Ribosomes translate the genetic code into proteins in all living cells with extremely high effici... more Ribosomes translate the genetic code into proteins in all living cells with extremely high efficiency, owing to their inherent flexibility and to their spectacular architecture. During the last 6 decades, extensive effort has been made to elucidate the molecular mechanisms associated with their function, and a quantum jump has been made in recent years, once the three dimensional structures of ribosomes and their functional complexes have been determined. These illuminated key issues in ribosome function, confirmed various biochemical, genetic, and medical findings, and revealed mechanistic details beyond previous expectation, thus leading to conceptual revolutions, and turning old myths into actual facts.
A feasible scenario for the emergence of life requires the spontaneous materialization and sustai... more A feasible scenario for the emergence of life requires the spontaneous materialization and sustainability of a proto-ribosome that could have catalysed the formation of the first peptides. Models of proto-ribosomes were derived from the ribosomal Peptidyl Transferase Centre (PTC) region, but the poor prebiotic copying abilities give rise to the question of their mode of replication. Here, complementarity is demonstrated in bacterial ribosomes, between nucleotides that constitute the two halves of the PTC cavity. The complementarity corroborates the dimeric nature of the proto-ribosome and is likely to underlie the symmetry of the PTC region. Furthermore, it indicates a simple and efficient replication mode; the strand of each monomer could have acted as a template for the synthesis of its counterpart, forming a self-replicating ribozyme.
The ribosome is a molecular machine whose manner of controlling the progression of the nascent ch... more The ribosome is a molecular machine whose manner of controlling the progression of the nascent chain through the ribosomal exit tunnel is currently unknown. A novel model for the mechanism driving the nascent chain motion is hereby presented, in which the ∼180° rotatory motion performed by each C-terminal amino acid of the nascent chain during its translocation from the A site to the P site, is suggested to twist the newly formed peptide bond into cis conformation. By catalyzing the cis to trans isomerization, the ribosome is proposed to release the potential energy stored within the cis conformer and to utilize it to push the chain down the tunnel, thus operating as a molecular motor. This hypothetical isomerization mechanism is supported by its ability to provide an explanation for the peculiar conduct observed in translational events of nascent chains with C-terminal prolines: the slow peptide bond formation with puromycin, translation arrest, and tmRNA tagging.
Journal of Inclusion Phenomena and Macrocyclic Chemistry, Mar 1, 1991
The crystalline complex dipotassium 1,3-diammonio-2-propanone tetrachloroplatinate trihydrate, fo... more The crystalline complex dipotassium 1,3-diammonio-2-propanone tetrachloroplatinate trihydrate, formulated as K2[(NH3CHzC)20][PtCI4]2' 3H20, has been prepared while synthesizing complexes analogous to cis-dichlorodianfine Pt(II). Its structure is orthorhombic, space group Pnma, Mr=896.1 , F(000)=1640, a=8.428(4), b=20.360(10), c=12.141(7)/1t, v=2083.3A 3, z=4, D~ =2.860gcm-3, #(MoK~)= 196cm-1, final R =0.083 for 1379 unique reflections. The structure consists of a very extended hydrogen bonded network which involves half of the PtC142-ions, the organic molecules and the water molecules of hydration, leaving large oval channels which accommodate potassium cations and tetrachloroplatinate anions. Within the channel the potassium and tetrachloroplatinate ions pack in columns extending along the a axis. The dense packing is rather reminiscent of that found in the neat salt where each potassium ion is surrounded by six chlorine atoms which describe a polyhedron of a distorted trigonal prism with K+...CI distances ranging between 3.163-3.187A. There is some indirect evidence for the formation of 1,3-diammonio-2propanone dications which counterbalance the charges of half of the PtCI 2-ions by protonation of each of the amino nitrogens. The carbonyl oxygens, however, are involved only in a very weak O...C~-O interactions along the a axis,
International Journal of Molecular Sciences, Jun 30, 2009
A symmetric pocket-like entity, composed of two L-shaped RNA units, encircles the peptide synthes... more A symmetric pocket-like entity, composed of two L-shaped RNA units, encircles the peptide synthesis site within the contemporary ribosome. This entity was suggested to be the vestige of a dimeric proto-ribosome, which could have formed spontaneously in the prebiotic world, catalyzing non-coded peptide bond formation and elongation. This structural element, beyond offering the initial step in the evolution of translation, is hypothesized here to be linked to the origin of life. By catalyzing the production of random peptide chains, the proto-ribosome could have enabled the formation of primary enzymes, launching a process of co-evolution of the translation apparatus and the proteins, thus presenting an alternative to the RNA world hypothesis.
