scholarly journals The conformation of abscisic acid by n.m.r. and a revision of the proposed mechanism for cyclization during its biosynthesis

1984 ◽  
Vol 220 (1) ◽  
pp. 325-332 ◽  
Author(s):  
B V Milborrow
Keyword(s):  
Abscisic Acid ◽  
Hydrogen Atom ◽  
Pulse Sequence ◽  
Chair Conformation ◽  
Crystalline Form ◽  
Side Chain ◽  
Hydrogen Atoms ◽  
Methyl Group ◽  
Nuclear Overhauser Enhancement ◽  
Nuclear Overhauser

The n.m.r. spectrum of abscisic acid (ABA) formed from [1,2-13C2]acetate by the fungus Cercospora rosicola shows 13C-13C coupling between C-6′ (41.7 p.p.m.; 36 Hz) and the downfield 6′-methyl group (6′-Me) (24.3 p.p.m, 36 Hz). This 6′-Me, therefore, is derived from C-3′ of mevalonate [Bennett, Norman & Maier (1981) Phytochemistry 20, 2343-2344]. An i.n.e.p.t. (insensitive nuclei enhanced by polarization transfer) pulse sequence demonstrated that the downfield 13C signal is produced by the 6′-Me that gives rise to the upfield 1H 6′-Me signal (23.1 d). The absolute configuration of this, the equatorial 6′-Me group, was determined as 6′-pro-R by decoupling and n.O.e. (nuclear-Overhauser-enhancement) experiments at 300 MHz using ABA, ABA in which the axial 6′-pro-S 5′-hydrogen atom had been exchanged with 2H in NaO2H and the 1′,4′-cis- and 1′,4′-trans-diols formed from these samples. The configuration at C-1′ and at C-6′ are now compatible with a chair-folded intermediate during cyclization, as proposed for beta- and epsilon-rings of carotenoids. ABA in solution exists, as in the crystalline form, with the ring in a pseudo-chair conformation. The side chain is axial and the C-3 Me and the C-5 hydrogen atoms are predominantly cis(Z).

Untersuchungen über Nebenreaktionen und den sterischen Verlauf bei der Direkt-Markierung einfacher Verbindungen mit Tritium-Gas

1968 ◽  
Vol 23 (6) ◽  
pp. 778-784
Author(s):  
Helmut Simon ◽  
Klaus Schmidt ◽  
Gerhard Müllhofer ◽  
Walter Eder ◽  
Ramiro Medina
Keyword(s):  
Hydrogen Atom ◽  
Tartaric Acid ◽  
Malic Acid ◽  
Glycolic Acid ◽  
Free Acid ◽  
Hydrogen Atoms ◽  
Methyl Group ◽  
Tartaric Acids ◽  
Carbon Carbon Bond ◽  
High Degree

Free tartaric acids, various tartrates, malic, hydroxymaleic, fumaric and aspartic acid, as well as propane diol-1.2 have been labelled by the Wilzbach method. In propane diol, hydrogen attached at C-2 is substituted by tritium three times faster than a hydrogen atom of the methyl group whereas that at C-l is substituted two times faster. Splitting of a carbon-carbon bond and fixation of tritium is eight to nine times more probable than the substitution of hydrogen by tritium. The OH group in propane diol as well as in tartaric and malic acid is more often replaced by atomic tritium than hydrogen atoms fixed at carbon. During the formation of malic acid from tartaric acid the second center of asymmetry is not attacked. The replacement of the OH group occurs in the cases of the free acid and the dimethylester to a high degree under inversion. Is the calcium or copper salt exposed retention prevails. In meso tartaric acid the carbon bound hydrogen is substituted with 12% inversion. When glycolic acid is formed from copper L-tartrate 29% inversion takes place.

scholarly journals The role of a cholesta-8,14-dien-3β-ol system in cholesterol biosynthesis

1969 ◽  
Vol 111 (5) ◽  
pp. 757-761 ◽  
Author(s):  
M. Akhtar ◽  
I. A. Watkinson ◽  
A. D. Rahimtula ◽  
D. C. Wilton ◽  
K. A. Munday
Keyword(s):  
Hydrogen Atom ◽  
Cholesterol Biosynthesis ◽  
Tritiated Water ◽  
Mevalonic Acid ◽  
Hydrogen Atoms ◽  
Steroid Biosynthesis ◽  
Methyl Group

The biosynthesis of cholesterol from squalene and tritiated water is described. Degradation of the cholesterol indicated that C-15 may be involved in cholesterol biosynthesis. In accordance with this view it is shown that in the conversion of [2RS−3H2]mevalonic acid into cholesterol one of the hydrogen atoms at C-15 is removed. A mechanism for the removal of the 14α-methyl group in steroid biosynthesis that involves the labilization of a C-15 hydrogen atom is outlined. In accordance with the requirement of this scheme it is shown that 4,4′-dimethyl-cholesta-8,14-dien-3β-ol is converted into cholesterol.

