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Showing papers on "Intramolecular force published in 1995"


Journal ArticleDOI
TL;DR: A new approach was used to provide the initial structural data on an amyloid fibril—comprising a peptide from the C-terminus of the β-amyloid protein—based on measurement of intramolecular 13C–13C distances and 13C chemical shifts by solid-state 13C NMR and individual amide absorption frequencies by isotope-edited infrared spectroscopy.
Abstract: Amyloids are a class of noncrystalline, yet ordered, protein aggregates. A new approach was used to provide the initial structural data on an amyloid fibril—comprising a peptide (β34–42) from the C-terminus of the β-amyloid protein—based on measurement of intramolecular 13C–13C distances and 13C chemical shifts by solid-state 13C NMR and individual amide absorption frequencies by isotope-edited infrared spectroscopy. Intermolecular orientation and alignment within the amyloid sheet was determined by fitting models to observed intermolecular 13C–13C couplings. Although the structural model we present is defined to relatively low resolution, it nevertheless shows a pleated antiparallel β-sheet characterized by a specific intermolecular alignment.

402 citations


Journal ArticleDOI
TL;DR: A photoinduced intramolecular electron transfer process that causes quenching of the fluorescence of the adduct and strong decrease triplet population in polar solvents is shown in this paper.
Abstract: A covalently functionalized fullerene comprising an electron donating aniline group coupled to the fullerene unit by a saturated heterocyclic bridge is shown to undergo a photoinduced intramolecular electron transfer process that causes quenching of the fluorescence of the adduct and strong decrease triplet population in polar solvents. VIS-absorption, fluorescence and phosphorescence at 77 K, triplet-triplet absorption, time resolved fluorescence and redox potentials of the fullerene adduct are presented. Analysis of the solvent dependence of the energetics of the intramolecular electron transfer is given and is in good agreement with the experimental results. 17 refs., 6 figs., 3 tabs.

346 citations


Journal ArticleDOI
TL;DR: In this article, a light-emitting square complex, 1; was prepared by combining the chromophore fac-Re(CO)F1(4,4'-bpy)2 (2)6 (105 mg, 0.170 m o l ) (dppp = 1,3-(diphenylphosphino)propane) in 50 mL of dry CH2C12.
Abstract: Rigid macrocycles based on cis bridging ligation of transition metals represent an unusual,'-4 and largely new,5 class of compounds having tremendous promise in host-guest, inclusion, and molecular recognition chemistry. Most of the available systems are (1) derived from pt(I1) or Pd(I1) triflate species, (2) tetranuclear with respect to metal content, and therefore, (3) arranged in square or box-like geometries, with metal atoms defining the comers. We report a simple, but potentially very useful, variant and extension of this theme: incorporation of visible-light-addressable, luminescent metal-ligand components within a square assembly. Induction of photoluminescent characteristics is particularly attractive in the context of eventual molecular sensing applications because it suggests an altemative to 'H NMR spectroscopy for detection of guest inclusion. Induction of luminescence also opens up the possibility of electronic excited state reactivity and possible manipulation of reactivity by encapsulated guests. The light-emitting square complex, 1; was prepared by combining the chromophore fac-Re(CO)F1(4,4'-bpy)2 (2)6 (105 mg, 0.170 "01) (4,4'-bpy = 4,4'-bipyridine) with Pd(dppp)(triflate)zId (139 mg, 0.170 m o l ) (dppp = 1,3-(diphenylphosphino)propane) in 50 mL of dry CH2C12. After 12 h of stirring, the pale yellow product was collected as a powder on a glass frit, washed with methylene chloride, and dried under vacuum. The yield was 83%. Elemental analysis data are consistent with the formation of either 1 or a larger assembly with 1:l Re:Pd stoichiometry.' FAB+ mass spectral measurements (m-NBA) yielded intense peaks at m/z = 2871.7 (2871.2 expected for l*H+), 2721.0 (1 triflate) and 2564.9 (1 triflate 4,4'bpy), thereby establishing the tetranuclear metal content of the compound. No peaks for compounds of higher mass (e.g., hexanuclear or octanuclear complexes) were found. FT-IR and 'H NMR measurements (supporting information) yielded additional data supportive of molecular square formation.* Figure 1 shows that the lowest energy transition in the electronic absorption spectrum of 2 is shifted substantially red by formation of the square. The shift is similar to that observed upon ligand protonation in 26 and is consistent with assignment

