bluuebunny/crossref_metadata_2025_split huggingface dataset:
Brønsted βNuValues and Leaving Group Effects in SN2 Reactions. Tests of the Reactivity—Selectivity Postulate
Authors: Bordwell Frederick G. , Branca John C. , Cripe Thomas A. 
Published: November 2013

AbstractThe rate constants for reactions of a family of 19 carbanions, ArCHSO2Ph−(derived from benzyl phenyl sulfones) withn‐butyl chloride have been measured in Me2SO solution. A plot of logkvs. the pKaof the conjugate acids for 12 of these carbanions give a linear plot (R2= 0.999) with a Brønsted coefficient of βNu= 0.402. Points for para electron‐withdrawing substituents, SPh, SOPh, SO2Ph. COPh, CN and NO2, deviate substantially from the plot. The deviations are attributed to the enhanced solvation of these remote substituents in the anion which leads to rate retardation. A curved Brønsted plot can be drawn through all the points, which would be consistent with the predictions of the Hammond–Leffler postulate (HLP) and the reactivity–selectivity postulate (RSP), but this interpretation is rejected. Instead, it is suggested that the apparent curvature in Brønsted plots for acid–base reactions – upon which HLP and RSP are based – is also caused by deviations due to solvent effects, donor atom effects in the bases, mechanistic changes and/or the failure to keep electronic and steric effects constant. A reactivity–selectivity plot for reactions of 9 ArCHSO2Ph−ions withn‐butyl bromide andn‐butyl chloride indicated constant selectivity. A similar plot for 4 carbanions derived from α‐methylbenzyl phenyl sulfones reacting withn‐butyl bromide andn‐butyl chloride also showed constant selectivity. Based on these results and an examination of the literature, it is concluded that there is no firm experimental basis for HLP and RSP.
An Examination of the Relationship Between Transition State Geometry in Hydron Transfer Reactions and the Temperature Dependence of the Primary Kinetic Isotope Effect
Authors: Mclennan Duncan J. , Gill Peter M.W. 
Published: November 2013

AbstractSeveral recent papers by Kwart feature hydron transfer reactions in which the primary kinetic isotope effect is temperature independent over wide temperature ranges. It has been asserted (without theoretical justification) that this phenomenon is associated with transition state X….H….Y bond angles which are significantly less than 180°, whereas normal temperature dependence may be associated with transition states in which the angle is close to 180°. This work employs model calculations of isotope effects, principally for [1,5] H‐shifts in 1,3‐pentadiene, to examine Kwarts's hypothesis. No model tested yielded a temperature‐independent isotope effect of substantial magnitude. The transition state (angle = 133°) was located on the MNDO potential energy surface and the isotope effect, calculated by a new and fast computational procedure, was again temperature dependent. Model calculations of secondary hydrogen isotope effects were carried out. When bending motions of the non‐reacting hydrogens were coupled to C….H stretching modes in the force constant matrix, it was found that kH/kD was greater than the equilibrium secondary isotope effect, kH/KD, and that kH/kD for hydrogen transfer exceeded kH/kD for deuteron transfer when simple tunnel corrections were incorporated. A novel interpretation to account for the observation of some temperature‐independent isotope effects is advanced.
The Application of 3‐Amino‐2H‐Azirines as Amino Acid Equivalents
Authors: Heimgartner Heinz 
Published: November 2013

Abstract3‐amino‐2H‐azirines undergo a number of ring opening reactions. The cleavage of the azirine‐N(1), C(3) double bond leads thereby to synthons which can be considered as amino acid equivalents. The reaction of 3‐amino‐2H‐azirines with carboxylic acids yields N‐acyl amino acid amides, which can be converted by a selective amide cleavage to the corresponding N‐acyl amino acids. 2‐Oxazolin‐5‐ones are intermediates of this amide cleavage. This reaction sequence has been used for the extension of peptide chains as well as for a number of heterocycle syntheses. Likewise, the described synthesis of cyclic depsipeptides and lactones by direct amide cyclization proceeds via 2‐oxazolin‐5‐one intermediates. The selective amide cleavage was also applied to a novel method for the resolution of enantiomeric amino acid derivatives.
Reaction of Coumarins with Superoxide Anion Radical (O2): Facile Entry to o‐Coumarinic Acid Systems
Authors: Frimer Aryeh A. , Aljadeff Gladis , Gilinsky‐Sharon Pessia 
Published: November 2013

