bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photoinitiated Electron Transfer in Carotenoporphyrin‐Quinone Triads: Enhanced Quantum Yields via Control of Reaction Exergonicity
Authors: Moore Thomas A. , Gust Devens , Hatlevig Susan , Moore Ana L. , Makings Lewis R. , Pesski Peter J. , De Schryver F.C. , van Der Auweraer M. , Lexa Doris , Bensasson René V. , Rougée Michel 
Published: November 2013

AbstractCarotenoporphyrin‐quinone triad molecules have previously been found to successfully mimic many aspects of photosynthetic electron and energy transfer. These molecules generate long‐lived, energetic charge‐separated states via a biomimetic multistep electron transfer scheme. In many of these molecules, the overall quantum yield for charge separation has been limited by an unfavorable partitioning of an intermediate charge‐separated state between charge recombination and further reaction to yield the desired species. One strategy for overcoming this limitation is based on the fact that, in general, electron transfer rate constants do not depend linearly on reaction free energy change. In the triads, raising the energy of the intermediate charge‐separated state would be expected to speed up the desired electron transfer (“normal” Marcus behavior), but slow down charge recombination (which formally lies in the “inverted” region). Laser flash photolysis and transient fluorescence measurements on two new triad molecules reveal enhanced quantum yields of long‐lived charge‐separated states which are consistent with this expectation.
Formation Dynamics, Decay Kinetics, and Singlet–Triplet Splitting of the (Bacteriochlorophyll Dimer)+ (Bacteriopheophytin)− Radical Pair in Bacterial Photosynthesis
Authors: Bixon M. , Michel‐Beyerle M.E. , Jortner Joshua 
Published: November 2013

AbstractWe explore the interrelationship between (i) the kinetic data for the primary electron transfer (ET) from the excited singlet state (1P*) of the bacteriochlorophyll dimer (P) to bacteriopheophytin (H) in the reaction centers (RCs) of purple photosynthetic bacteria and (ii) the energetics and dynamics of the primary radical pair, P+H, i.e., its singlet‐triplet splitting rate, J, and triplet recombination rate, kT. We present a critical scrutiny of the ET mechanisms with regard to the role of the accessory bacteriochlorophyll (B). On the basis of the very weak temperature dependence of J, which sets a lower limit to the energy of (hypothetic) transient short‐lived ionic intermediate states, we argue against the applicability of sequential ET mechanisms involving P+B− H or P+BH−. The nonadiabatic/adiabatic ET mechanism is fraught with difficulties pertaining to a very low medium reorganization energy for the nonadiabatic crossing and an exceedingly high rate of the adiabatic crossing, together with a predicted order‐of‐magnitude enhancement of the low‐temperature (77 K) fluorescence quantum yield in high electric fields (5–10 mV/Å) which seem not to be borne out by experiment We propose that primary ET occurs via the unistep superexchange 1P*BH r̊ P+H−, which is mediated by electronic coupling with P+B−H. The analysis of the competition between superexchange and thermally activated ET resulted in large values of the intermolecular electronic couplings at the equilibrium nuclear configuration of 1P*BH. We suggest that the structural relaxation of one of the prosthetic groups (H) accompanying the primary ET results in the decrease of the intermolecular electronic coupling at the equilibrium configuration of P+BH−. It was established that the superexchange‐predicted kinetic data are consistent with the magnetic data for the radical pair. The small value of J for P+BH− originates from the cumulative effect of: (i) an essential, model‐independent cancellation between the triplet contribution, which is related to kT by a self‐consistency relationship, and the singlet contribution; (ii) the effect of configurational relaxation on the singlet contribution. Our analysis supports the superexchange mechanism for primary ET in bacterial photosynthetic RCs.
The Cytochrome‐c Binding Surface of Reaction Centers from Rhodobacter sphaeroides
Authors: Tiede David M. , Chang Chong‐Hwan 
Published: November 2013