In the conceptualization presented here, a possible scenario concerned with the emergence of code... more In the conceptualization presented here, a possible scenario concerned with the emergence of coded life in nature is inferred from a model that merges computer science concepts with prebiotic chemistry. In this "digital" model, sets of strings composed of letters, such that each letter represents a molecular building block, are located within compartments. Some of the sets of strings together with their reactions form "autocatalytic sets". Some of the strings in the autocatalytic sets play the role of catalysts of reactions and others play the role of templates for replication processes. We find several unique sets of strings, comprised of two types of letters $$r_i$$ and $$p_j$$ representing nucleotides and amino acids respectively, with some inherent asymmetry in their properties, that prompt the emergence of a code. By identifying such "code prompting" autocatalytic sets, our abstract model suggests novel models for artificial life, and a possible explanation for the emergence and the fixation of the genetic code in life as we know it.
The Diels-Alder reaction between l l-cyano-l,6methano[10]annulene and 4-methyl-l,2,4-triazoline-3... more The Diels-Alder reaction between l l-cyano-l,6methano[10]annulene and 4-methyl-l,2,4-triazoline-3,5-dione yields the exo and endo isomers of the adduct 2,3,5,5a,9a,10-hexahydro-2-methyl-1,3-dioxo-5,10-etheno-5a,9a-methano-1 H-[ 1,2,4] triazolo[ 1,2-b]ph thalazine-14-carbonit rile (C M AT). CM AT(II) (the crystal of the exo isomer) transforms to CMAT(I) (the crystal of the endo isomer) during a thermally induced phase transition in the solid state. Two modes of transition were observed in crystals of CMAT(II) taken from different batches: a single-crystal-tosingle-crystal transition and a single-crystal-topolycrystal transition. The identification of the factor that determines the mode of transition was based on a structural comparison at room temperature and at circa 380 K, on thermal-motion analyses at both temperatures and on an examination of the defect content using X-ray topography. No substantial differences, concerning these aspects, were detected between crystals from the two batches and the various modes of transition were attributed to different sites of nucleation. A single nucleus at the edge of the crystal plates results in a single-crystal product, while nucleation at holes on the plate surface leads to a polycrystalline material.
Journal of the American Chemical Society, Feb 1, 1987
Single-crystal X-ray determination established that the spontaneous cyclodimer of butatriene is a... more Single-crystal X-ray determination established that the spontaneous cyclodimer of butatriene is a syn-head-to-head bis(alleny1)cyclobutane (5) and the cyclodimer of the hexapentaene 0 (CH3)pC = C =C=C=C=C is a unique syn-head-to-head cyclyne, 1,3,7,9-~yclododecatetrayne (18). In both dimers 5 and 18 the cyclohexane positions of the [4.1.0]bicycloheptane groups are found to be disordered between two half-chair conformations. The central diacetylenic units in cyclyne 18 are part of a twelve-membered ring with a noncrystallographic twofold symmetry. These results, along with a plausible mechanism of formation, are discussed. Strained organic molecules2 have fascinated chemists for nearly a century. Because of their high energy and associated strain, these molecules generally have unusual chemical, spectral, and physical attributes that are of theoretical interest2 and hence are a synthetic challenge. Among the intriguing, strained, class of molecules are the cumulenes, 1, a family of hydrocarbons with cumulated double bonds, with known members from the simple three carbon allenes (1: n = 1) through the 12-carbon dodecaudecaene (1: n = data and