AbstractCoumarins 1–5 were reacted with O2 in aprotic media. The course of the reaction was followed by using methyl iodide to “Cap” (trap) the oxy‐anions generated. Abstraction of the enolic hydrogen (where available) proved to be the most facile process followed by “saponification” of the lactone linkage. The cis‐stereochemistry of the original coumarin was maintained allowing easy access to a variety of o‐coumarinic acid systems.
Chiral Induction in Originally Racemic Amino Acids via 5‐Acyl and 5‐Acyloxyaminooxazoles
Authors: Lipshutz Bruce H. , Hungate Randall W. , Mccarthy Keith E. 
Published: November 2013

AbstractA variety of chiral 2,4‐disubstituted‐5‐trifluoroacetamido‐,5‐carbobenzyloxyamino, and 5‐β,β,β,‐trichloroethoxycarbonylaminooxazoles have been prepared from diamide, dipeptide, or amide nitrile precursors. Conversion to their (poly)amide equivalents via a deacylation/hydration sequence is described. The extent of induction of chirality realized upon ring opening in each case is determined by HPLC.
Synthesis of Aminopyridazines from Aminocyclopropenylium Salts and Diazomethyl Compounds [1]
Authors: Heydt Heinrich , Eisenbarth Philipp , Feith Karin , Urgast Klaus , Maas Gerhard , Regitz Manfred 
Published: November 2013

AbstractAmino‐substituted cyclopropenylium salts (11a,b) react with diazomethyl compounds (12a‐i) in dichloromethane or acetonitrile in the presence of a base (ethyldiisopropylamine, 1,5‐diazabicyclo[4.3.0] non‐5‐ene) to form 4‐aminopyridazines (13a‐i) specifically. The reaction is interpreted in terms of an initial electrophilic diazoalkane substitution to give diazomethylcyclopropenes (11 + 12 → 14) which undergo a [1.5]‐cyclisation to form the betaines 16 which, in turn, isomerise with opening of the bridging bond to give the aminopyridazines 13. In a similar manner, the tris[amino] cyclopropenylium salts 8a,b and 10a‐c are converted to the 3,4,5‐tris[amino]‐pyridazines (21a‐k) by treatment with diazomethyl compounds (12a, c‐f).
New Approaches to Solvent‐Mediated Molecular Interactions
Authors: Montgomery Pettitt B. , Rossky Peter J. 
Published: November 2013

AbstractRecently developed integral equation methods for the evaluation of intermolecular and intramolecular interactions in a polar solvent medium are discussed and representative applications are presented. Examples include solvent modification of interionic interactions between atomic or molecular ions in water and liquid environment induced shifts in conformational equilibria for both non‐polar and polar flexible molecules in water. Emphasis is placed on the features of these phenomena which can be traced specifically to the molecularity of the solvent and hence cannot be adequately reproduced by a simple continuum dielectric solvent representation.
Homogeneous Catalyzed Generation of Formamides and Amines with Syngas as a Basic Building Block
Authors: Knifton John F. , Alexander David C. 
Published: November 2013

AbstractN‐Alkylformamides, polymeric polyamines, and tertiary aliphatic and araliphatic amines, have been prepared using ruthenium homogeneous catalysis and synthesis gas as a basic building block. N,N‐dimethylformamide, for example, has been generated by a new synthesis route, directly from syngas plus ammonia, using the concept of ruthenium “melt” catalysis. Polymeric polyamine derivatives of 1,2‐polybutadienes have been prepared via the aminomethylation reaction using a two‐phase medium to facilitate product‐catalyst separation.
π‐π Interactions, Architecture, and Bandwidths in “Molecular Metals”. A Synthetic and Electronic Structure Study of Monomeric and Cofacially Joined Dimeric Germanium Phthalocyanines
Authors: Doris K. A. , Ciliberto E. , Fragala I. , Ratner M. A. , Marks T. J. 
Published: November 2013

AbstractSmall molecule spectroscopy on M(Pc)(OR)2 and ROM(Pc)OM(Pc)OR molecular fragments (M = Si, Ge; Pc = phthalocyaninato) of the conductive polymer [M(Pcp+)O]n provides a unique probe of the band structure‐forming π‐π interactions. We describe here the synthesis and characterization of the M = Ge monomer and dimer where R = (t‐Bu)Me2Si‐. The lowest energy He I photoelectron spectroscopic feature of the dimer is a doublet with a splitting of 0.19(3) eV. This splitting can be associated with the cofacial HOMO‐HOMO Pc‐Pc interaction and translates into a tight‐binding bandwidth in the polymer of 0.38(6) eV. This result is in good agreement with an experimental bandwidth of 0.48(5) eV obtained from a Drude analysis of the {[Ge(Pc)0]I1.1}n reflectance spectrum and a bandwidth of 0.50 eV derived from theoretical calculations.
Bisaxially Substituted 2,3‐Naphthalocyaninatoiron Compounds
Authors: Deger Sonja , Hanack Michael 
Published: November 2013