AbstractIn this paper, we describe the water‐exposed, periplasmic surface of the bacterial photosynthetic reaction center from Rhodobacter sphaeroides R‐26. This surface contains binding sites for c‐type cytochromes. The approximate two‐fold structural symmetry is evident in the folding of the L and M subunits of the reaction center on this surface. In spite of this, the distribution of charged residues shows a marked deviation from symmetry. One group of charged residues is distributed symmetrically. For this group, equivalently charged residues are located at symmetry‐related positions on both L and M subunits. However, charged residues are also found at positions which do not follow the two‐fold symmetry. Seven charged residues appear on the L and M subunits at positions which do not have a charged residue at the corresponding position on the other subunit. These symmetry‐breaking charged residues appear in a single region, or “patch”, on the surface of the reaction center. We propose that this patch may function as a site for cytochrome c docking. A model of the Rb. sphaeroides cytochrome c2‐reaction center electron transfer complex has been built. Preliminary energy calculations show that this configuration is at least a local energy minimum.
Quantitative Analysis of Magnetic Resonance Signals in the Time Domain
Authors: De Beer R. , van Ormondt D. , Pijnappel W.W.F. , van der Veen J.W.C. 
Published: November 2013

AbstractThis paper is concerned with quantification of magnetic resonance signals in terms of physically significant parameters such as amplitudes, frequencies, damping factors (line widths) and phases. The approach considered here is to fit an appropriate model function, in the same domain as that in which the data are collected. Important advantages of this approach are 1) absence or rejection of data points need not impair the quantification and 2) non‐uniform sample distributions aimed at resolution enhancement can be accommodated in a natural fashion. Aspects given attention are singular value decomposition (SVD) of the data matrix, SNR enhancement, linear prediction (LPSVD), state space formalism (HSVD), variable projection method (VARPRO) in various forms, exploitation of prior knowledge, precision of the fitted parameters, experimental design and 2‐D time domain model function fitting.
Rotary Resonance Recoupling in Heteronuclear Spin Pair Systems
Authors: Levitt Malcolm H. , Oas Terrence G. , Griffin Robert G. 
Published: November 2013

AbstractIn solid state magic angle spinning experiments on a polycrystalline sample containing dilute heteronuclear spin pairs, the usual spectrum is insensitive to small heteronuclear couplings between the spins, and to the relative orientations of the various interaction tensors. Applying a radio frequency of an intensity chosen to satisfy a rotary resonance condition can restore the heteronuclear splittings in the spectrum. Theory and numerical simulations of this effect are presented. The sensitivity of the spectra to the intensity of the radio frequency field and to the relative orientations of tensors is explored. To fully explain the experimental results, it is necessary to postulate a small phase modulation induced by the motion of the sample through a radio frequency field of non‐uniform direction.
Applications of Two‐Dimensional 1H NMR Methods to Photo‐Chemically Induced Dynamic Nuclear Polarisation Spectroscopy
Authors: Stob Sijtze , Scheek Ruud M. , Boelens Rolf , Dukstra Klaas , Kaptein Robert 
Published: November 2013

AbstractPhoto‐CIDNP is a useful technique to identify surface residues in proteins and to assign resonances in their 1H NMR spectra by the spectral simplification that is obtained. In spite of this simplification, conventional photo‐CIDNP spectra can be quite complex due to spectral overlap. For this reason two‐dimensional (2D) methods are welcome. We present combinations of photo‐CIDNP with 2D J‐correlated spectroscopy (CIDNP‐COSY) and with 2D NOE spectroscopy (CIDNP‐NOESY). In these experiments we insert a saturation pulse sequence and a short laser‐irradiation period at the beginning of the preparation period to generate CIDNP, leaving the remaining of the pulse scheme essentially unaltered. The CIDNP‐COSY and CIDNP‐NOESY spectra of a hen egg‐white (HEW) lysozyme are presented. The CIDNP‐NOESY spectrum of lysozyme is compared with the results from 1D cross‐polarisation experiments on the same protein. CIDNP‐NOESY spectra of lysozyme with and without inhibitor NAG are compared.
Enhancement of Contrast in Magnetic Resonance Images by Paramagnetic Agents: Possibilities and Problems
Authors: Koenig Seymour H. 
Published: November 2013

