Showing papers in "Israel Journal of Chemistry in 1981"
TL;DR: In this paper, the phases for the x-ray data for crystals of azurin from P. aeruginosa have been refined and extended from 3 A to 2.7 A resolution by a method based on the direct space averaging of the electron density of the four molecules in the asymmetric unit.
Abstract: The phases for the x-ray data for crystals of azurin from P. aeruginosa have been refined and extended from 3 A to 2.7 A resolution by a method based on the direct space averaging of the electron density of the four molecules in the asymmetric unit. The electron density in the present map is much improved over the earlier one, and most of the side-chains are now evident. In the detailed fitting of a model to the present map, an alternative interpretation, conforming to the plastocyanin model and reversing the chain directions in strands one and two of the original azurin model, was found to better represent certain features of the present map. The surface of the molecule closest to the copper consists of an extended region of invariant or semiconserved hydrophobic residues. There is not a region of conserved charge that is not paired with a compensating charge.
229 citations
TL;DR: It is suggested that ubiquinone and o-Phen compete for the B-site in a mutually exclusive fashion, and that their relative binding strengths are modulated by the redox and protonation state of QA.
Abstract: The oxidation-reduction midpoint potential (Em) of the primary quinone (QA) of the acceptor quinone complex of bacterial photosynthetic reaction centers has been measured as a function of pH in the presence and absence of ubiquinone and o-phenanthroline (o-Phen). Reaction centers, isolated from Rhodopseudomonas sphaeroides, were incorporated into egg phosphatidylcholine vesicles. Contrary to earlier reports, the Em was found to exhibit a pH-dependence very similar to that observed in chromatophores, with a slope of - 60 mV/pH up to a p K for Q−A/Q−A(H+) at pH 9.5–10.0. In the presence of ubiquinone to reconstitute the secondary quinone (QB), the Em/pH curve of QA was shifted to lower potentials, indicating that the binding of Qn (actually QBH2) was suppressed by reduction of QA. o-Phen, an inhibitor of electron transport from QA to QB, raised the pK of Q−A/Q−A(H+) and, at pH-values below but not above this pK, reversed the effects of QB. In the absence of QB, o-Phen lowered the Em of QA above the pK but had no effect below it. These results are discussed in terms of interactions between the binding sites for QA and QB (A- and B-sites). It is suggested that ubiquinone and o-Phen compete for the B-site in a mutually exclusive fashion, and that their relative binding strengths are modulated by the redox and protonation state of QA. In preparations with low quinone content, o-Phen inhibits photochemistry suggesting that it can also compete with ubiquinone at the A-site. Competitive displacement of quinone from the B-site by o-Phen and other inhibitors is suggested as the primary mode of action of a broad class of herbicides active in Photosystem II of plants. The relative binding affinities of the various redox states of QB are also discussed and it is concluded that the order of binding strength is: Q−B → QB → QBH2. This accounts for the atypical stability of the semiquinone and the lower average Em for reduction to the quinol, compared to free ubiquinone in the quinone pool. It may also be significant in the functioning of quinones in communicating reducing equivalents from the reaction center to other electron transport complexes in the intact membrane.
223 citations
TL;DR: In this paper, the triplet state of Chl a+ was formed by reaction of 3Chl a with a quinone; it has an absorption band at 840 nm (in cyclohexanol) with a shoulder around 740 nm and no absorption between 1000 and 1650 nm.