literature analogies we assigned the cyclodimers as the head-to-tail [4]radialene isomers 8.4 However, on the basis of the spectral data alone we could not, of course, be sure of this assignment, nor could we say anything about the syn-8 or anti-8' forms. Likewise, we subsequently reported' the unsaturated carbene, 11, derived spontaneous cyclodimerization of hexapentaene 12 to a novel unsymmetrical cyclyne, 13, a 1,3,7,9cyclododecatetrayne. >C=(=C=),=C< 1: n = 1, 2...10 (CH3)pC=C=C=C=C =C: Recently one of us reported4 the cyclodimerization of butatrienes 3, derivedS from unsaturated carbene(s) 2. Cyclodimerization 11 b: R=CH3 Once again a large number of cyclodimers are possible, including perhaps 2 + 2 dimers, such as the [4]radialene, 15, reported by Hartzler* as the sole product in the thermal cyclodimerization of tetra-tert-butylhexapentaene 14: Although the product cyclyne 1cyclodimer b: R*CH3 4a: R=H CR&= C=C=C 3a:R.H b: R=CH3 occurred spontaneously under the reaction conditions at-20 "C. Considering the unsymmetrical nature of the precursor butatienes 3, a priori, there are 15 possible isomeric cyclodimers, 4, consisting of three pairs of head-to-head 5, 7, 9 and head-to-tail 6, 8, 10 dimers, about the three different double bonds, each with syn and anti pairs, respectively, as shown in Chart I (isomers 5', 7', and 9' can exist as enantiomeric pairs). Interestingly, despite the large number of possible cyclodimers, only a single compound was isolated from both the parent system 3a and the dimethyl butatriene 3b.436 On the basis of spectral ~ ~ (1) (a) Technion. (b) Fulbright Postdoctoral Fellow, Utah. (c) Utah. (2) Greenberg, A,; Liebman, I. F. Strained Organic Molecules; Academic: New York, 1978. (3) For reviews and references on cumulenes see: Fischer, H. The Chemisrry of Alkenes;
Acta Crystallographica Section C-crystal Structure Communications, Aug 15, 1992
C 18 H 15 N 3 O 4 , isomere 1, cristallise dans P2 1 2 1 2 1 avec a = 23,866, b = 9,571 et c = 6,... more C 18 H 15 N 3 O 4 , isomere 1, cristallise dans P2 1 2 1 2 1 avec a = 23,866, b = 9,571 et c = 6,686 A, Z = 4; affinement jusqu'a R = 0,059. L'isomere 2 cristallise dans P2 1 2 1 2 1 avec a = 24,104, b = 9,774 et c = 6,463 A, Z = 4; affinement jusqu'a R = 0,095. C 20 H 19 N 3 O 4 , isomere 3, cristallise dans P2 1 2 1 2 1 avec a = 13,096, b = 12,653 et c = 10,696 A, Z = 4; affinement jusqu'a R = 0,066. L'isomere 4 cristallise dans P2 1 /c avec a = 26,724, b = 11,985 et c = 11,453 A, β = 99,96 °, Z = 8; affinement jusqu'a R = 0,036. C 20 H 19 N 3 O 4 cristallise dans P2 1 /c avec a = 13,474, b = 11,422 et c = 11,534 A, β = 97,73 °, Z = 4; affinement jusqu'a R = 0,070.
ChemInform Abstract Crystal and molecular structures of (I)-(VII); space groups: P21/n for (I)-(I... more ChemInform Abstract Crystal and molecular structures of (I)-(VII); space groups: P21/n for (I)-(IIIa) (Z=4); P21/c for (IIIb) (Z=4); P2, for (IVa) (Z=2); PT for (IVb) (Z=4) and (VII) (Z=2); P212121 for (IVc) and (Va) (Z=4); C2/c for (Vb) (Z=4); Pnma for (VI) (Z=4).
Acta Crystallographica Section C-crystal Structure Communications, May 15, 1993
Introduction. Many Diels-Alder reactions between propellanes containing one or two cyclohexadiene... more Introduction. Many Diels-Alder reactions between propellanes containing one or two cyclohexadiene rings and bridged [10]annulenes with triazolinediones show a preference for the endo mono-adducts (Kalo,
Vertical ionization potentials (measured by photoelectron spectroscopy) and formal oxidation pote... more Vertical ionization potentials (measured by photoelectron spectroscopy) and formal oxidation potentials (cyclic voltammetry) are reported for (I)‐(VI); X‐ray crystal structures are given for (III) (space group P2/n with Z=2) and (V) (space group P21/n), ESR data are given for the cation radicals of (IV)‐(VI).
