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Molecular Structure Determination (molecular + structure_determination)

Distribution by Scientific Domains

Chemistry	80%

Selected Abstracts

Molecular structure determination of Ni(II) diimine complex and DMA analysis of Ni(II) diimine-based polyethenes ,

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2001
J. O. Liimatta
Abstract Dynamic mechanical thermoanalysis showed that polyethene, prepared under suitable polymerization conditions with the Brookhart-type catalyst dibromo- N,N,-1,2-acenaphthylenediylidenebis[2,6-bis(1-methylethyl)benzeneamine]Ni(II)/methylaluminoxane (MAO), behaved like an elastomer, even though no comonomer was added. A structural characterization showed that the polymers contained methyl to hexyl branches and some longer branches. The effect of the polymerization conditions on branching was investigated through variations in the pressure and temperature of the polymerization. Depending on the degree and type of branching, polyethene was either quite amorphous or highly crystalline with a high melting temperature. The solid-state structure of the catalyst dibromo- N,N,-1,2-acenaphthylenediylidenebis[2,6-bis(1-methylethyl)benzeneamine]Ni(II) consisted of two centrosymmetrically related monomeric moieties, where Ni atoms were bridged by two bromide ligands. The Ni atom was five-coordinated, with a square pyramidal coordination polyhedron. The sixth coordination site of the octahedral geometry was effectively blocked by the isopropyl groups of the 2,6-C6H3(i -Pr) substituents of the diimine ligand. In solution in the presence of MAO, the longer bridging NiBr bonds broke, and the complex dissociated to a monomeric species. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1426,1434, 2001 [source]

Enhancement of bound-state residual dipolar couplings: Conformational analysis of lactose bound to Galectin-3

PROTEIN SCIENCE, Issue 7 2006
Tiandi Zhuang
Abstract Residual dipolar couplings (RDCs) have proven to be a valuable NMR tool that can provide long-range constraints for molecular structure determination. The constraints are orientational in nature and are, thus, highly complementary to conventional distance constraints from NOE data. This complementarity would seem to extend to the study of the geometry of ligands bound to proteins. However, unlike transferred NOEs, where collection, even with a large excess of free ligand, results in measurements dominated by bound contributions, RDCs of exchanging ligands can be dominated by free-state contributions. Here we present a strategy for enhancement of RDCs from bound states that is based on specifically enhancing the alignment of the protein to which a ligand will bind. The protein is modified by addition of a hydrophobic alkyl tail that anchors it to the bicelles that are a part of the ordering medium needed for RDC measurement. As an illustration, we have added a propyl chain to the C terminus of the carbohydrate recognition domain of the protein, Galectin-3, and report enhanced RDCs that prove consistent with known bound-ligand geometries for this protein. [source]

Renaissance in chiroptical spectroscopic methods for molecular structure determination

THE CHEMICAL RECORD, Issue 2 2007
Prasad L. Polavarapu
Abstract Two of the chiroptical spectroscopic methods, namely optical rotatory dispersion (ORD) and electronic circular dichroism (ECD), have been around for several decades. But their use in determining the absolute configuration and predominant conformation is gaining renewed interest with the availability of quantum mechanical methods for predicting ORD and ECD. Two other methods, namely vibrational circular dichroism (VCD) and vibrational Raman optical activity (VROA), are relatively new and offer convenient approaches for deducing the structural information in chiral molecules. With the availability of quantum mechanical programs for predicting VCD and VROA, these methods have attracted numerous new researchers to this area. This review summarizes the latest developments in these four areas and provides examples where more than one method has been used to confirm the information obtained from individual methods. © 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 125,136; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20117 [source]

Determining Molecular Structures and Conformations Directly from Electron Diffraction using a Genetic Algorithm

CHEMPHYSCHEM, Issue 2 2006
Scott Habershon Dr.
Abstract A global optimization strategy, based upon application of a genetic algorithm (GA), is demonstrated as an approach for determining the structures of molecules possessing significant conformational flexibility directly from gas-phase electron diffraction data. In contrast to the common approach to molecular structure determination, based on trial-and-error assessment of structures available from quantum chemical calculations, the GA approach described here does not require expensive quantum mechanical calculations or manual searching of the potential energy surface of the sample molecule, relying instead upon simple comparison between the experimental and calculated diffraction pattern derived from a proposed trial molecular structure. Structures as complex as all- trans retinal and p -coumaric acid, both important chromophores in photosensing processes, may be determined by this approach. In the examples presented here, we find that the GA approach can determine the correct conformation of a flexible molecule described by 11 independent torsion angles. We also demonstrate applications to samples comprising a mixture of two distinct molecular conformations. With these results we conclude that applications of this approach are very promising in elucidating the structures of large molecules directly from electron diffraction data. [source]

N - n -Alkyl N,N -dimethylammonioacetic acid bromides: the first complete series of crystal and molecular structure determinations of an amphiphilic compound with alkyl chain lengths n = 1,..., 16

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2000
Rainer Rudert
The molecular and crystal structures of 16 N - n -alkyl N,N -dimethylammonioacetic acid bromides with chain lengths between n = 1 and n = 16 have been determined. All compounds from n = 5 to n = 16 form bilayers with interdigitated chains. The even-numbered chains display the chain packing type M2II. The chain packing of the odd-numbered chain compounds is less regular. The head groups of all compounds are connected via electrostatic N+,Br, interactions, and by OH,Br, hydrogen bonds. The compounds with short chains are packed in different ways. Their molecular conformation depends on the crystal packing. [source]