Abstract: Absorption spectra in the near infra-red have been obtained for transient species obtained from chlorophyll a, by flash absorption spectroscopy. The triplet state of Chl a presents an absorption band around 760 nm (ϵ = 7,500 M−1 cm−1) and a very broad band (ϵ = 1,700 M−1 cm−1) around 1100 nm. At low temperature, evidence was obtained for the formation of the triplet state of aggregated Chl a. Its t1/2 of decay is around 0.5 ms instead of 1 ms for the monomeric species. The radical-cation Chl a+ was formed by reaction of 3Chl a with a quinone; it has an absorption band at 840 nm (in cyclohexanol) with a shoulder around 740 nm and no absorption between 1000 and 1650 nm. The cation has also been formed in vivo, as the oxidized state of the photosynthetic primary electron donors, P680 and P700. The spectrum of P+680 resembles that of Chl a+; its absorption maximum is at 820 nm. The spectrum of P+680, maximum at 810 nm, is significantly broader. P+700 has no absorption between 1000 and 1650 nm. These spectra are in favor of recent suggestions on a monomeric nature of P680 and of a dimeric nature for P700.
125 citations
TL;DR: In this article, the authors report the measurement of the room-temperature optical properties of antenna chlorophyll proteins (prepared with sodium dodecyl sulfate membrane solubilization) from six plant species.
Abstract: We report the measurement of the room-temperature optical properties of antenna chlorophyll proteins (prepared with sodium dodecyl sulfate membrane solubilization) from six plant species of five different orders: pea (Pisum sativum), soy (Glycine max), barley (Hordeum vulgare), spinach (Spinacea oleracea), yew (Taxus canadensis), and fern (Onoclea sensibilis). All six species' antenna protein preparations exhibit optical spectra which are similar in great detail, implying similar internal chromophore arrangements from species to species. Features of the spectra thought due to chromophore interactions disappear upon heat denaturation. A model proposed by Van Metter for spinach chlorophyll protein has been revised and is consistent with all the data presented here.
42 citations
TL;DR: In this article, the authors show that the large number of approaches using apparently unrelated strainless increments for unsubstituted alicyclic hydrocarbons in the literature are neither mathematically nor conceptually unique.
Abstract: In this paper we show that the large number of approaches using apparently unrelated strainless increments for unsubstituted alicyclic hydrocarbons in the literature are neither mathematically nor conceptually unique. We additionally demonstrate that if the strain energy assigned to a compound by any three sets of increments is known, the strain energy any other approach would assign can automatically be determined without considering any further details of the structure of the compound. Equivalently, there are but three mathematically distinct, i.e. linearly independent, strainless incremental approaches for these compounds. Thus the choice of which method to employ in one's own reasoning relative to a chemical problem must be based on personal, rather than strictly chemical or mathematical criteria. We proceed by presenting our criteria and their molecular realization, the concept of diagonal reference states. Diagonal reference states are defined from hydrocarbons composed solely of the group of interest. In particular, >C CH–, –CH2– and –CH3 are derived from diamond, cubane, cyclohexane and ethane with the accompanying enthalpies of formation, +1.9, –9.7, –20.6. and –42.0kJ mol−1. The virtues and debits of this method are presented in support of our conclusion that this approach is preferable because it is diagonal.
38 citations
TL;DR: In this article, the reaction mechanism can be understood to proceed via initial protonation of the aminoazirine, subsequent attack of the nitrogen nucleophile on the iminiumaziridine and ring expansion of the three-membered ring to form a zwitterionic intermediate.
Abstract: 3-Dimethylamino-2,2-dimethyl-2H-azirine (1) reacts with a number of NH-acidic compounds to give adducts of interesting structural variety. In analogy to the reaction of OH-acidic substrates, the reaction mechanism can be understood to proceed via initial protonation of the aminoazirine, subsequent attack of the nitrogen nucleophile on the iminiumaziridine and ring expansion of the three-membered ring to form a zwitterionic intermediate. Depending very much on the charge-distribution delivered by the variety of substituents, this zwitterionic intermediate is going to be stablised by different routes, e.g. water elimination, ring enlargement or rearrangement, to yield new N-heterocyclic compounds.
34 citations
TL;DR: It is suggested that the chromium is coordinated to Lys-85 and Glu-91, and this assignment and the analysis of the three dimensional model of azurin serve in delineating an electron transfer pathway.