AbstractDespite the beauty and excellent spatial resolution of magnetic resonance images of native tissue, the improvement in contrast achievable by the introduction of exogenous paramagnetic agents often more than offsets the toxicity risks, and several such agents are currently in clinical trials. The efficacy of any agent relates to the changes it induces in tissue relaxation rates per unit concentration of agent (its relaxivity), which in turn — as is well known—depends both on magnetic field strength and on the physical and chemical environment of the agent. The latter depends on in vivo biochemistry, and may vary from one tissue to another. In addition, current imaging (including animal studies) is done in the range 0.2–200 MHz proton Larmor frequency and, at the highest fields, water and lipid protons can be distinguished and imaged separately. Thus there is much to consider, several classes of problems and related paramagnetic agents exist: (1) Agents that remain in the intact circulation until excreted through the kidney. These include small chelate complexes of Fe3+ and Gd3+ as well as conjugates, both covalent and hydrophobic, with plasma proteins, generally albumin or immunoglobulins. (2) Agents that are scavenged by the liver, ultimately to be excreted, via the bile and gall bladder, into the intestine. There is a search now for safe agents of this class, which is complicated by liver biochemistry that can release the metal ions and increase toxicity. (3) It has been discovered that some non S‐state ions, usually regarded as shifters rather than relaxers, can have high relaxivities at high fields, in part because of directed orbitals. Mn3+‐porphyrin is an example. (4) Aggregates and particulates of magnetic material are becoming popular, in part because they influence 1/T2 preferentially at imaging fields, and in part because their fate in vivo is rather benign. (5) Lipophilic agents, such as nitroxide free radicals, may enhance the relaxation rates of adipose tissue preferentially. It has recently been shown that neat triolein is a good model for such tissue, providing a sizable area for new investigations. (6) Nuclei other than protons may be important, perhaps the most interesting being fluorine because of its presence in synthetic blood substitutes. We present an overview of our recent collaborative work in these many areas, results now under the rubric of “relaxometry”. This work is made possible because of continued advances in the specialized instrumentation (“field‐cycling relaxometers”) required for such investigations.
Cation Radicals of Covalently Linked Porphyrin Dimers and their Monomeric Constituents: An ESR and ENDOR Study
Authors: Huber Martina , Galili Tamar , Möbius Klaus , Levanon Haim 
Published: November 2013

AbstractThe spin density distributions in the cation radicals of various covalently linked porphyrin dimers have been studied by liquid phase ESR and ENDOR methods to find out whether these systems show intramolecular electron delocalization. Such a delocalization is known to occur in the bacteriochlorophyll dimer (“special pair”) in the photosynthetic reaction center. The dimers that were studied in this work were derived from zinc mesotetratolylporphyrin (ZnTTP) and linked at the ortho or para positions of one phenyl ring with varying bridge lengths. 1H and I4N hyperfine coupling constants could be measured for the dimer cation radicals and compared with those of monomeric ZnTTP as well as ZnTTP derivatives that carry alkoxy or hydroxy substituents to mimic the bridges of the dimers. By comparing the hyperfine data of the monomer and dimer cation radicals it is concluded that in the present dimers the unpaired electron is localized on one porphyrin unit.
Recent ENDOR and Pulsed Electron Paramagnetic Resonance Studies of Cytochrome c Peroxidase ‐ Compound I and Its Site‐Directed Mutants
Authors: Scholes Charles P. , Liu Yajun , Fishel Laurence A. , Farnum Martin F. , Mauro J. Matthew , Kraut Joseph 
Published: November 2013

AbstractEPR, ENDOR and pulsed EPR were applied to the radical signals of yeast cytochrome c peroxidase‐compound I from this protein expressed in E. coli and two variants prepared after site‐directed mutagenesis. The compound I complexes studied were first, that of the parent peroxidase expressed in E. coli, and next, those of the following amino acid mutants as specific probes for the radical in compound I: 1. Tryptophan‐51 → Phenylalanine; 2. Tryptophan‐191 → Phenylalanine. Tryptophan‐51 lies near the heme on the distal (H2O2‐binding) side of cytochrome c peroxidase, while Tryptophan‐191 lies in contact with the proximal heme ligand, Histidine‐175. EPR above 20 K and saturation recovery T1 measurements at liquid He temperatures showed the unusual temperature‐dependent relaxation of compound I's axially symmetric signal (g__ = 2.035, g⟂ = 2.00), which has comprised the major radical species in bakers' yeast wildtype compound I and here in all but the Tryptophan‐191 → Phenylalanine mutant. The ENDOR of the major radical species was characterized by the substantial hyperfine couplings previously proposed to be at a sulfur‐based radical. The mutation Tryptophan‐51 → Phenylalanine does not perturb these ENDOR features. Pulse field‐sweep EPR resolved for the first time distinct proton hyperfine couplings due to the features observed by ENDOR. A narrow, more typical organic free radical signal at g = 2.001 with resolved hyperfine structure was the only species observed from the Tryptophan‐191 → Phenylalanine mutant and was a minor feature in all samples. The Tryptophan‐191 → Phenylalanine mutation does perturb or destabilize the majority radical species of compound I. Thus, Tryptophan‐191 is either intimately involved with the kinetics of radical formation or participates intimately in the majority radical site, despite not being a sulfur‐containing amino acid.
Propellanes 100. The Nature of the Stereoelectronic Effect in the Highly Stereospecific Hydroxyl Elimination from syn, anti‐[4.3.3]Propellane‐8,11‐diol upon Isobutane Chemical Ionization
Authors: Ashkenazi P. , Domon B. , Gutman A.L. , Mandelbaum A. , Müller D. , Richter W.J. , Ginsburg D. 
Published: November 2013