243 citations


Journal ArticleDOI
TL;DR: In this paper, the first step in the twofold 1,1-organoboration is an intermolecular 1, 1-vinyloboration, followed by an intramolecular 2, 1-, 1, 2-vinYLoboration which leads exclusively, in the case of M�Si and Ge, to siloles and germoles.

207 citations



Journal ArticleDOI
TL;DR: In this article, a transition state model was proposed to predict enantioselectivity in intramolecular cyclopropanations of 3-allcyl/aryl-2-Z)-alken-l-y1 diazoacetates.
Abstract: Diazo decomposition of allylic and homoallylic diazoacetates loa-p and 22a-j catalyzed by chiral dirhodium(I1) tetrakis[methyl 2-pyrrolidone-S(S)-carboxylate], Rhz(SS-MEPY)4 (7), and its enantiomer, Rhz(5RMEPY)4 (S), produces the corresponding intramolecular cyclopropanation products l la-p and 23a-j in good to excellent yields and with exceptional enantioselectivity. Higher enantiocontrol is observed with allylic diazoacetates than with their homoallylic counterparts, but allylic diazoacetates are subject to greater variations in enantioselectivities with changes in substitution patterns on the carbon-carbon double bond. For example, the enantioselectivities in the intramolecular cyclopropanations of 3-allcyl/aryl-2(Z)-alken-l-y1 diazoacetates are generally 194%, whereas the cyclizations of the homologous 4-alkyl/aryl-3(Z)-alken1-yl diazoacetates are typically in the range of 70-90% ee. The corresponding 3-alkyYaryl-2(E)-alken-l-y1 and 4-alkyl/aryl-3(E)-alken-l-yl diazoacetates undergo cyclization with slightly lower ee’s (54-85%). Although the Rh@-MEPY)4-catalyzed cyclization of the 2-methallyl diazoacetate 1Oc proceeds with only 7% ee, alternative chiral dirhodium(I1) catalysts, including those with methyl N-acylimidazolidin-2-one-4(S)-carboxylate ligands such as Rh2(4S-MACIM)4 (14) and Rhz(4S-MPAIM)d (E) , may be employed to increase the level of enantiocontrol to 78 and 65%, respectively. Some allylic diazoacetamides also undergo highly enantioselective cyclization to form cyclopropyl lactams as illustrated by the diazo decomposition of N-allyl diazoacetamide (19) in the presence of dirhodium(II) tetrakis[methyl 2-oxazolidinone-4(S)-carboxylate], Rhz(4SMEOXk, to give the 3-azabicyclo[3.1.0]hexan-2-one 20 in 98% ee. The absolute configuration and the level of enantiocontrol in these intramolecular cyclopropanations have been interpreted by a transition state model in which the important determinants are (i) the preferred conformation about the rhodium-carbon bond; (ii) the trajectory of approach of the double bond to the metallocarbene center; and (iii) the orientation of the double bond with respect to the chiral face of the catalyst.