Abstract: Pseudomonas aeruginosa azurin is stoichiometrically reduced by chromium (II) ions to yield a relatively inert chromium (III) complex with azurin. When this derivative is proteolytically digested by either trypsin or chymotrypsin, only a single, yet different peptide labeled with chromium is obtained upon chromatographic separations. Amino acid analysis of these fragments shows that the tryptic peptide consists of residues Val-80 to Lys-92, while the chymotryptic peptide consists of the Thr-84 to Val-95 fragment. Based on the above findings, the chemical nature of the amino acid side chains of these peptides and the crystallographic model of azurin, it is suggested that the chromium is coordinated to Lys-85 and Glu-91. This assignment and the analysis of the three dimensional model of azurin serve in delineating an electron transfer pathway. A resonance transfer from an opening in the peptide sheath to Cu(II) through an extended relay of the aligned imidazoles of His-35 and His-46 is proposed.
34 citations
TL;DR: In this article, the optical and EPR properties of the ferric form of the iron containing superoxide dismutase from E. coli B, at pH values ranging from 4.5 to 10.9, have revealed two reversible structural transitions affecting the Fe:" ion.
Abstract: Examination of the optical and EPR properties of the ferric form of the iron containing superoxide dismutase from E. coli B, at pH values ranging from 4.5 to 10.9, has revealed two reversible structural transitions affecting the Fe:" ion. The apparent pK. values of these transitions are 5.1 ±0.3 and 9.0±0.3. The binding of azide has been studied over the pH range 4.5 to 10.7; the affinity of the Fe" for N~ is independent of pH from 4.5 to -7.5, after which the dissociation constant decreased hy a factor of 10 per unit increase in pH. The apparent pK. which affects N~ binding to the iron is 8.6±0.2. The association of N.~ with the iron has been examined using the temperature-jump method at pH 7.4 and 9.3. The kinetics of ligand association were shown to conform to the minimal mechanism: K, P-Fe" +
31 citations
TL;DR: Radiationless energy transfer studies with a fluorescent substrate demonstrate that zinc is not required for substrate binding and does not mediate the activating effect of chloride, thus, zinc functions solely in the hydrolytic step of catalysis.
Abstract: Angiotensin converting enzyme is a chloride-dependent zinc metalloexo-peptidase. The zinc atom is essential for enzymatic activity. Removal of zinc by dialysis against the chelating agent 1,10-phenanthroline abolishes activity toward the chromophoric substrates furanacryloyl-Phe-Gly-Gly. Reconstitution of the resultant apoenzyme with either zinc, cobalt or manganese restores activity to 100, 55 or 25% of that of the native enzyme, respectively. Spontaneous dissociation of zinc from the enzyme occurs below pH 7 and this can be prevented by adding excess metal to the buffer. Radiationless energy transfer studies with a fluorescent substrate demonstrate that zinc is not required for substrate binding and does not mediate the activating effect of chloride. Thus, zinc functions solely in the hydrolytic step of catalysis.
30 citations
TL;DR: In this paper, the optical, paramagnetic and redox properties of the reduced primary acceptors of photosystems (PS) I and II with those of chlorophyll and pheophytin anions are compared.
Abstract: Comparison of the optical, paramagnetic and redox properties of the reduced primary acceptors of Photosystems (PS) I and II with those of chlorophyll and pheophytin anions suggests that chlorophyll is the primary reduced product in PS I and pheophytin in PS II. Molecular orbital calculations provide a description of the electronic profiles of the radicals, and indicate that the protein environment of the chromophores in the reaction center may impose specific orientations and induce hydrogen bonding to some of the chlorophyll substituent groups. Examination of all the primary acceptors postulated for green plants and purple bacteria suggests that (bacterio)chlorophyll-like acceptors may be obligatory to effect the rapid primary charge separation, because they allow favorable orbital overlap between donor and acceptor. Extrapolation of these results to green bacteria suggests that bacteriochlorophyll a and bacteriopheophytin c may act as transient electron acceptors in these organisms.