Abstract2,3‐Naphthalocyaninatoiron(II) (2,3‐NcFe) (1) is reacted with L = 1,4‐diisocyanobenzene (dib), tert‐butylisocyanide (t‐buNC), cyclohexylisocyanide (c‐hxNC), benzylisocyanide (bzNC), pyrazine (pyz), 4,4′‐bipyridine (bpy) and pyridine (py) to form the corresponding bis‐axially coordinated 2,3‐naphthalocyaninatoiron(II) compounds, 2,3‐NcFeL2 3, 4, 5, 6, 7, 8, 9. All 2,3‐NcFeL2‐complexes are characterized by IR‐, and if soluble enough by 1H‐NMR‐ and UV/VIS‐spectroscopy, TG/DTA‐measurements and elemental analyses. The dc‐room temperature conductivities of the 2,3‐NcFeL2‐complexes are measured and found to be several orders of magnitude higher than the corresponding phthalocyaninato‐iron‐complexes PcFeL2.
Artificial Allosteric System 5. Allosteric Gain, Total Cooperativity and Local Cooperativity as Appropriate Indices for Allosteric Effect
Authors: Tabushi Iwao , Nishiya Takako 
Published: November 2013

AbstractMagnitude of cooperativity of a hypothetical m‐subunit allosteric system of hemoglobin type is discussed, based on computation of the amount of O2 to be transferred to the corresponding monomeric nonallosteric system of myoglobin type. For quantitative discussion, “allosteric gain” is defined as the difference between the amount of O2 to be transferred from the allosteric to the nonallosteric system and that from the hypothetical system of average O2 affinity to the nonallosteric system. These two quantities, allosteric gain and amount of O2 transfer, are quite sensitive to equilibrium constant ratio, especially Km/K1, (Ki+1/Ki)max, and i. Hill's coefficient appropriately describes neither the amount of O2 transferred nor allosteric gain. Allosteric gain or the amount of O2 transfer which is best characterized by the total cooperativity index (Km/K1) and the local cooperativity indices (Ki+1/Ki) seem to be appropriate measures of the magnitude of cooperativity.
A Thermal Expansion Model for the Special Pair of the Bacterial Reaction Center
Authors: Won Youngdo , Friesner Richard A. 
Published: November 2013

AbstractThe temperature dependence of the longwavelength absorption band of the Rhodopseudomonas viridis reaction center is investigated with a thermal expansion model of the special pair. The validity of the model is demonstrated through the reproduction of optical lineshapes measured at various temperatures and through the consistency of the energetic parameters obtained in this fashion. The connection with the temperature dependence of the primary electron transfer process is briefly discussed.
Directed Mutations Affecting the Putative Bacteriochlorophyll‐Binding Sites in the Light‐Harvesting I Antenna of Rhodobacter capsulatus
Authors: Bylina Edward J. , Robles Steven J. , Youvan Douglas C. 
Published: November 2013

AbstractOligonucleotide‐mediated mutagenesis of the light‐harvesting I (LHI; B880 complex) antenna genes from Rhodobacter capsulatus has been used to study the bacteriochlorophyll‐peptide interactions in this photosynthetic membrane complex. A system of deletion strains and complementing plasmids has been used to assay the effects of changing single amino acid residues in the a structural polypeptide of LHI. Fourteen mutations have been introduced in the putative bacteriochlorophyll‐binding site: Ala‐X‐X‐X‐His. Residues with side chains smaller than valine can substitute for alanine, but the LHI complex is lost from the membrane if alanine is replaced by residues larger than valine, or if histidine is replaced by arginine, asparagine, aspartic acid, glutamine, proline, or threonine. The block in biogenesis is not transcriptional, since the associated reaction center, whose genes are known to be downstream from the LHI genes, continues to be inserted into the membrane. The membrane‐bound reaction center is photochemically active in these LHI‐depleted strains. Such strains may be useful for directly observing electron transfer properties of the reaction center in the membrane. This system is a model for studying the assembly of integral membrane proteins, since easily assigned optical transitions of the prosthetic groups signal proper assembly.
A Comparison of the Manganese Center Responsible for Photosynthetic Water Oxidation in O2‐Evolving Core Particles and Photosystem II Enriched Membranes: EPR of the S2 State
Authors: Sivaraia M. , Charles Dismukes G. 
Published: November 2013