185 citations


Journal ArticleDOI
TL;DR: In this article, the topology of the electron density within the theory of atoms in crystals has been used to define primary and secondary interactions in terms of the topological properties of crystal lattices.
Abstract: The heavier halide molecules form layered crystals indicative of the presence of a specific directed intermolecular interaction. It is shown that this interaction within the crystal can be defined and characterized using the topology of the electron density within the theory of atoms in crystals. It is also shown that its presence in the crystal and the resulting geometry of the layered structure can be predicted in terms of the topology of the Laplacian distribution of an isolated Cl2 molecule, as it relates to the definition of Lewis acid and base sites within the valence shell of an atom. The generality of the definition of both primary and secondary interactions in terms of the topology of the electron density is demonstrated for all types of crystal. The electron density of solid molecular chlorine was determined by fitting the experimental X-ray structure factors and by theoretical calculation and its topology determined. Each Cl atom is found to be linked by bond paths, lines of maximum electron density, to twelve other atoms in the crystal: to four atoms in the same layer parallel to the bc plane, one of which defines the intramolecular bond of the Cl2 group, to six atoms in the four neighbouring molecules lying in the same stack parallel to the b axis and to two atoms in molecules situated in a neighbouring stack.

180 citations




Journal ArticleDOI
TL;DR: In this article, the authors show that the influence of the cyano substituents on the ICT reaction in the excited state can be understood by considering the redox potentials of the anthryl groups.
Abstract: The energy of the charge transfer (CT) emission maximum of a series of dual fluorescent 4-aminobenzonitriles in diethyl ether and acetonitrile does not show a correlation with the redox potentials of the amino (D) and benzonitrile (A) subgroups. It is therefore concluded that these electron donor and acceptor subgroups cannot be treated in the same way as the A and D molecules in exciplexes 1(A−D+). This means that in the intramolecular charge transfer (ICT) state of the aminobenzonitriles, a considerable electronic coupling between the A− and D+ parts exists, in contradiction with the twisted intramolecular charge transfer (TICT) hypothesis. For 9,9′-bianthryl, 10-cyano-9,9′-bianthryl and 10,10′-dicyano-9,9′-bianthryl, photostationary and time-resolved measurements are presented, which show that the influence of the cyano substituents on the ICT reaction in the excited state can be understood by considering the redox potentials of the anthryl groups. The introduction of an additional cyano group in an aminobenzonitrile, however, leads to the disappearance of ICT and dual fluorescence, indicating that the photophysics of these molecules cannot be understood on the basis of the redox potentials of the D and A constituents.

