Revision of the structure assigned to a monoterpene isolated from

Unambiguous synthesis of 2-methyl-3-isopropenylanisole (Image ) and 2-isopropenyl-3-methylanisole (Image ) has led to revision, from (Image ) to (Image ), of the structure assigned to a monoterpene phenol ether isolated from

Aromatic amines and their derivatives. Part 3. The synthesis and crystal structure of 4,4-NN-tetramethyl-NN-dinitroso-2,2-methylenedianiline

The synthesis of 4,4,N,N-tetramethyl-NN-dinitroso-2,2-methylenedianiline (1) by the route p-MeC6H4NH2+ HCHO + OH–(p-MeC6H4NMe)2CH2(7b); (7b)+ acid at 70 °C 4,N-dimethyl-6-(N-methyl-p-toluidinomethyl)aniline (4b); (4b)+ acid at 130 °C 4,4,NN-tetramethyl-2,2-methylenedianiline (3b); (3b)+ HNO2(1), is described. Aspects of the 1H n.m.r. spectra of the above and related compounds are discussed. A crystal-structure analysis of compound (1) shows one of the N-nitroso-groups to be disordered with the endo-form being in preponderance (4 : 1) over the exo-form. The other N-nitroso-group is exclusively exo in the solid state. There is little or no resonance between the benzene ring and the nitroso-group attached to the ring, the two groups being almost perpendicular to each other. In one of the N-nitroso-groups, the nitrogen atom deviates significantly from the plane of the benzene ring to which it is attached. Both amide nitrogen atoms show some pyramidal character.

Molecular structure of nicotinamide adenine dinucleotide

NICOTINAMIDE adenine dinucleotide (NAD) has a fundamental role in metabolic processes as an electron transport molecule. Although its chemical structure was elucidated1 in 1934, its detailed conformation remains still to be established in spite of numerous physicochemical applications2. NAD analogues with a variety of substitutions on the bases are known to retain considerable activity of the natural coenzyme as long as the pyrophosphate diester group has been retained3,4. The geometry of this backbone moiety is therefore indispensable to our understanding of the conformation and function of the coenzyme. We have so far no experimental evidence on this in NAD or any other nucleotide coenzyme molecule. X-ray studies have been possible only on those analogues5,6 where the nicotinamide and adenine rings are linked by a trimethylene bridge. The results are conflicting and it is difficult to use them to provide a structural basis for the NAD molecule itself, particularly as the phosphate backbone is absent from these analogues.

An uncommon nucleotide conformation shown by molecular structure of deoxyuridine-5-phosphate and nucleic acid stereochemistry

CRYSTAL structure determinations of nucleic acid fragments have shown that several of the conformational features found in the monomeric building blocks are also manifested at the nucleic acid level. Stereochemical variations between thymine and uracil nucleotides are therefore of interest as they can provide a structural basis for some of the differences between the conformations of DNA and RNA. X-ray studies have so far not shown any major dissimilarities between these two nucleotide species although the sugar ring of deoxyribonucleotides is found to possess greater flexibility than that in ribonucleotides. We report here the molecular structure of deoxyuridine-5'-phosphate (dUMP-5') which is not a common monomer unit of DNAs as it is replaced by its 5-methyl analogue deoxythymidine-5'-phosphate (dTMP-5'). The investigation was undertaken to help determine whether or not this implied a fundamental difference between the geometries of these two molecules.

Stabilization of the collagen structure by hydroxyproline residues

The molecular structure of collagen is now accepted to be based on a triple-stranded coiled-coil, in which the three strands are held together predominantly by hydrogen bonds. Recent experimental evidence has shown that the presence of hydroxyproline residues in the third position of the repeating tripeptide unit lends additional stability to the collagen structure. In this paper, we report a model structure, which is supported by these observations. In a model structure proposed earlier, there are two hydrogen bonds per tripeptide unit, one of which is a direct interchain hydrogen bond, while the second hydrogen bond can be formedvia a water molecule. It has now been shown that the same water molecule can also form a hydrogen bond with the oxygen of theγ-hydroxyl group of hydroxyproline in the third position in the sequence (Gly-R2-R3). This hydroxyl group can also take part in an inter-triple-helix hydrogen bond. Our studies thus show the role played by hydroxyproline residues in the structure and stability of collagen.

