Vernacular Architecture as Self-Determination: Venturi, Scott Brown and the Controversy over Philadelphia's Crosstown Expressway, 1967-1973

Between 1967 and 1972 Robert Venturi and Denise Scott Brown worked on two projects simultaneously. One culminated in Learning from Las Vegas, their renowned contribution to architectural theory; the other reflected their political engagement in the so-called Crosstown Controversy. In this project, Venturi and Scott Brown spoke on behalf of a citizens’ initiative opposing a proposed inner-city expressway in the South Street area of Philadelphia. The aesthetic criticism exemplified in their study of Las Vegas found parallels in the political critique embodied by the alternative scheme they developed for the Philadelphia citizen group. This scheme proposed to revalorise the vernacular architecture of the existing neighbourhood, against the wholesale demolition implied in the official plans for the Crosstown Expressway. This article investigates the connections between aesthetic ideals and social concern in the work of Venturi and Scott Brown. Embedding the discussion on architectural theory in the concrete context of urban history reveals important links between intellectual discourse and political action. Venturi and Scott Brown’s repeated reference to social aspects of architecture can only be understood, so this article demonstrates, if we analyse the concrete engagements in which they developed their projects. The South Street project offers a concrete occasion wherein the architects were forced to adopt a political position

Photogrammetric and Glaciological Studies of Salmon Glacier

Reports results of work as geodesist-glaciologist on Univ. of Toronto-National Research Council of Canada Second Salmon Glacier Expedition, 1957, to this glacier on the Alaska-British Columbia border. A dense and accurate ground control network was established to ensure accuracy for photogrammetric processing of photographs. Surveying and mapping was done by aerial photogrammetry; the methods, and such for supplementary terrestrial photogrammetry are described. General physical setting is outlined. Recession of the Salmon and associated glaciers is described, for the period 1949-1957, and tentative data for 1920-1957 recession are tabulated. Compilation of a map at scale 1:25,000 provided an accurate basis for glaciological investigations. A second mapping in 1962, with relative flying elevations not to exceed 2,500 m is recommended

Investigation of an atypical protoporphyric family in South Africa

Includes bibliographical references (leaves 171-184).Affected members of the family investigated in this dissertation presented with photosensitivity and raised red cell protoporphyrin concentrations, indicative of protoporphyria. Further examination of this family revealed features that were atypical of erythropoietic protoporphyria. These included a highly penetrant disease, disease severity as expressed by more prevalent hepatic complications, a preponderance of protoporphyrin in its zinc chelated form, a therapeutic response to iron supplementation, and an absence of mutations in the ferrochelatase gene or haplotype markers associated with erythropoietic protoporphyria. We have reviewed clinical data from this family, established a ferrochelatase enzyme assay in our laboratory, and shown normal ferrochelatase enzyme activity in affected subjects

The rates of proton uptake and electron transfer at the reducing side of photosystem II in thylakoids

AbstractProton and electron transfer at the reducing side of photosystem II of green plants was studied under flashing light, the former at improved time resolution by using Neutral red. The rates of electron transfer within QAFeQB were determined by pump-probe flashes through electrochromic transients. The extent of proton binding was about 1 H+/e−. The rates of proton transfer were proportional to the concentration of Neutral red (collisional transfer), whereas the rates of electron transfer out of Q−A and from QAFeQ−B to the cytochrome b6f complex were constant. The half-rise times of electron transfer (τe) and the apparent times of proton binding (τh) at 30 μM Neutral red were: Q−A ⇒ FeIIIQB (τe ⩽ 100 μs, τ, 230 μs); Q−A ⇒ FeIIQB (τe = 150 μs, τh = 760 μs); and Q−A ⇒ FeIIQ−B (τe = 620 μs, τh = 310 μs)

Evidence for impaired hydrogen-bonding of tyrosine YZ in calcium-depleted Photosystem II

