Electron Transfer in Proteins

Electron-transfer (ET) reactions are key steps in a diverse array of biological transformations ranging from photosynthesis to aerobic respiration. A powerful theoretical formalism has been developed that describes ET rates in terms of two parameters: the nuclear reorganization [lambda] energy (1) and the electronic-coupling strength (HAB). Studies of ET reactions in ruthenium-modified proteins have probed [lambda] and HAB in several metalloproteins (cytochrome c, myoglobin, azurin). This work has shown that protein reorganization energies are sensitive to the medium surrounding the redox sites and that an aqueous environment, in particular, leads to large reorganization energies. Analyses of electronic-coupling strengths suggest that the efficiency of long-range ET depends on the protein secondary structure: [beta]sheets appear to mediate coupling more efficiently than [alpha]-helical structures, and hydrogen bonds play a critical role in both

Exploratory investigation of sound pressure level in the wake of an oscillating airfoil in the vicinity of stall

Wind tunnel tests were performed on two oscillating two-dimensional lifting surfaces. The first of these models had an NACA 0012 airfoil section while the second simulated the classical flat plate. Both of these models had a mean angle of attack of 12 degrees while being oscillated in pitch about their midchord with a double amplitude of 6 degrees. Wake surveys of sound pressure level were made over a frequency range from 16 to 32 Hz and at various free stream velocities up to 100 ft/sec. The sound pressure level spectrum indicated significant peaks in sound intensity at the oscillation frequency and its first harmonic near the wake of both models. From a comparison of these data with that of a sound level meter, it is concluded that most of the sound intensity is contained within these peaks and no appreciable peaks occur at higher harmonics. It is concluded that within the wake the sound intensity is largely pseudosound while at one chord length outside the wake, it is largely true vortex sound. For both the airfoil and flat plate the peaks appear to be more strongly dependent upon the airspeed than on the oscillation frequency. Therefore reduced frequency does not appear to be a significant parameter in the generation of wake sound intensity

Hydrogen Generation Catalyzed by Fluorinated Diglyoxime−Iron Complexes at Low Overpotentials

Fe^(II) complexes containing the fluorinated ligand 1,2-bis(perfluorophenyl)ethane-1,2-dionedioxime (dAr^FgH_2; H = dissociable proton) exhibit relatively positive Fe^(II/I) reduction potentials. The air-stable difluoroborated species [(dAr^FgBF_2)_2Fe(py)_2] (2) electrocatalyzes H_2 generation at −0.9 V vs SCE with i_(cat)/i_p ≈ 4, corresponding to a turnover frequency (TOF) of ~ 20 s^(–1) [Faradaic yield (FY) = 82 ± 13%]. The corresponding monofluoroborated, proton-bridged complex [(dArFg2H-BF2)Fe(py)2] (3) exhibits an improved TOF of ~ 200 s^(–1) (i_(cat)/i_p ≈ 8; FY = 68 ± 14%) at −0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of 2 suggest rate-limiting protonation of an Fe“0” intermediate (k_(RLS) ≈ 200 M^(–1) s^(–1)) that undergoes hydride protonation to form H_2. Complex 3 likely reacts via protonation of an Fe^I intermediate that subsequently forms H_2 via a bimetallic mechanism (k_(RLS) ≈ 2000 M^(–1) s^(–1)). 3 catalyzes production at relatively positive potentials compared with other iron complexes

Proton-Coupled Electron Flow in Protein Redox Machines

Electron transfer (ET) reactions are fundamental steps in biological redox processes. Respiration is a case in point: at least 15 ET reactions are required to take reducing equivalents from NADH, deposit them in O_2, and generate the electrochemical proton gradient that drives ATP synthesis. Most of these reactions involve quantum tunneling between weakly coupled redox cofactors (ET distances > 10 Å) embedded in the interiors of folded proteins. Here we review experimental findings that have shed light on the factors controlling these distant ET events. We also review work on a sensitizer-modified copper protein photosystem in which multistep electron tunneling (hopping) through an intervening tryptophan is orders of magnitude faster than the corresponding single-step ET reaction.If proton transfers are coupled to ET events, we refer to the processes as proton coupled ET, or PCET, a term introduced by Huynh and Meyer in 1981. Here we focus on two protein redox machines, photosystem II and ribonucleotide reductase, where PCET processes involving tyrosines are believed to be critical for function. Relevant tyrosine model systems also will be discussed

Mechanism of H_2 Evolution from a Photogenerated Hydridocobaloxime

Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged Co^I-diglyoxime complex, forming Co^(III)H, was monitored via transient absorption. The second-order rate constant for Co^(III)H formation is in the range (3.5−4.7) × 10^9 M^(−1) s^(−1), with proton transfer coupled to excited-state deactivation of the photoacid. Co^(III)H is subsequently reduced by excess Co^I-diglyoxime in solution to produce Co^(II)H (k_(red) = 9.2 × 10^6 M^(−1) s^(−1)), which is then protonated to yield Co^(II)-diglyoxime and H_2

An automatic lightning detection and photographic system

Conventional 35-mm camera is activated by an electronic signal every time lightning strikes in general vicinity. Electronic circuit detects lightning by means of antenna which picks up atmospheric radio disturbances. Camera is equipped with fish-eye lense, automatic shutter advance, and small 24-hour clock to indicate time when exposures are made

The effect of NOM characteristics and membrane type on microfiltration performance

Efforts to understand and predict the role of different organic fractions in the fouling of low-pressure membranes are presented. Preliminary experiments with an experimental apparatus that incorporates automatic backwashing and filtration over several days has shown that microfiltration of the hydrophilic fractions leads to rapid flux decline and the formation of a cake or gel layer, while the hydrophobic fractions show a steady flux decline and no obvious formation of a gel or cake layer. The addition of calcium to the weakly hydrophobic acid (WHA) fraction led to the formation of a gel layer from associations between components of the WHA. The dominant foulants were found to be the neutral and charged hydrophilic compounds, with hydrophobic and small pore size membranes being the most readily fouled. The findings suggest that surface analyses such as FTIR will preferentially identify hydrophilic compounds as the main foulants, as these components form a gel layer on the surface while the hydrophobic compounds adsorb within the membrane pores. Furthermore, coagulation pre-treatment is also likely to reduce fouling by reducing pore constriction rather than the formation of a gel layer, as coagulants remove the hydrophobic compounds to a large extent and very little of the hydrophilic neutral components

DIRECT MARKETING IN THE 1990S: TENNESSEE'S NEW FARMERS' MARKETS

Marketing,

SUPERMARKET PATRONAGE: AN ANALYSIS OF CUSTOMER COUNTS AMONG OUTLETS WITHIN A GEOGRAPHIC AREA

As new supermarket management tools are introduced, the need for an improved understanding of store patronage is growing. Weekly customer counts for five supermarkets located in a Southeastern metropolitan area covering 261 weeks are analyzed. Descriptive statistics indicate that food shopper patterns vary by outlet. Regression equations are estimated for each location. Results point to store specific relationships. They indicate that evaluation of television and radio ads and double couponing can be quite involved.Consumer/Household Economics,