Journal of the American Chemical Society, Jul 1, 1986
The crystal structures of twelve N,N'-acylated cyclic hydrazines are described. In five adducts o... more The crystal structures of twelve N,N'-acylated cyclic hydrazines are described. In five adducts of 1,2,44riazolidine-3,Sdione with various cyclopentadiene derivatives, it is shown that the configuration of the triazolidinedione moiety with respect to the bicycloheptyl rings is highly sensitive to very small differences in steric effects. The degree of flattening of the N atoms at the N-N bond is determined by the steric repulsions of the substituents. The average valence angle (a,,") at the nitrogens ranges between 110.5 and 112.0'. Increasing the size of N,N' rings and replacing the urea-type nitrogen of the triazolinedione ring by carbon is shown to cause significant flattening at the hydrazine nitrogens, and five of the other seven structures described here are nearly flat at nitrogen (aav greater than 119'). N(2)-C(8) N(3)-C(4) N(3)-C(7) C(l)-c(2) C(I)-c(5) C(3)-C(4) C(4)-C(5) N(3)-N(1)-C(1) N(3)-N(l)-C(6) C(1)-N(1)-C(6) C(6)-N(2)-C(7) C(6)-N(Z)-C(8) C(7)-N(2)-C(8) N(1)-N(3)-C(4) N(1)-N(3)-C(7) C(4)-N(3)-C(7) N (1)-c(l)-W) N(1)-C(1)-C(5) C(2)-C(1)-C(5) C(2)-C(3)-C(4) C(3)-C(4)-C(5) O(1)-C(6)-N(2) ~(2)-~(7)
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
copy (cryo-EM) have also revealed new features of ribosomal particles and their complexes (Gabash... more copy (cryo-EM) have also revealed new features of ribosomal particles and their complexes (Gabashvili et al., 1999b, 2000; Stark et al., 2000). In parallel, the structures of several ribosomal protein-RNA complexes have been determined at high resolution (Conn et al., 1999; Wim
Ribosomes translate the genetic code into proteins in all living cells with extremely high effici... more Ribosomes translate the genetic code into proteins in all living cells with extremely high efficiency, owing to their inherent flexibility and to their spectacular architecture. During the last 6 decades, extensive effort has been made to elucidate the molecular mechanisms associated with their function, and a quantum jump has been made in recent years, once the three dimensional structures of ribosomes and their functional complexes have been determined. These illuminated key issues in ribosome function, confirmed various biochemical, genetic, and medical findings, and revealed mechanistic details beyond previous expectation, thus leading to conceptual revolutions, and turning old myths into actual facts.
A feasible scenario for the emergence of life requires the spontaneous materialization and sustai... more A feasible scenario for the emergence of life requires the spontaneous materialization and sustainability of a proto-ribosome that could have catalysed the formation of the first peptides. Models of proto-ribosomes were derived from the ribosomal Peptidyl Transferase Centre (PTC) region, but the poor prebiotic copying abilities give rise to the question of their mode of replication. Here, complementarity is demonstrated in bacterial ribosomes, between nucleotides that constitute the two halves of the PTC cavity. The complementarity corroborates the dimeric nature of the proto-ribosome and is likely to underlie the symmetry of the PTC region. Furthermore, it indicates a simple and efficient replication mode; the strand of each monomer could have acted as a template for the synthesis of its counterpart, forming a self-replicating ribozyme.
The ribosome is a molecular machine whose manner of controlling the progression of the nascent ch... more The ribosome is a molecular machine whose manner of controlling the progression of the nascent chain through the ribosomal exit tunnel is currently unknown. A novel model for the mechanism driving the nascent chain motion is hereby presented, in which the ∼180° rotatory motion performed by each C-terminal amino acid of the nascent chain during its translocation from the A site to the P site, is suggested to twist the newly formed peptide bond into cis conformation. By catalyzing the cis to trans isomerization, the ribosome is proposed to release the potential energy stored within the cis conformer and to utilize it to push the chain down the tunnel, thus operating as a molecular motor. This hypothetical isomerization mechanism is supported by its ability to provide an explanation for the peculiar conduct observed in translational events of nascent chains with C-terminal prolines: the slow peptide bond formation with puromycin, translation arrest, and tmRNA tagging.