136 citations






Journal ArticleDOI
TL;DR: In this paper, the structure and solvation of aquocobalamin (vitamin Biza, 1+) in the crystal and in aqueous solution were investigated using synchrotron radiation in combination with an imaging plate detector.
Abstract: Experiments are described to elucidate the structure and solvation of aquocobalamin (vitamin Biza, 1+) in the crystal and in aqueous solution. Aquocobalamin (1+) is the B12 derivative in which a water molecule replaces the axially coordinating organic substituent (methyl or 5'-desoxyadenosyl) at the P-side of the cobalt of the B12 coenzymes. (1) A single-crystal structure analysis of aquocobalamin perchlorate (l'ClO,), using synchrotron radiation in combination with an imaging plate detector, yielded the most accurate structural data ever determined for a B12 molecule. l'C10, crystallizes in the orthorhombic space group P212121, a = 15.042(11) A, b = 23.715(14) A, c = 25.104(12) A, with four 1'CIOi moieties plus about 100 solvent water molecules per unit cell; 22 867 independent and 20 942 significant intensity data were recorded to a nominal resolution of 0.8 A, and refinement against quantities led to a conventional R-value of 0.050 for all 22 867 observations and to a structural model with an average ESD for all carbon-carbon bonds of 0.003 A. In the crystal, the aquocobalamin ion has a very short axial bond between cobalt and the dimethylbenzimidazole (DMB) base of 1.925 (0.002) A, which is rationalized as resulting from the very weak trans axial donor (water). Steric repulsion between the DMB base and the corrin ring induced by this short Co-DMB bond leads to a relatively large "butteffly" deformation with an "upward" fold angle of 18.7 l(0.07)". The relevance of this observation for the "upward conformational deformation" hypothesis for the initiation of Co-C bond homolysis in coenzyme B12 dependent enzymes is discussed. (2) EXAFS spectra were taken from an authentic sample of the aquocobalamin perchlorate crystals used for the X-ray structure analysis, as well as from 1+C104 dissolved in a 1:l mixture of watedethylene glycol. Absorption spectra were recorded at 20 K between 7300 and 8700 eV, and the k3-weighted EXAFS was extracted for a k-value between 2.7 and 14.4 A-l. EXAFS spectra for solid and dissolved l'C10, agree closely, establishing identical cobalt coordination for the aquocobalamin ion in solution and in the solid state. A curve-fitting analysis on the Fourier filtered first-shell data yields a coordination number of 6 and an average distance of 1.90 A for both samples. There is no evidence for a longer Co-N distance. This refutes data published by Sagi and Chance (J. Am. Chem. Soc. 1992, 114, 8061). (3) NMR experiments are described, constituting the first detailed NMR investigations on a Bl2 derivative in H20, including 2D homo- and heteronuclear studies on aquocobalamin chloride (l+Cl-) and assignment of signals due to the exchangeable amide protons of all nitrogens as well a measurements of amide proton exchange rates. The NMR data confirm the occupation of the axial coordination site at the Co(II1) center by water, as well as the occurrence in solution of an intramolecular hydrogen bond to the axially coordinating water molecule, as observed in the crystal structure of l'C10,. However, significant differences of the structure of 1+ in crystals of l'C10, and of 1+ in aqueous solution are indicated from NOE data concerning the time-averaged conformation of the hydrogen-bonding c-acetamide side chain.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the effect of the length of the polyenic chain linking the donor and acceptor end groups of donor-acceptor polyenes on the ICT absorption band.

Journal ArticleDOI
TL;DR: In this paper, a model system delineates one process by which heterocyclic C-N bonds are cleaved and offer new insight as to how nitrogen heterocycles may be further degraded after C -N bond cleavage in hydrodenitrogenation catalysis.
Abstract: The reaction of the {eta}{sup 2}(N,C)-pyridine complex with LiBEt{sub 3}H affords the C-N bond scission product (2). The reactions of (1) with carbon nucleophiles RLi or RMgX provide the alkyl derivatives [R = Me (3), Et (4), {sup n}Pr (5), {sup n}Bu (6), and CH{sub 2}SiMe{sub 3} (7)]. Kinetic and mechanistic studies of the 3 {yields} 8 rearrangement reveal that methyl migration is strictly intramolecular. The reactions of this model system delineate one process by which heterocyclic C-N bonds are cleaved and offer new insight as to how nitrogen heterocycles may be further degraded after C-N bond cleavage in hydrodenitrogenation catalysis. 74 refs., 7 figs., 5 tabs.

Journal ArticleDOI
TL;DR: A semi-empirical model which describes the state of the 31-mer as a function of pH, sodium ion concentration, and temperature is developed and may serve as a prototype for developing a method to predict the phase diagrams of intramolecular triple helix systems.

Journal ArticleDOI
TL;DR: In this article, a simple enantioselective synthesis of sirenin (bioactive form) has been developed which depends on the intramolecular cycloaddition reaction of diazo ester 5 to form bicyclic ester 6 (95 : 5 enantiosity) using a new and remarkably stable monomeric chiral Cu(I) complex whose structure was proven by X-ray diffraction analysis.