Crystal and molecular structure of allo-4-hydroxy-L-proline dihydrate

allo-4-Hydroxy-L-proline crystallizes from an aqueous solution as the dihydrate. The crystals are orthorhombic, space group P212121, with a=7.08 (2), b=22.13 (3), c= 5"20 (2) A,. The structure was solved by direct methods and refined by block-diagonal least squares. The final R for 733 observed reflexions is 0.054. The molecule exists as a zwitterion with hydroxyl and carboxyl groups cis to the pyrrolidine ring. The latter is puckered at the fl-carbon atom, which deviates by -0.54 A, from the best plane formed by the four remaining atoms. The molecules are held together by a network of hydrogen bonds, the water molecules playing a dominant role in the stability of the structure.

Structure and Conformation of a Cyclic Tripeptide

MANY cyclic peptides have interesting biological functions and the details of their molecular structure and conformation have been the subject of extensive investigations. Cyclic dipeptides such as diketopiperazine have been synthesised and shown to occur with the peptide units in the cis configuration1,2. In the case of a tripeptide, cyclisation can take place only if all three units are in the cis configuration3. In cyclic peptides with four units also, cis peptides are found4,5. As the number of the peptide units increases, the more stable trans configuration is generally more common6,7. We report here the main results of our X-ray crystallographic investigations of the cyclic tripeptides L-Pro-L-Pro-L-Pro and L-Pro-L-Pro-L-Hyp (hereafter called CTP 1 and CTP 2, respectively). CTP 1 was synthesised by Rothe et al. 8 and its derivatives have been prepared by Blout and his collaborators9.

Structure and Bonding in N-Methylacetamide Complexes of Alkali and Alkaline Earth Metals

A detailed crystallographic investigation of N-methylacetamide complexes of Li, Na, K, Mg and Ca has been made in view of its importance in the coordination chemistry and biochemistry of alkali and alkaline earth metals. The metal ions bind to the amide oxygen causing an increase in the carbonyl distance and a proportionate decrease in the central C-N bond distance. The decrease in the central C-N distance is accompanied by an increase in the distance of the adjacent C-C bond and a decrease in the adjacent C-N bond distance. The metal ion generally deviates from the direction of the lone pair of the carbonyl oxygen and also from the plane of the peptide, the out-of-plane deviation varying with the ionic potential of the cation. The metal-oxygen distance in alkali and alkaline earth metal complexes of a given coordination number also varies with the ionic potential of the cation, as does the strength of binding of the cations to the amide. The amide molecules are essentially planar in these complexes, as expected from the increased bond order of the central C-N bond. The NH bonds of the amide are generally hydrogen bonded to anions. The structures of the amide complexes are compared with those of other oxygen donor complexes of alkali and alkaline earth metals. The structural study described here also provides a basis for the interpretation of results from spectroscopic and theoretical investigations of the interaction of alkali and alkaline earth metal cations with amides.

Crystal and molecular structure of 15-bromolongibornane-8,9-dione

The crystal and molecular structure of the title compound (1) has been determined by the heavy-atom method from 1038 observed three-dimensional photographic data. Crystals are orthorhombic, with a = 20.07 ± 0.02, b= 10.05 ± 0.02, c= 7.31 ± 0.01 Å, space group P212121, with Z= 4. The structure was refined by block diagonal leastsquares to R 0.099. The conformation of the norbornane moiety is discussed. The seven-membered ring portion of the molecule adopts an approximate chair conformation. The packing of the molecules in the crystal is mainly a consequence of van der Waals interactions.