Journal of Inclusion Phenomena and Macrocyclic Chemistry, Mar 1, 1991
The crystalline complex dipotassium 1,3-diammonio-2-propanone tetrachloroplatinate trihydrate, fo... more The crystalline complex dipotassium 1,3-diammonio-2-propanone tetrachloroplatinate trihydrate, formulated as K2[(NH3CHzC)20][PtCI4]2' 3H20, has been prepared while synthesizing complexes analogous to cis-dichlorodianfine Pt(II). Its structure is orthorhombic, space group Pnma, Mr=896.1 , F(000)=1640, a=8.428(4), b=20.360(10), c=12.141(7)/1t, v=2083.3A 3, z=4, D~ =2.860gcm-3, #(MoK~)= 196cm-1, final R =0.083 for 1379 unique reflections. The structure consists of a very extended hydrogen bonded network which involves half of the PtC142-ions, the organic molecules and the water molecules of hydration, leaving large oval channels which accommodate potassium cations and tetrachloroplatinate anions. Within the channel the potassium and tetrachloroplatinate ions pack in columns extending along the a axis. The dense packing is rather reminiscent of that found in the neat salt where each potassium ion is surrounded by six chlorine atoms which describe a polyhedron of a distorted trigonal prism with K+...CI distances ranging between 3.163-3.187A. There is some indirect evidence for the formation of 1,3-diammonio-2propanone dications which counterbalance the charges of half of the PtCI 2-ions by protonation of each of the amino nitrogens. The carbonyl oxygens, however, are involved only in a very weak O...C~-O interactions along the a axis,
International Journal of Molecular Sciences, Jun 30, 2009
A symmetric pocket-like entity, composed of two L-shaped RNA units, encircles the peptide synthes... more A symmetric pocket-like entity, composed of two L-shaped RNA units, encircles the peptide synthesis site within the contemporary ribosome. This entity was suggested to be the vestige of a dimeric proto-ribosome, which could have formed spontaneously in the prebiotic world, catalyzing non-coded peptide bond formation and elongation. This structural element, beyond offering the initial step in the evolution of translation, is hypothesized here to be linked to the origin of life. By catalyzing the production of random peptide chains, the proto-ribosome could have enabled the formation of primary enzymes, launching a process of co-evolution of the translation apparatus and the proteins, thus presenting an alternative to the RNA world hypothesis.
In the conceptualization presented here, a possible scenario concerned with the emergence of code... more In the conceptualization presented here, a possible scenario concerned with the emergence of coded life in nature is inferred from a model that merges computer science concepts with prebiotic chemistry. In this "digital" model, sets of strings composed of letters, such that each letter represents a molecular building block, are located within compartments. Some of the sets of strings together with their reactions form "autocatalytic sets". Some of the strings in the autocatalytic sets play the role of catalysts of reactions and others play the role of templates for replication processes. We find several unique sets of strings, comprised of two types of letters $$r_i$$ and $$p_j$$ representing nucleotides and amino acids respectively, with some inherent asymmetry in their properties, that prompt the emergence of a code. By identifying such "code prompting" autocatalytic sets, our abstract model suggests novel models for artificial life, and a possible explanation for the emergence and the fixation of the genetic code in life as we know it.
The Diels-Alder reaction between l l-cyano-l,6methano[10]annulene and 4-methyl-l,2,4-triazoline-3... more The Diels-Alder reaction between l l-cyano-l,6methano[10]annulene and 4-methyl-l,2,4-triazoline-3,5-dione yields the exo and endo isomers of the adduct 2,3,5,5a,9a,10-hexahydro-2-methyl-1,3-dioxo-5,10-etheno-5a,9a-methano-1 H-[ 1,2,4] triazolo[ 1,2-b]ph thalazine-14-carbonit rile (C M AT). CM AT(II) (the crystal of the exo isomer) transforms to CMAT(I) (the crystal of the endo isomer) during a thermally induced phase transition in the solid state. Two modes of transition were observed in crystals of CMAT(II) taken from different batches: a single-crystal-tosingle-crystal transition and a single-crystal-topolycrystal transition. The identification of the factor that determines the mode of transition was based on a structural comparison at room temperature and at circa 380 K, on thermal-motion analyses at both temperatures and on an examination of the defect content using X-ray topography. No substantial differences, concerning these aspects, were detected between crystals from the two batches and the various modes of transition were attributed to different sites of nucleation. A single nucleus at the edge of the crystal plates results in a single-crystal product, while nucleation at holes on the plate surface leads to a polycrystalline material.