AbstractWe have characterized the electron donors to Photosystem II (PS II) in an O2‐evolving reaction center core preparation from spinach (Ghanotakis, D.F.; Yocum, C.F. FEBS Lett., 1986, 197: 244–248) using EPR spectroscopy of the manganese center involved in water oxidation and of the tyrosine donor responsible for signal II. Both the 16‐line and the 19‐line form of the S2 multiline EPR signal can be observed in the core particles under conditions similar to those which produce these signals in PS II membranes. Consequently, the structure of the coupled cluster of three or four Mn ions remains intact in the core particles. The Kok parameters which characterize the number of reaction centers capable of advancing by 0, 1, or 2 equivalents in response to a short laser pulse are found to be the same in PS II membranes and in the core particles. Also, there is no evidence for partially inactivated centers which reach the S2 or S3 states, but which do not advance to O2 release. Thus the new class of highly resolved O2‐evolving particles appears to be well suited for biophysical studies directed at the mechanism of water oxidation.
Magnetic Resonance and Molecular Orbital Studies of the Primary Donor Cation Radical P+.960 in the Photosynthetic Bacterium Rhodopseudomonas viridis
Authors: Plato Martin , Lubitz Wolfgang , Lendzian Friedhelm , Möbius Klaus 
Published: November 2013

AbstractThe light‐induced radical cation of the primary electron donor, P+.960, in photosynthetic reaction centers (RCs) from Rhodopseudomonas viridis has been investigated by various magnetic resonance techniques (ESR, ENDOR, TRIPLE). Comparison of the measured hyperfine couplings with those of the cation radical of monomeric bacteriochlorophyll b (BChl b) and with RHF‐INDO/SP molecular orbital calculations, performed on P+.960 using coordinates of an X‐ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P960. The derived spin density ratio is 2.8 in favor of the dimer half bound to the L protein subunit. The possible relevance of this result for the primary electron transfer step in the RC is briefly discussed.
Solvent Polarity Dependent Photophysics of a Fixed‐Distance, Symmetric Chlorophyll Dimer. A Model of the Special Pair in Photosynthetic Reaction Centers
Authors: Johnson Douglas G. , Svec Walter A. , Wasielewski Michael R. 
Published: November 2013

AbstractThe synthesis and properties of a fixed‐distance symmetric bis‐pyrochlorophyllide‐a molecule are described. The molecule, 1, consists of two methyl pyrochlorophyllide‐a (MePChlide‐a) moieties which share a common vinyl group at the 2‐position of each macrocycle. The two chlorophylls are trans to one another across the vinyl linkage. The dihedral angle between the plane of each macrocycle and the vinyl linkage is about 50°. The Qy absorption band of this dimer occurs at 689 nm and is essentially independent of solvent. The band is red‐shifted by about 27 run relative to that of MePChlide‐a. The fluorescence maximum of 1 occurs at 722 nm. This is a red‐shift of 49 nm relative to methyl MePChlide‐a. The circular dichroism spectrum of 1 shows strong, nonconservative bands in the red region. The fluorescence quantum yield, fluorescence lifetimes, and picosecond transient absorbance kinetics all exhibit a very strong dependence on solvent polarity. The fluorescence quantum yields of 1 in toluene, butyronitrile, and N, N‐dimethyl formamide (DMF) are 0.18, 0.008, and 0.002, respectively. The bleach of the 689 nm band (following excitation with a 1.5‐ps, 610‐nm laser pulse) recovers with single exponential time constants of 2.51 ns, 100 ps, and 64 ps in toluene, butyronitrile, and DMF, respectively. Possible mechanisms for the polarity‐dependent increase of the nonradiative decay rate of 1 are considered.
Electron‐Nuclear Multiple Resonance on Stable and Transient Radicals
Authors: Möbius Klaus , Plato Martin , Lubitz Wolfgang , Lendzian Friedhelm 
Published: November 2013

AbstractThe physical principles underlying steady‐state electron‐nuclear double resonance (ENDOR) and electron‐nuclear‐nuclear triple resonance (TRIPLE) experiments on radicals in solution are discussed. The gain in spectral resolution achievable by these techniques, as compared with standard ESR, allows hyperfine coupling constants to be measured even for large and low‐symmetry radicals. ENDOR/TRIPLE is applied to cation radicals related to bacterial photosynthesis. The measured hyperfine couplings are interpreted by means of the all‐valence electron MO method RHF‐INDO/SP. Information about the spatial structure of chromophores in the reaction centers of photosynthetic bacteria, Rb. sphaeroides and Rps. viridis, is obtained from an analysis of the spin density distribution and from minimization of the total energy. The results are compared with recently available structural data of the primary donor obtained from X‐ray diffraction experiments. The spin density distribution in the primary donor cation radical P+960, which consists of two bacteriochlorophyll b molecules, is discussed in terms of symmetry and perturbations from the protein environment.
Two‐Dimensional Deuterium NMR Exchange Spectroscopy in Liquid Crystalline Solutions
Authors: Boeffel C. , Luz Z. , Poupko R. , Vega A.J. 
Published: November 2013