27 citations
TL;DR: In this article, the Schiff base chelates potentiate the absorption of inorganic chromium by cells of the intestine, kidney and liver, and the relationship of these complexes to the glucose tolerance factor is discussed.
Abstract: The poor gastrointestinal absorption of inorganic chromium has hampered study of the bioavailability, biodistribution and function of this essential trace element. New coordination compounds of Cr(III) — most notably the Schiff base complexes of pyridoxal, dipeptides and nicotinic acid — have been synthesized and characterized for metabolic studies in mice. Data are presented to document the whole-body absorption, excretory pathways and organ distribution of orally administered 51Cr complexes. These compounds are well absorbed but retention is low because of rapid urinary excretion. The liver accumulates more Cr from the pyridoxylidene complexes than from inorganic or other coordination compounds. These results suggest that Schiff base chelates potentiate the absorption of Cr by cells of the intestine, kidney and liver. Coordination by nicotinic acid increased the stability of the Schiff base chelates but did not affect the biodistribution significantly. The relationship of these complexes to the “glucose tolerance factor” is discussed.
TL;DR: In this paper, the optical properties and orientation of reaction center pigments of green photosynthetic bacterium Prosthecochloris aestuarii were investigated and the Qy transitions of the long-wave bacteriochlorophylls were found to be oriented more or less parallel to the membrane.
Abstract: Experiments are reported on the optical properties and orientation of reaction center pigments of the green photosynthetic bacterium Prosthecochloris aestuarii. Analysis of the light-induced absorption difference spectra of P840, the primary electron donor, and of reaction center triplet formation indicates that the bleaching of the P840 band at 836 nm is accompanied by band shifts of bacteriochlorophylls absorbing at 797, 816 and 833 nm at 80 K. The Qy transitions of the long-wave bacteriochlorophylls were found to be oriented more or less parallel to the membrane, the Qx transitions were more or less perpendicularly oriented. Fluorescence emission spectra of bacteriochlorophyll a in intact cells and isolated pigment-protein complexes are presented for the temperature range 295-4 K. Experiments with different wavelengths of excitation indicate that in the PP and RCPP complexes light absorbed by bacteriochlorophyll a produces mainly emission by light-harvesting bacteriochlorophyll, whereas light absorbed by pigments absorbing at 670 nm (bacteriopheophytin c or a possibly related pigment) predominantly excites long-wave fluorescence, which appears to be emitted by pigments more closely associated with the photochemical reaction center. The emission spectra of the light-harvesting bacteriochlorophyll a complex at various temperatures indicated temperature equilibrium between the emitting species. In intact cells, an increase in bacteriochlorophyll c fluorescence upon cooling indicates a decrease in the efficiency of energy transfer between the chlorosome and bacteriochlorophyll a.
TL;DR: It is suggested that the formation of oxygen free-radicals, catalyzed by iron bound to nucleotides, may contribute to cellular alterations involving membranes as well as other biological constituents.
Abstract: Nucleotide pyrophosphate-iron complexes (NPP-Fe), which presumably function in the intracellular transport of iron, may also serve as substrates for cytochrome P450 reductase. The reduction of these complexes via this flavin enzyme leads to the formation of oxygen radicals which have the capacity to initiate peroxidation reactions in phospholipids. It is suggested that the formation of oxygen free-radicals, catalyzed by iron bound to nucleotides, may contribute to cellular alterations involving membranes as well as other biological constituents.
TL;DR: The most widely accepted formulation is that of an he(IV) porphyrin p--cation radical (S = 1/2), which has been reviewed and assessed in this article.
Abstract: The highest oxidation states of catalase and horseradish peroxidase contain two oxidizing equivalents more than the resting ferrihemoproteins. While numerous electronic configurations for these primary complexes have been suggested, the most widely accepted formulation is that of an he(IV) (S = 1) porphyrin p--cation radical (S = 1/2). The evidence for and against these electronic configurations is reviewed and assessed.