Dipole moments and structure of organophosphine and organoarsine compounds

The formal charge distributions in and the dipole moments of some organophosphines and arsines have been calculated, and the dipole moments of (p-chlorophenyl)dichlorophosphine (2.28 D) and (p-bromophenyl)dichlorophosphine (2.04 D) have been determined in benzene at 35° C. The differences between the observed and the calculated moments are explained in terms of dπ---pπ back-bonding and hyperconjugative effects in alkylhaloarsines. The mesomeric effects operating in the aromatic systems are evaluated by comparing the moments with those for the corresponding aliphatic systems. In unsaturated compounds the differences are attributed to mesomeric effects involving the expansion of arsenic valence shell.

Crystal structure of ferroelectric sodium meta vanadate, NaVO3

The crystal structure of ferroelectric sodium meta vanadate, NaVO3 has been solved using three dimensional X-ray data and refined to an R-value of 0.077 for 375 observed reflections. The crystal belongs to the monoclinic system with space group Cc and with unit cell dimensions a = 10.494 (9) Aring, b = 9.434 (7) Aring, c = 5.863 (6) Aring and β = 108° 48' in the room temperature ferroelectric phase. The unit cell dimensions in the high temperature paraelectric phase (above 380°C) are a = 10.595 (15) Aring, b = 9.671 (10) Aring, c = 5.926 (8) Aring and β = 108° 45' with space group C2/c. The crystal structure may be viewed as consisting of alternate channels of sodium polyhedra and VO4 tetrahedra.

Crystal structure from packing studies: application to the triclinic structure of the cyclic hexapeptide cyclo-(gly-tyr-gly)2

The paper describes a novel method of finding the position and orientation of a relatively rigid molecule in the unit cell from criteria concerning allowed contact distances between atoms. On application to the crystal structure of a hexapeptide, C25H31N6O8.2H2O, it was possible to solve the structure from this starting point, by a series of SFLS refinements with an increasingly larger number of reflexions at successive stages. The packing analysis succeeded, even though the water molecules were not included to start with.

Crystal structure and semiconductor-metal transition of the quasi-two-dimensional transition metal oxide, La2NiO4

Abstract is not available.

Neutron-diffraction study of structure and conformation of nucleotide uridine-5 phosphate (disodium salt)

Acta Crystallographica Section A: Foundations of Crystallography covers theoretical and fundamental aspects of the structure of matter. The journal is the prime forum for research in diffraction physics and the theory of crystallographic structure determination by diffraction methods using X-rays, neutrons and electrons. The structures include periodic and aperiodic crystals, and non-periodic disordered materials, and the corresponding Bragg, satellite and diffuse scattering, thermal motion and symmetry aspects. Spatial resolutions range from the subatomic domain in charge-density studies to nanodimensional imperfections such as dislocations and twin walls. The chemistry encompasses metals, alloys, and inorganic, organic and biological materials. Structure prediction and properties such as the theory of phase transformations are also covered.

Hyperfine structure in the ESR spectra of exchange-coupled pairs—copper diethyidithiocarbamate

A study of the hyperfine interaction in the ESR of coupled Cu---Cu pairs in single crystals of copper diethyldithiocarbamate as a function of temperature has shown distinct differences in the hyperfine structure in the two fine-structure transitions at 20 K; the spectrum does not have the usual binomial hyperfine pattern for the fine-structure transition of the low field in contrast to that of the high field. The details of the structure of both fine-structure transitions in the 20-K spectrum can be explained by recognizing the fact that the mixing of the nuclear spin states caused by the anisotropic hyperfine interaction affects the electron spin states |+1 and |−1 differently. The anomalous hyperfine structure is found to become symmetric at 77 and 300 K. It is proposed that the reason for this lies in the dynamics of spin-lattice interaction, which limits the lifetime of the spin states in each of the electronic levels |−1 , |0 , and |+1 . The estimate of spin-lattice relaxation time in the temperature range where the changes are observed agrees with those indicated by other studies. The model proposed here for the hyperfine interaction of pairs in the electronic triplet state is of general validity.