Journal of the American Chemical Society, Feb 1, 1987
Single-crystal X-ray determination established that the spontaneous cyclodimer of butatriene is a... more Single-crystal X-ray determination established that the spontaneous cyclodimer of butatriene is a syn-head-to-head bis(alleny1)cyclobutane (5) and the cyclodimer of the hexapentaene 0 (CH3)pC = C =C=C=C=C is a unique syn-head-to-head cyclyne, 1,3,7,9-~yclododecatetrayne (18). In both dimers 5 and 18 the cyclohexane positions of the [4.1.0]bicycloheptane groups are found to be disordered between two half-chair conformations. The central diacetylenic units in cyclyne 18 are part of a twelve-membered ring with a noncrystallographic twofold symmetry. These results, along with a plausible mechanism of formation, are discussed. Strained organic molecules2 have fascinated chemists for nearly a century. Because of their high energy and associated strain, these molecules generally have unusual chemical, spectral, and physical attributes that are of theoretical interest2 and hence are a synthetic challenge. Among the intriguing, strained, class of molecules are the cumulenes, 1, a family of hydrocarbons with cumulated double bonds, with known members from the simple three carbon allenes (1: n = 1) through the 12-carbon dodecaudecaene (1: n = data and literature analogies we assigned the cyclodimers as the head-to-tail [4]radialene isomers 8.4 However, on the basis of the spectral data alone we could not, of course, be sure of this assignment, nor could we say anything about the syn-8 or anti-8' forms. Likewise, we subsequently reported' the unsaturated carbene, 11, derived spontaneous cyclodimerization of hexapentaene 12 to a novel unsymmetrical cyclyne, 13, a 1,3,7,9cyclododecatetrayne. >C=(=C=),=C< 1: n = 1, 2...10 (CH3)pC=C=C=C=C =C: Recently one of us reported4 the cyclodimerization of butatrienes 3, derivedS from unsaturated carbene(s) 2. Cyclodimerization 11 b: R=CH3 Once again a large number of cyclodimers are possible, including perhaps 2 + 2 dimers, such as the [4]radialene, 15, reported by Hartzler* as the sole product in the thermal cyclodimerization of tetra-tert-butylhexapentaene 14: Although the product cyclyne 1cyclodimer b: R*CH3 4a: R=H CR&= C=C=C 3a:R.H b: R=CH3 occurred spontaneously under the reaction conditions at-20 "C. Considering the unsymmetrical nature of the precursor butatienes 3, a priori, there are 15 possible isomeric cyclodimers, 4, consisting of three pairs of head-to-head 5, 7, 9 and head-to-tail 6, 8, 10 dimers, about the three different double bonds, each with syn and anti pairs, respectively, as shown in Chart I (isomers 5', 7', and 9' can exist as enantiomeric pairs). Interestingly, despite the large number of possible cyclodimers, only a single compound was isolated from both the parent system 3a and the dimethyl butatriene 3b.436 On the basis of spectral ~ ~ (1) (a) Technion. (b) Fulbright Postdoctoral Fellow, Utah. (c) Utah. (2) Greenberg, A,; Liebman, I. F. Strained Organic Molecules; Academic: New York, 1978. (3) For reviews and references on cumulenes see: Fischer, H. The Chemisrry of Alkenes;
Acta Crystallographica Section C-crystal Structure Communications, Aug 15, 1992
C 18 H 15 N 3 O 4 , isomere 1, cristallise dans P2 1 2 1 2 1 avec a = 23,866, b = 9,571 et c = 6,... more C 18 H 15 N 3 O 4 , isomere 1, cristallise dans P2 1 2 1 2 1 avec a = 23,866, b = 9,571 et c = 6,686 A, Z = 4; affinement jusqu'a R = 0,059. L'isomere 2 cristallise dans P2 1 2 1 2 1 avec a = 24,104, b = 9,774 et c = 6,463 A, Z = 4; affinement jusqu'a R = 0,095. C 20 H 19 N 3 O 4 , isomere 3, cristallise dans P2 1 2 1 2 1 avec a = 13,096, b = 12,653 et c = 10,696 A, Z = 4; affinement jusqu'a R = 0,066. L'isomere 4 cristallise dans P2 1 /c avec a = 26,724, b = 11,985 et c = 11,453 A, β = 99,96 °, Z = 8; affinement jusqu'a R = 0,036. C 20 H 19 N 3 O 4 cristallise dans P2 1 /c avec a = 13,474, b = 11,422 et c = 11,534 A, β = 97,73 °, Z = 4; affinement jusqu'a R = 0,070.