TL;DR: In this paper, the effect of amino-substituents on cyclopropenium ion is discussed and the properties of diaminocyclopropenethiones are characterized.
Abstract: Several diaminocyclopropenethiones (1) are prepared by the reaction of diaminochlorocyclopropenium or triaminocyclopropenium salts with NaSH, and physical properties are characterized by IR, UV, PMR and CMR spectra. Reactions of 1 with electrophiles afford various diaminothiocyclopropenium salts. Diaminocyclopropenium salts (10) are prepared via reaction of 1 with oxidizing agents and characterized. The effect of amino-substituents on cyclopropenium ion is discussed.
TL;DR: The synthesis and reactions of silylcyclopropanes are reviewed from a preparative viewpoint, these strained systems can be obtained by the addition of Silyl-substituted carbenes to olefins and of carbeness or carbenoids to vinylsilanes, by condensation of cyclopropyllithiums with chlorosilanes, and by ring opening of sallyl-ssubstitized bicyclobutanes and by trimethylsilylmethylenesulfurane addition to α,β-unsaturated ketones as mentioned in this paper
Abstract: The synthesis and reactions of silylcyclopropanes are reviewed From the preparative viewpoint, these strained systems can be obtained by the addition of silyl-substituted carbenes to olefins and of carbenes or carbenoids to vinylsilanes, by condensation of cyclopropyllithiums with chlorosilanes, by ring opening of silyl-substituted bicyclobutanes and by trimethylsilylmethylenesulfurane addition to α,β-unsaturated ketones The susceptibility of this class of molecules to acid- and base-promoted ring openings, electrophilic substitution and thermal rearrangement is detailed These latter topics and a summary of the use of silylcyclopropanes as cyclopropene precursors are presented in a manner which stresses the unique synthetic utility conveyed by the presence of the silicon atom
TL;DR: The kinetics of interaction of Ca2+ ions with M ConA were followed by using the quenching of fluorescence of 4-methylumbelliferyl-α-D-mannopyranoside when bound to protein to interpret kinetic data in terms of a mechanism in which a species initially formed (constant K1) transforms slowly (k2) to the final form.
Abstract: The kinetics of interaction of Ca2+ ions with M ConA (M = Co2+, Mn2+, Ni2+ and Zn2+) were followed by using the quenching of fluorescence of 4-methylumbelliferyl-α-D-mannopyranoside when bound to protein. The kinetic data can be interpreted in terms of a mechanism in which a species (MCaP)1 initially formed (constant K1) transforms slowly (k2) to the final form. Values of k1, ΔH1 and ΔS1 and (particularly) k2, ΔH2≠ and ΔS2≠ are very similar for all metals, but the ΔH2≠-values are much lower than previously reported by other investigators. The kinetics of binding of 4-methylumbelliferyl-α-D-galactopyranoside to Bandeiraea simplicifolia I lectin were studied by stopped-flow fluorescence methods. The formation rate constant (∼ 105 M−1 s−1) is similar to that of sugars with other lectins. EDTA removes the sugar binding ability of B. simplicifolia I lectin with t1/2 = 24 min (pH = 7.2, 25°). The addition of Ca2+ ions to apolectin to give a final product appears controlled by a conformational change.
TL;DR: The key aggregation steps for C-phycocyanin consist of denatured subunits (α and β polypeptide chains), monomers, trimers, hexamers and stacks.
Abstract: The key aggregation steps for C-phycocyanin consist of denatured subunits (α and β polypeptide chains), monomers, trimers, hexamers and stacks. For allophycocyanin the steps are denatured subunits, monomers and trimers. Ultimately these assembly states lead to the formation of phycobilisomes in blue-green and red algae. The spectroscopic changes that accompany each stage in the assembly can be monitored as a function of the ratio of visible to near-UV absorption. For C-phycocyanin, monomers and hexamers are the important levels in the modulation of tetrapyrrole conformation by protein-chromophore interaction. The effect of protein aggregation on the protein-chromophore interactions is the key to solar-energy harvesting and excitation-energy migration in photosynthesis.