Journal ArticleDOI
TL;DR: In this article, a combination of the instanton approach with quantum-chemically computed potentials and force fields is used to calculate hydrogen tunneling splittings, which are due to intramolecular hydrogen transfer in symmetric double-minimum potentials in molecules such as malonaldehyde and the hydrogenoxalate anion.
Abstract: Calculations of hydrogen tunneling splittings are reported based on a combination of the instanton approach with quantum‐chemically computed potentials and force fields. The splittings are due to intramolecular hydrogen transfer in symmetric double‐minimum potentials in molecules such as malonaldehyde and the hydrogenoxalate anion. Potential‐energy curves along the tunneling coordinates and harmonic force fields at the stationary points are calculated at the HF/6‐31G** and HF/6‐31+G** level of theory, and combined to yield a complete multidimensional surface. All modes that are displaced between the equilibrium configuration and the transition state are included in the calculation. In the formalism, these modes are linearly coupled to the tunneling mode, the couplings being proportional to the displacements in dimensionless units. These couplings modify the instanton trajectory and subject it to fluctuations. It is argued that within the accuracy of the available potential‐energy surfaces, direct calculat...



Journal ArticleDOI
TL;DR: Substituted furans have been synthesized in the presence of ruthenium catalysts under neutral and mild conditions via cyclization of primary and secondary enynols of type (Z)-HC C-C(Me)=CH-CH(R)OH containing a terminal triple bond as mentioned in this paper.




Journal ArticleDOI
TL;DR: The potential energy surface of o-hydroxyacetophenone was calculated at the ab initio SCF and CIS levels, respectively, using a split-valence basis set to which was added polarization functions on atoms involved in the intramolecular H-bond as mentioned in this paper.
Abstract: The potential energy surface of the ground state and lowest lying singlet of o-hydroxyacetophenone was calculated at the ab initio SCF and CIS levels, respectively, using a split-valence basis set to which was added polarization functions on atoms involved in the intramolecular H-bond. Whereas these potentials each contain one minimum corresponding to the normal form and a second well for the proton-transferred tautomer, only one minimum appears in these potentials after correlation is added. SO S1 excitation hence induces the tautomerization associated with intramolecular proton transfer from the hydroxyl to the carbonyl oxygen. The excited state proton transfer may involve a nonradiative transition from the Sl(nn*) state to Sl(nn*). The vibrational spectra of the two tautomers have certain strong differences. Anharmonic adiabatic treatment of the OH* 9 segment of the molecule leads to improved reproduction of experimentally observed v ( 0 9) and v(0H) frequencies.


Journal ArticleDOI
TL;DR: In this article, a new and efficient synthesis of the pyrrolo[2,1-c][1,4]benzodiazepine ring system has been carried out using an intramolecular aza Wittig reaction of the appropriately substituted N-(2-azidobenzoyl)pyrrolidine-2-carboxaldehydes.

Journal ArticleDOI
TL;DR: In this paper, three dyad molecules, each consisting of a porphyrin (P) linked to a quinone (Q) through a rigid bicyclic bridge, have been prepared, and their photochemistry has been investigated using time-resolved fluorescence and absorption techniques.
Abstract: Three dyad molecules, each consisting of a porphyrin (P) linked to a quinone (Q) through a rigid bicyclic bridge, have been prepared, and their photochemistry has been investigated using time-resolved fluorescence and absorption techniques. In all three molecules, photoinduced electron transfer from the porphyrin first excited singlet state to the quinone occurs with rate constants of approximately 10{sup 12} s{sup -1} in solvents ranging in dielectric constant from approximately 2.0 to 25.6 and at temperatures from 77 to 295 K. The transfer rate is also relatively insensitive to thermodynamic driving force changes up to 0.4 eV. This behavior is phenomenologically similar to photosynthetic electron transfer. The rapid rate of photoinduced electron transfer and its lack of dependence on environmental factors suggests that transfer is governed by intramolecular vibrations. Charge recombination of P{sup {center_dot}+} {sup -}Q{center_dot}, on the other hand, is substantially slower than charge separation and sensitive to both driving force and environmental conditions. Thus, by changing conditions, charge recombination rates can be varied over a wide range while photoinduced electron transfer rates are relatively unaffected. This suggests that rigid dyads of this general type may be useful building blocks for more complex molecular devices. 76 refs., 9 figs., 1more » tab.« less