AbstractA stereospecific 18O‐labelling study shows that the syn‐hydroxy group is preferentially lost from the syn, anti‐title diol upon i‐C4H10 chemical ionization, indicating occurrence of a stereoelectronic effect in this process. A collision‐induced dissociation study of deuterium‐ and 18O‐labelled analogs shows that a symmetrical ether structure is not formed in the above process by an SNi mechanism. This result leads to the conclusion that the stereoelectronic assistance of the anti‐OH group in the loss of the syn‐hydroxyl is expressed either in a weak transition state interaction or by other processes such as β‐elimination via hydrogen transfer with concomitant double bond formation.
Chiral Synthesis via Organoboranes 23. Enantioselective Ring Opening of meso‐Epoxides with B‐Halodiisopinocampheylboranes. The First General Synthesis of Optically Active 1,2‐Halohydrins
Authors: Srebnik M. , Joshi N.N. , Brown Herbert C. 
Published: November 2013

AbstractB‐Halodiisopinocampheylboranes, Ipc2BX, where X is Cl, Br, or I, cleave cyclohexene oxide at low temperatures to give the 1,2‐halohydrins, in 22, 84, and 91% enantiomeric excess (ee), respectively. 1,2‐Epoxycyclohex‐4‐ene is converted to the bromohydrin and iodohydrin in 84 and 91% ee. cis‐2,3‐Epoxybutane, cis‐3,4‐epoxyhexane, and cyclopentene oxide were also studied with all three halides. In general, optical induction increases in the order I > Br > Cl for any given epoxide. Thus cis‐2,3‐epoxybutane furnishes the corresponding chlorohydrin in 35% ee, the bromohydrin in 69% ee, and the iodohydrin in 78% ee. In certain cases recrystallization provides essentially optically pure material, e.g., (1R, 2R)‐2‐bromocyclohexanol, (1R, 2R)‐2‐iodocyclohexanol and (1R, 2R)‐2‐iodocyclohex‐4‐en‐1‐ol. In all cases examined dIpc2BX (derived from (+)‐α‐pinene) provide (1R, 2R) halohydrins, in which the enantiotopic S C—O bond is cleaved. Ring cleavage occurs in an anti‐periplanar manner, consistent with an SN2 type reaction pathway. Modified B‐chloromonoisopinocampheylboranes, IpcBCl(OR), cleave cyclohexene oxide at a slower rate but furnish the chlorohydrin in up to 35% ee (R = Bnz). This study is not only a novel application of chiral haloboranes, but constitutes the first general synthesis of symmetrical optically active 1,2‐halohydrins.
[4.4.1]Propellatetraene and Related Propellanes
Authors: Frauenrath Herbert , Kapon Moshe , Rubin Mordecai B. 
Published: November 2013

AbstractReaction of dichlorovinylene carbonate with 1,6‐methano[10]annulene (2) followed by hydrolysis of the intermediate adduct afforded α‐diketone 3. Photolysis of 3 at liquid nitrogen temperature yielded the title compound, 1. The activation parameters for the thermal isomerization 1 → 2 were ΔH+ = 5.1 kcal/mole and ΔS−= −19 eu (100 K). The structure of 3 was established by X‐ray analysis. This analysis and MM2 calculations on a number of related compounds provide additional examples of the so‐called “Klammer” effect in propellanes.
The Stereochemistry of the Cope Rearrangement: Qualitative Theory (OCAMS) and Computation (AM1)
Authors: Halevi E. Amitai , Rom Rami 
Published: November 2013