TL;DR: In this article, the plastoquinone analogue, DBMIB, was added to chloroplasts and the appearance of an EPR signal at g = 1.94 was observed at cryogenic temperatures.
Abstract: Addition of the plastoquinone analogue, DBMIB, to chloroplasts results in the appearance of an EPR signal at g = 1.94 which is observed at cryogenic temperatures. Studies with subchloroplast fragments which either contain or lack the Rieske iron-sulfur center indicate the presence of this electron carrier is required for the DBMIB-induced EPR signal. A correlation between the disappearance of the Rieske center signal at g = 1.89 and the appearance of the DBMIB-induced signal at g = 1.94 suggests DBMIB alters the Rieske center to produce the new signal. Similar effects were observed in photosynthetic membrane fragments from a blue-green alga and a photosynthetic bacterium. It has been possible to use the DBMIB-induced signal to test for interaction of other inhibitors in the region of the Rieske center. These results are considered in terms of models in which quinone and the Rieske center function as a donor-acceptor pair during physiological electron transport.
TL;DR: The exploration of reactions in vesicles is started in order to understand the biological mechanism for the production of solids such as bone and to generate new observations about inorganic reactions in small spaces.
Abstract: Many bio-inorganic reactions, especially of magnesium and calcium, take place in small vesicles. Chemistry in such small volumes has not been studied. Here we start an exploration of reactions in vesicles in order to understand the biological mechanism for the production of solids-Such as bone. At the same time this study has generated new observations about inorganic reactions in small spaces. The products of reaction can be designed to provide new probes in electron microscopy and NMR spectroscopy or to carry drugs to locations.
TL;DR: In this article, the authors examined the efficiency of light-induced electron transfer across membranes on a molecular level and showed that a conduction chain of stacked acceptors immersed in a low dielectric membrane cannot be expected to provide an efficient photosynthetic system.
Abstract: The efficiency of light induced electron transfer across membranes is examined on a molecular level. The activation energies that determine this efficiency are evaluated by pure electrostatic considerations in terms of the membrane dielectric and the relaxation of the dielectric at the sites of the electron donors and acceptors. These considerations allow one, for the first time, to assess the efficiency of photosynthetic systems in terms of their molecular components. It is demonstrated that a conduction chain of stacked acceptors immersed in a low dielectric membrane cannot be expected to provide an efficient photosynthetic system. On the other hand, efficient photosynthetic systems can be obtained when the acceptors are placed in polar active sites of proteins and arranged in order of increasing redox potentials.
TL;DR: In this paper, a model for the mutual orientation of the chlorophyll pigments and the consequent energy transfer processes in terms of a Forster mechanism is proposed, which yields a value of 23 A for the Forster R0.
Abstract: The fluorescence and force-area behavior of chlorophyll a (Chl a) monomolecular films diluted with hydrocarbon on an aqueous subphase is reported. It is found that the diluent hexadecane, instead of conventional fatty alcohols, results in a substantial increase in the fluorescence yield of Chl a. The luminescence intensity varies nonmonotonically with respect to the intermolecular distance. A model for the mutual orientation of the chlorophyll pigments and the consequent energy transfer processes in terms of a Forster mechanism is proposed. For a random distribution of the Chl a molecules in the monolayer (k2 = 2/3) an average value of 43° is calculated for the angle between the molecular planes and the water surface. This model yields a value of 23 A for the Forster R0. Also, a value of 58° is derived for the angle between the Q-band transition moment and the line of intersection between the molecular plane and water surface. When a mixture of Chl a and Chl b in the monolayers is being excited at the absorption band of the latter pigment, typical fluorescence of Chl a is being detected. This sensitization process can also be interpreted in terms of the orientational model of the Chl molecules as presented in this work.