Formation of surface layer of hydrogen in pure aluminum

2019 ◽  
Vol 484 (1) ◽  
pp. 56-60
Author(s):  
D. A. Indejtsev ◽  
E. V. Osipova
Keyword(s):  
Surface Layer ◽  
Hydrogen Atom ◽  
Ab Initio ◽  
Pure Aluminum ◽  
Hydrogen Atoms ◽  
Energy Characteristics ◽  
Aluminum Crystal

Hydrogen atom behavior in pure aluminum is described by ab initio modelling. All main energy characteristics of the system consisting of hydrogen atoms in a periodic aluminum crystal are found.

scholarly journals Effect of side-chain length in lignin model compound on MnO2 oxidation: comparison of oxidations between C6-C2- and C6-C1-type compounds

2021 ◽  
Vol 67 (1) ◽  
Author(s):  
Shirong Sun ◽  
Tomoya Yokoyama
Keyword(s):  
Isotope Effects ◽  
Aromatic Nucleus ◽  
Side Chain ◽  
Direct Oxidation ◽  
Initial Oxidation ◽  
Methyl Group ◽  
Lignin Model Compound ◽  
Oxidation Rates ◽  
Kinetic Isotope ◽  
Lignin Model

AbstractMonomeric C6-C2-type lignin model compounds with a p-hydroxyphenyl (H), guaiacyl (G), syringyl (S), or p-ethylphenyl (E) nucleus (1-phenylethanol derivatives) were individually oxidized by MnO2 at a pH of 1.5 and room temperature. The results were compared with those of the corresponding C6-C1-type benzyl alcohol derivatives obtained in our recent report to examine the effect of the presence of the β-methyl group on the oxidation. The presence decelerated the oxidation regardless of the type of aromatic nucleus, although it did not change the order of the oxidation rates: G > S >> H > E. This deceleration results from the steric factor of the β-methyl group in the C6-C2-type compounds. The MnO2 oxidations of the corresponding C6-C2-type compounds deuterated at their α-(benzyl)positions showed that the magnitudes of the kinetic isotope effects are smaller than those observed in the oxidations of the corresponding C6-C1-type compounds, regardless of the type of aromatic nucleus. These smaller magnitudes suggest that the presence of the β-methyl group shifts the initial oxidation mode of MnO2 from direct oxidation of the benzyl position to one-electron oxidation of the aromatic nucleus. Only the S-type compounds afforded products via degradation of the aromatic nuclei.

scholarly journals NMR solution conformation of heparin-derived tetrasaccharide

1996 ◽  
Vol 318 (1) ◽  
pp. 93-102 ◽  
Author(s):  
Dmitri MIKHAILOV ◽  
Kevin H. MAYO ◽  
Ioncho R. VLAHOV ◽  
Toshihiko TOIDA ◽  
Azra PERVIN ◽  
...  
Keyword(s):  
Chair Conformation ◽  
Coupling Constants ◽  
Solution Conformation ◽  
1H And 13C Nmr ◽  
Small Deviations ◽  
J Coupling Constants ◽  
Nuclear Overhauser ◽  
Nuclear Overhauser Effects ◽  
Dynamics Simulations ◽  
Ring Conformations

The solution conformation of the homogeneous, heparin-derived tetrasaccharide ΔUA2S(1 → 4)-α-d-GlcNpS6S(1 → 4)-α-l-IdoAp2S(1 → 4)-α-d-GlcNpS6S (residues A, B, C and D respectively, where IdoA is iduronic acid) has been investigated by using 1H- and 13C-NMR. Ring conformations have been defined by J-coupling constants and inter-proton nuclear Overhauser effects (NOEs), and the orientation of one ring with respect to the other has been defined by inter-ring NOEs. NOE-based conformational modelling has been done by using the iterative relaxation matrix approach (IRMA), restrained molecular dynamics simulations and energy minimization to refine structures and to distinguish between minor structural differences and equilibria between various ring forms. Both glucosamine residues B and D are in the 4C1 chair conformation. The 6-O-sulphate group is oriented in the gauche–trans configuration in the D ring, whereas in the B ring the gauche–gauche rotomer predominates. Uronate (A) and iduronate (C) residues are mostly represented by 1H2 and 2S0 twisted boat forms, respectively, with small deviations in expected coupling constants and NOEs suggesting minor contributions from other A and C ring conformations.

Crystal and molecular structure of heptacain hydrochloride

1986 ◽  
Vol 51 (2) ◽  
pp. 264-270 ◽  
Author(s):  
František Pavelčík ◽  
Milan Remko ◽  
Jozef Čižmárik ◽  
Jaroslav Majer
Keyword(s):  
Molecular Structure ◽  
Least Squares Method ◽  
Crystal And Molecular Structure ◽  
Three Dimensional ◽  
Chair Conformation ◽  
Piperidine Ring ◽  
Side Chain ◽  
Ring Plane ◽  
Benzene Ring Plane ◽  
Carbamate Group

The crystal and molecular structure of heptacain hydrochloride was determined from three-dimensional diffractometric data. The carbamate group was found to be rotated out of the benzene ring plane by 15.5°, whilst the piperidine ring is in a more stable chair conformation. The heptyloxy side chain is in a stable all-trans conformation. The structure was refined by the full matrix least-squares method to a final R value 0.1127 for the observed reflections.

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