AbstractPhotosystem II (PS II) evolves oxygen from two bound water molecules in a four-stepped reaction that is driven by four quanta of light, each oxidizing the chlorophyll moiety P680 to yield P+680. When starting from its dark equilibrium (mainly state S1), the catalytic center can be clocked through its redox states (S0…S4) by a series of short flashes of light. The center involves at least a Mn4-cluster and a special tyrosine residue, named YZ, as redox cofactors plus two essential ionic cofactors, Cl− and Ca2+. Centers which have lost Ca2+ do not evolve oxygen. We investigated the stepped progression in dark-adapted PS II core particles after the removal of Ca2+. YZ was oxidized from the first flash on. The difference spectrum of YZ→YoxZ differed from the one in competent centers, where it has been ascribed to a hydrogen-bonded tyrosinate. The rate of the electron transfer from YZ to P+680 was slowed down by three orders of magnitude and its kinetic isotope effect rose up from 1.1 to 2.5. Proton release into the bulk was now a prerequisite for the electron transfer from YZ to P+680. On the basis of these results and similar effects in Mn-(plus Ca2+-)depleted PS II (M. Haumann et al., Biochemistry, 38 (1999) 1258–1267) we conclude that the presence of Ca2+ in the catalytic center is required to tune the apparent pK of a base cluster, B, to which YZ is linked by hydrogen bonds. The deposition of a proton on B within close proximity of YZ (not its release into the bulk!) is a necessary condition for the reduction in nanoseconds of P+680 and for the functioning of water oxidation. The removal of Ca2+ rises the pK of B, thereby disturbing the hydrogen bonded structure of YZB

The molecular proceedings of biological hydrogen turnover

Over the past two decades, the bioinorganic chemistry of hydrogenases has attracted much interest from basic and applied research. Hydrogenases are highly efficient metalloenzymes that catalyze the reversible reduction of protons to molecular hydrogen (H2) in all domains of life. Their iron- and nickel-based cofactors represent promising blueprints for the design of biomimetic, synthetic catalysts. In this Account, we address the molecular proceedings of hydrogen turnover with [FeFe]-hydrogenases. The active site cofactor of [FeFe]-hydrogenases (“H-cluster”) comprises a unique diiron complex linked to a [4Fe-4S] cluster via a single cysteine. Since it was discovered that a synthetic analogue of the diiron site can be incorporated into apoprotein in vitro to yield active enzyme, significant progress has been made toward a comprehensive understanding of hydrogenase catalysis. The diiron site carries three to four carbon monoxide (CO) and two cyanide (CN–) ligands that give rise to intense infrared (IR) absorption bands. These bands are sensitive reporters of the electron density across the H-cluster, which can be addressed by infrared spectroscopy to follow redox and protonation changes at the cofactor. Synthetic variation of the metal-bridging dithiolate ligand at the diiron site, as well as site-directed mutagenesis of amino acids, provides access to the proton pathways toward the cofactor. Quantum chemical calculations are employed to specifically assign IR bands to vibrational modes of the diatomic ligands and yield refined H-cluster geometries. Here, we provide an overview of recent research on [FeFe]-hydrogenases with emphasis on experimental and computational IR studies. We describe advances in attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) and protein film electrochemistry, as well as density functional theory (DFT) calculations. Key cofactor species are discussed in terms of molecular geometry, redox state, and protonation. Isotope editing is introduced as a tool to evaluate the cofactor geometry beyond the limits of protein crystallography. In particular, the role of proton-coupled electron transfer (PCET) in the generation of catalytically relevant redox species is addressed. We propose that site-selective protonation of the H-cluster biases surplus electrons either to the [4Fe-4S] cluster or to the diiron site. Protonation of the [4Fe-4S] cluster prevents premature reduction at the diiron site and stabilizes a reactive, terminal hydride. The observed H-cluster species are assigned to rapid H2 conversion or to reactions possibly involved in activity regulation and cellular H2 sensing. In the catalytic cycle of [FeFe]-hydrogenases, an H-cluster geometry is preserved that features a bridging CO ligand. PCET levels the redox potential for two steps of sequential cofactor reduction. The concept of consecutive PCET at a geometrically inert cofactor with tight control of electron and proton localization may inspire the design of a novel generation of biomimetic catalysts for the production of H2 as a fuel