AbstractA theoretical analysis is given of deuterium 2D exchange spectroscopy using Zeeman‐ and quadrupole‐order experiments in liquid crystalline solutions. For the analysis, a phenomenological vector model for spin I = 1 nuclei under the effect of the quadrupole Hamiltonian and rf pulses is employed, which makes it easy to follow the evolution of the various coherences during the pulse sequences. In both experiments several types of cross‐peaks are obtained; however, only part of them are relevant to the exchange process. It is shown that proper combination of the signals from the Zeeman‐ and quadrupole‐order experiments can lead to 2D exchange spectra with “pure” exchange cross‐peaks as the only off‐diagonal signals. These exchange cross‐peaks can provide estimates for the exchange rates and indicate the relative signs of the quadrupole interactions in the exchanging sites. The method is applied to two compounds which undergo ring inversion, i.e., cyclohex‐ ane‐d12 and 1,4‐dioxane‐d8, dissolved in liquid crystalline solvents. In the Appendices procedures for phase cycling are discussed, and peak intensities for a general two‐site 2D exchange spectrum are derived.
Strategies for Obtaining Biochemical and Clinically Diagnostic Information from Human Subjects Using 31P Magnetic Resonance Spectroscopy
Authors: Styles Peter , Radda George K. 
Published: November 2013

AbstractThe examination of human subjects by 31P magnetic spectroscopy with a view to obtaining biochemical or diagnostic information requires that some well defined strategy be employed, both in performing the investigation and interpreting the results. The various approaches that can be utilised are reviewed in the light of experience gained in the Oxford laboratory. It is shown how resting metabolite concentration may be interpreted in terms of known enzymatic and physiological parameters, and how stress tests can be used to offer additional insight into metabolic response and control. Illustrations of the applications of these techniques include studies of human muscle, heart, liver and brain.
Hyperfine and Structural Isotope Effects in Muonated Cyclohexadienyl and Cyclopentyl Radicals
Authors: Roduner Emil , Reid Ivan D. 
Published: November 2013

AbstractIncorporation of muonium, the ultra‐light isotope of hydrogen with a mass of only one‐ninth the mass of H, leads to pronounced isotope effects in hyperfine coupling constants of radicals. They reflect the mass dependence of the internal dynamics. A detailed analysis is presented for the cyclohexadienyl and the cyclopentyl radicals. It is concluded that the isotope dependent bond length in an anharmonic potential accounts quantitatively for the observed effects in rigid species.
An ENDOR Study of Nitrosyl Myoglobin Single Crystals
Authors: Kappl R. , Hüttermann J. 
Published: November 2013

Abstract14N, 15N and 1H ENDOR spectra were taken from single crystals of 15NO ligated myoglobin at about 5 K in order to probe the spatial arrangement of the protons and nitrogens from the environment interacting with the Fe‐N‐O complex and to analyze its bonding configuration. The 14N hyperfine (20.4 MHz, 15.2 MHz and 16.8 MHz) and quadrupolar tensors (+1.67 MHz, −0.86 MHz, −0.98 MHz) of NE2 of the proximal histidine expand the previously reported picture from ESR analysis showing a distorted NO bond configuration. 15N data of the N(NO) ligand indicate, however, an orientation of the NO ligand differing from previous ESR reports. The single crystal spectra are applied to an interpretation of the more complex powder‐type spectra. Nine different 1H interaction tensors have been analyzed and assigned on the basis of theoretical simulation of the single crystal gamut of line positions. For the distal NE2 bound histidine proton, a small position distribution, combined with a strong anisotropy of interaction, is shown to broaden the ENDOR patterns beyond detection. The consequences of this finding with respect to the tetrameric molecule hemoglobin are discussed.
ENDOR Study of Biradicals Randomly Oriented in a Rigid Matrix
Authors: van Willigen Hans , Kirste Burkhard 
Published: November 2013

AbstractAn ENDOR study has been made of the structure of ketyl dimers with triplet ground state randomly oriented in a rigid matrix. The measurements give signs and magnitudes of hyperfine coupling constants and also give information on 23Na quadrupole couplings. The ENDOR results can be accounted for using the structural model previously proposed for these dimers.