence decays may be explained as a result of electron transfer from an ensemble of rotational conformations. The nonexponentiality demonstrates the sensitivity of the electron transfer rate to the precise magnitude of the electronic matrix element, which supports the expectation that electron transfer is nonadiabatic in this system. The addition of a second bicyclooctane moiety (15 Å vs. 18 Å edge-to-edge between porphyrin and quinone) reduces the transfer rate by at least a factor of 500-1500. Porphyrinquinones with variously substituted quinones allowed an examination of the dependence of the electron transfer rate constant κ_ET on reaction driving force. The classical trend of increasing rate versus increasing exothermicity occurs from 0.7 eV≤ |ΔG^0'(R)| ≤ 1.0 eV until a maximum is reached (κ_ET = 3 x 10⁸ sec^-1 rising to 1.15 x 10¹⁰ sec^-1 in acetonitrile). The rate remains insensitive to ΔG⁰ for ~ 300 mV from 1.0 eV≤ |ΔG^0’(R)| ≤ 1.3 eV, and then slightly decreases in the most exothermic case studied (cyanoquinone, κ_ET = 5 x 10⁹ sec^-1).

Thermopower in Two-Dimensional Electron Systems

The subject of this thesis is the measurement and interpretation of thermopower in high-mobility two-dimensional electron systems (2DESs). These 2DESs are realized within state-of-the-art GaAs/AlGaAs heterostructures that are cooled to temperatures as low as T = 20 mK. Much of this work takes place within strong magnetic fields where the single-particle density of states quantizes into discrete Landau levels (LLs), a regime best known for the quantum Hall effect (QHE). In addition, we review a novel hot-electron technique for measuring thermopower of 2DESs that dramatically reduces the influence of phonon drag.

Early chapters concentrate on experimental materials and methods. A brief overview of GaAs/AlGaAs heterostructures and device fabrication is followed by details of our cryogenic setup. Next, we provide a primer on thermopower that focuses on 2DESs at low temperatures. We then review our experimental devices, temperature calibration methods, as well as measurement circuits and protocols.

Latter chapters focus on the physics and thermopower results in the QHE regime. After reviewing the basic phenomena associated with the QHE, we discuss thermopower in this regime. Emphasis is given to the relationship between diffusion thermopower and entropy. Experimental results demonstrate this relationship persists well into the fractional quantum Hall (FQH) regime.

Several experimental results are reviewed. Unprecedented observations of the diffusion thermopower of a high-mobility 2DES at temperatures as high as T = 2 K are achieved using our hot-electron technique. The composite fermion (CF) effective mass is extracted from measurements of thermopower at LL filling factor ν = 3/2. The thermopower versus magnetic field in the FQH regime is shown to be qualitatively consistent with a simple entropic model of CFs. The thermopower at ν = 5/2 is shown to be quantitatively consistent with the presence of non-Abelian anyons. An abrupt collapse of thermopower is observed at the onset of the reentrant integer quantum Hall effect (RIQHE). And the thermopower at temperatures just above the RIQHE transition suggests the existence of an unconventional conducting phase.