Alternating electron and proton transfer steps in photosynthetic water oxidation

Water oxidation by cyanobacteria, algae, and plants is pivotal in oxygenic photosynthesis, the process that powers life on Earth, and is the paradigm for engineering solar fuel–production systems. Each complete reaction cycle of photosynthetic water oxidation requires the removal of four electrons and four protons from the catalytic site, a manganese–calcium complex and its protein environment in photosystem II. In time-resolved photothermal beam deflection experiments, we monitored apparent volume changes of the photosystem II protein associated with charge creation by light-induced electron transfer (contraction) and charge-compensating proton relocation (expansion). Two previously invisible proton removal steps were detected, thereby filling two gaps in the basic reaction-cycle model of photosynthetic water oxidation. In the S2 → S3 transition of the classical S-state cycle, an intermediate is formed by deprotonation clearly before electron transfer to the oxidant (Graphic). The rate-determining elementary step (τ, approximately 30 µs at 20 °C) in the long-distance proton relocation toward the protein–water interface is characterized by a high activation energy (Ea = 0.46 ± 0.05 eV) and strong H/D kinetic isotope effect (approximately 6). The characteristics of a proton transfer step during the S0 → S1 transition are similar (τ, approximately 100 µs; Ea = 0.34 ± 0.08 eV; kinetic isotope effect, approximately 3); however, the proton removal from the Mn complex proceeds after electron transfer to Graphic. By discovery of the transient formation of two further intermediate states in the reaction cycle of photosynthetic water oxidation, a temporal sequence of strictly alternating removal of electrons and protons from the catalytic site is established

Identification of YdhV as the first molybdoenzyme binding a Bis-Mo-MPT cofactor in escherichia coli

The oxidoreductase YdhV in Escherichia coli has been predicted to belong to the family of molybdenum/tungsten cofactor (Moco/Wco)-containing enzymes. In this study, we characterized the YdhV protein in detail, which shares amino acid sequence homology with a tungsten-containing benzoyl-CoA reductase binding the bis-W-MPT (for metal-binding pterin) cofactor. The cofactor was identified to be of a bis-Mo-MPT type with no guanine nucleotides present, which represents a form of Moco that has not been found previously in any molybdoenzyme. Our studies showed that YdhV has a preference for bis-Mo-MPT over bis-W-MPT to be inserted into the enzyme. In-depth characterization of YdhV by X-ray absorption and electron paramagnetic resonance spectroscopies revealed that the bis-Mo-MPT cofactor in YdhV is redox active. The bis-Mo-MPT and bis-W-MPT cofactors include metal centers that bind the four sulfurs from the two dithiolene groups in addition to a cysteine and likely a sulfido ligand. The unexpected presence of a bis-Mo-MPT cofactor opens an additional route for cofactor biosynthesis in E. coli and expands the canon of the structurally highly versatile molybdenum and tungsten cofactors

Photosynthetic oxygen evolution: Net charge transients as inferred from electrochromic bandshifts are independent of proton release into the medium

AbstractThe manganese containing center of the oxygen evolving complex accumulates four oxidizing equivalents in the four stepped water oxidation cycle. Based on experiments on electrochromic absorption transients and the reduction rate of the primary electron donor, P680, it has been speculated that the oscillations of these variables reflect the net charge of the center as calculated from the difference between electron abstraction and proton release into the medium. We compared proton release with electrochromism in thylakoids and core particles, and under variation of the rate of proton release. We found no equivalent of the variations of the extents and the rates of proton release in electrochromism. The oscillatory pattern of the latter reflects the topological properties of the stepped charge storage relative to the position and orientation of electrochromically responsive pigments rather than responding to proton release from the periphery