AbstractContrary to common preconceptions, orbital correlation diagrams can be useful for deducing the mechanism and stereochemistry of degenerate sigmatropic rearrangements. The correlation lines have to be drawn between the reactant and the postulated transition state, rather than between the homomeric reactant and product. In the case of the Cope rearrangment, this is done most economically with Orbital Correspondence Analysis in Maximum Symmetry (OCAMS), by drawing correspondence lines between molecular orbitals of the reactant and those of a formal superposition (STS) of alternative transition structures (TS) in a symmetry point group that includes those of both as subgroups. The alternative boat (C2v) and chair (C2h) transition structures postulated for the degenerate Cope rearrangement of 1,5‐hexadiene are formally combined to an STS of D2h symmetry. OCAMS then selects its C2h component, indicating that the pathway via the chair‐like transition state is preferred, in agreement with experimental and computational results for both the Cope rearrangement and the isoelectronic Claisen rearrangement. Rearrangement through the less favored boat pathway is shown to arise from a changed electronic configuration in the transition state. Computational evidence is presented showing that the unexpectedly facile rearrangement of polycyclic molecules like semibullvalene and barbaralane, which are constrained to react via the boat pathway, is primarily due to steric and stereoelectronic factors that are explicable in molecular orbital terms, rather than to the incursion of a biradicaloid transition state.
Rotational Distributions in the Photodetachment of IHI− and in the I + HI Reaction: The Influence of IHI Transition State Resonances
Authors: Schatz George C. 
Published: November 2013

AbstractFranck‐Condon factors which determine IHI− photodetachment spectra have been calculated using a three‐dimensional coupled‐channel reactive scattering program. These Franck‐Condon factors show peaks as a function of energy that are in reasonable agreement with measured spectra due to Neumark and co‐workers. Some of these peaks are due to IHI transition state resonances and others are due to direct scattering reactive threshold effects. An analysis of the rotational distributions associated with the Franck‐Condon factors indicates that when direct scattering is dominant, the distributions peak at the rotational state which is nearest to its effective reaction threshold. At energies where IHI transition state resonances are important, the rotational distribution changes, thus providing a characteristic signature of resonance formation. The I + HI bimolecular rotational distributions are also considered, and they also show important differences between direct and resonant energies.
Quantitative Determination of HD Internal State Distributions via (2+1) REMPI
Authors: Rinnen Klaus‐Dieter , Kliner Dahv A.V. , Zare Richard N. , Huo Winifred M. 
Published: November 2013

AbstractThe relationship between quantum state populations and ion signals in (2+1) resonance‐enhanced multiphoton ionization (REMPI) detection of HD is investigated both experimentally, by measurements of hot‐nozzle population distributions, and theoretically, by calculations of the two‐photon E,F 1Σg+—X1Σg+ transition moments. The experimental and theoretical relative rotational cross‐sections are in good agreement for v″ = 0 and v″ = 1, but differ for v″ = 2 and for the relative vibrational cross‐sections. Arguments are presented that saturation of the E,F‐X transition is the main cause of these discrepancies. In spite of the presence of saturation, this calibration gives confidence in determining the relative internal state populations of HD from its (2+1) REMPI spectrum.
Photodissociation of Vinyl Bromide and the Heat of Formation of the Vinyl Radical
Authors: Wodtke Alec M. , Hintsa Eric J. , Somorjai John , Lee Yuan T. 
Published: November 2013

AbstractWe have performed measurements of the translational energy distributions and anisotropy parameters of the photodissociation products of vinyl bromide at 193 nm. Br‐atom and HBr elimination were observed with a branching ratio of 1.28 ± 0.05 between the two channels. Both processes occurred with a large release of translational energy, an average of about 2.0 eV for the Br‐atom channel and somewhat less for HBr elimination. The maximum release of translational energy for Br + C2H3 formation led to an upper limit of 77 ± 3 kcal/mol to D0(C2H3‐Br), from which an upper limit to the heat of formation of the vinyl radical, 71 ± 3 kcal/mol, was derived. This result was used to reexamine crossed molecular beams data relevant to the determination of the heat of formation of the vinyl radical. A metastable state of C2H3 observed in the photodissociation of vinyl bromide was interpreted as the formation of C2H3(Ã2A″).