ChemInform Abstract Crystal and molecular structures of (I)-(VII); space groups: P21/n for (I)-(I... more ChemInform Abstract Crystal and molecular structures of (I)-(VII); space groups: P21/n for (I)-(IIIa) (Z=4); P21/c for (IIIb) (Z=4); P2, for (IVa) (Z=2); PT for (IVb) (Z=4) and (VII) (Z=2); P212121 for (IVc) and (Va) (Z=4); C2/c for (Vb) (Z=4); Pnma for (VI) (Z=4).
Acta Crystallographica Section C-crystal Structure Communications, May 15, 1993
Introduction. Many Diels-Alder reactions between propellanes containing one or two cyclohexadiene... more Introduction. Many Diels-Alder reactions between propellanes containing one or two cyclohexadiene rings and bridged [10]annulenes with triazolinediones show a preference for the endo mono-adducts (Kalo,
Vertical ionization potentials (measured by photoelectron spectroscopy) and formal oxidation pote... more Vertical ionization potentials (measured by photoelectron spectroscopy) and formal oxidation potentials (cyclic voltammetry) are reported for (I)‐(VI); X‐ray crystal structures are given for (III) (space group P2/n with Z=2) and (V) (space group P21/n), ESR data are given for the cation radicals of (IV)‐(VI).
Journal of the American Chemical Society, Jul 1, 1986
The crystal structures of twelve N,N'-acylated cyclic hydrazines are described. In five adducts o... more The crystal structures of twelve N,N'-acylated cyclic hydrazines are described. In five adducts of 1,2,44riazolidine-3,Sdione with various cyclopentadiene derivatives, it is shown that the configuration of the triazolidinedione moiety with respect to the bicycloheptyl rings is highly sensitive to very small differences in steric effects. The degree of flattening of the N atoms at the N-N bond is determined by the steric repulsions of the substituents. The average valence angle (a,,") at the nitrogens ranges between 110.5 and 112.0'. Increasing the size of N,N' rings and replacing the urea-type nitrogen of the triazolinedione ring by carbon is shown to cause significant flattening at the hydrazine nitrogens, and five of the other seven structures described here are nearly flat at nitrogen (aav greater than 119'). N(2)-C(8) N(3)-C(4) N(3)-C(7) C(l)-c(2) C(I)-c(5) C(3)-C(4) C(4)-C(5) N(3)-N(1)-C(1) N(3)-N(l)-C(6) C(1)-N(1)-C(6) C(6)-N(2)-C(7) C(6)-N(Z)-C(8) C(7)-N(2)-C(8) N(1)-N(3)-C(4) N(1)-N(3)-C(7) C(4)-N(3)-C(7) N (1)-c(l)-W) N(1)-C(1)-C(5) C(2)-C(1)-C(5) C(2)-C(3)-C(4) C(3)-C(4)-C(5) O(1)-C(6)-N(2) ~(2)-~(7)
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
copy (cryo-EM) have also revealed new features of ribosomal particles and their complexes (Gabash... more copy (cryo-EM) have also revealed new features of ribosomal particles and their complexes (Gabashvili et al., 1999b, 2000; Stark et al., 2000). In parallel, the structures of several ribosomal protein-RNA complexes have been determined at high resolution (Conn et al., 1999; Wim
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