TL;DR: In this paper, the near-infrared, visible and near-ultraviolet absorption spectra of the nickel(II) derivatives of stellacyanin and Pseudomonas aeruginosa azurin are reported.
Abstract: The nickel(II) derivative of Rhus vernicifera stellacyanin has been prepared. The near-infrared, visible, and near-ultraviolet absorption spectra of the nickel(II) derivatives of stellacyanin and Pseudomonas aeruginosa azurin are reported. Charge transfer bands in the visible and near-ultraviolet regions are assigned as follows: nickel(II)-stellacyanin, 335 nm (29,900 cnr−1) (πN(his)Ni(II)), 410 nm (24,400 cm−1) (σS(cys) Ni(II)), and 470 nm (21,300 cm−1)(πS(cys) Ni(II)); nickel(II)-azurin, 355 nm (28,200 cm−1) (πN(his)Ni(II)), 440 nm (22,700 cm−1) (σS(cys) Ni(II)), and 500 nm (20,000 cm−1) (πS(cys) Ni(II)). Relatively weak bands at 550 (18,200) and 590 nm (16,900 cm−1) (Ni(II)St) and 540 (18,500) and 565 nm (17,700 cm−1) (Ni(II)Az) are attributed to d–d excitations. No evidence for lower energy d-d bands has been found for either Ni(II)–protein. The data now in hand raise the possibility that the Ni(II) site structure differs from the flattened tetrahedral geometry exhibited by Cu(II) in these proteins.
TL;DR: A brief review of the present knowledge of the mechanism by which higher plants and algae bring about the photooxidation of water is given in this paper, with emphasis placed on the role of manganese in this process and a summary is given of recent efforts from several laboratories to isolate the water splitting enzyme.
Abstract: A brief review is given of the present knowledge of the mechanism by which higher plants and algae bring about the photooxidation of water. Emphasis is placed on the role of manganese in this process and a summary is given of recent efforts from several laboratories to isolate the “water-splitting enzyme”. Experimental evidence is presented that, in contrast to earlier beliefs, the Mn not removed by washing chloroplast thylakoids with high concentrations of Tris (hydroxymethyl) amino methane could be involved in O2 evolution. This manganese is equivalent to about 2 Mn per photosystem two reaction centre. Also presented are details of the isolation of a heme-protein from spinach chloroplasts which may be intimately associated with the water splitting process. This protein has a native molecular weight of approximately 232 k Daltons and has characteristics similar to those of a catalase type enzyme. The possibility of involvement of peroxide formation during the water splitting process is discussed.
TL;DR: In this article, the synthesis of 1,1-dichloro and 1, 1-difluoronaphtho[b]cyclopropenes starting from 4,5-dimethylenecyclohexene and tetrachloro- or 1,2-diclopropene, respectively, is described.
Abstract: The synthesis of 1,1-dichloro and 1,1-difluoronaphtho[b]cyclopropenes 1 and 2 starting from 4,5-dimethylenecyclohexene and tetrachloro- or 1,2-dichloro-3,3- difluorocyclopropene, respectively, is described. Both 1 and 2 ionize in cold fluorosulfonic acid to yield stable aromatic cations 3 and 4. Upon quenching with water, 1-chloronaphtho[b]cyclopropenium ion 3 affords β-naphthoic acid.
TL;DR: In 1-vinyl substituted cyclopropanes hydrogenation with Pd/C leads mainly to simultaneous saturation of the side chain and specific ring opening at the adjacent C(1)-C(2) bond.
Abstract: In 1-vinyl substituted cyclopropanes hydrogenation with Pd/C leads mainly to simultaneous saturation of the side chain and specific ring opening at the adjacent C(1)-C(2) bond. 1,4-Addition of hydrogen is indicated by the detection of olefinic intermediates.
TL;DR: In this article, a number of amino acids and molecules of related functionality have been covalently connected to pyrochlorophyllide a (PChla) and bacteriopyro chlorophyllides a (BPChla), to determine their impact on the properties of the chromophore.
Abstract: A number of amino acids and molecules of related functionality have been covalently connected to pyrochlorophyllide a (PChla) and bacteriopyrochlorophyllide a (BPChla) to determine their impact on the properties of the chromophore. PChla and BPChla esterified with side chains containing the thio-ether functionality (methyl-6-hydroxyhexylsulfide, a Met analogue) were studied in detail by NMR. Intramolecular coordination at the central metal is observed for the Zn-PChla and Zn-BPChla thio-ether derivatives, but not for the Mg-PChla derivative. The NMR data for the Zn-PChla derivative is interpreted as providing evidence for two, distinguishable five-coordinate complexes, with the ligand on either side of the macrocycle. In contrast, the Zn-BPChla derivative appears to be coordinated exclusively on one side. It is proposed that stereoselective coordination may provide a simple mechanism for altering the spectroscopic properties of BChla when it binds to a protein. PChla was covalently attached to the methyl esters of L-Trp, L-Tyr, L-Phe and L-Val. Very large, similar chemical shift changes in the NMR spectra in the absence of a coordinating ligand indicate that all of these compounds form a novel dimer, in which the carbonyl group of the carbomethoxy group on the amino acid coordinates to the central metal, bridging between macrocycles. An average structure is proposed for this dimer in which the macrocycle planes are parallel, overlap in ring IV and have their x-axes antiparallel.
TL;DR: Unsaturated carbenes provide a ready means of entry into numerous, diverse small ring systems and have been employed to prepare strained alkylidenecyclopropanes that open up to the corresponding trimethylenemethanes or in one case m-xylylene as discussed by the authors.
Abstract: Unsaturated carbenes, provide a ready means of entry into numerous, diverse small ring systems. They have been employed to prepare strained alkylidenecyclopropanes that open up to the corresponding trimethylenemethanes or in one case m-xylylene. Triafulvenes, [3]-radialenes as well as [4]-radialenes and cyclynes have been prepared by addition of unsaturated carbenes to appropriate π-systems. They show great potential for the simple preparation of novel small ring heterocycles. Potential formation of allene oxides, allene episulfides, α-lactams and heterocumulene oxides are discussed.
TL;DR: In this paper, a relation for energy transfer at very low temperature is derived, leading to a (r 0R)−3 dependence of the rate of energy transfer on the distance r 0 of closest approach of donor and acceptor pigment and the diameter 2R of the antenna complex.
Abstract: With an optically detected magnetic resonance technique, Microwave Induced Fluorescence difference (MIF) spectra have been recorded at 1.5 K for a number of photosynthetic bacteria under reducing conditions. It is found that the MIF spectra coincide with the longest wavelength fluorescence band. A relation for energy transfer at very low temperature is derived, leading to a (r0R)−3 dependence of the rate of energy transfer on the distance r0 of closest approach of donor and acceptor pigment and the diameter 2R of the antenna complex. It is tentatively concluded that the photochemical trap in the photosynthetic reaction center has a distribution of 0-0 transitions similar to that of the antenna pigments.
TL;DR: In this paper, a review examines some recent developments in the organometallic chemistry of cyclopropanes, epoxides, episulfides and azirines, showing that transition metal organometalloy complexes effect a wide range of useful and interesting cleavage reactions of three-membered ring compounds.
Abstract: Transition metal organometallic complexes effect a wide range of useful and interesting cleavage reactions of three-membered ring compounds. This review examines some recent developments in the organometallic chemistry of cyclopropanes, epoxides, episulfides and azirines. Novel synthetic approaches to heterocyclic [e.g., β γ and δ-lactones, β-lactams, isoxazoles, pyrazoles, indoles] and acyclic compounds [β-diketones, vinyl isocyanates] have been achieved by means of these cleavage reactions.