Analysis ofN-amino-imidazolin-2-one peptide turn mimic 4-position substituent effects on conformation by X-ray crystallography

Biopolymers ◽  
2014 ◽  
Vol 102 (1) ◽  
pp. 7-15 ◽  
Author(s):  
Caroline Proulx ◽  
William D. Lubell
Keyword(s):  
Substituent Effects ◽  
X Ray ◽  
X Ray Crystallography ◽  
Turn Mimic

The geometry of 4-(1-ethoxyethylidene)-5-oxazolones and thiazolones: 1H and 13C studies and the crystal structure of 4-[(Z)-1-ethoxyethylidene]-2-phenyl-5-oxazolone

1985 ◽  
Vol 63 (12) ◽  
pp. 3618-3630 ◽  
Author(s):  
R. A. Bell ◽  
R. Faggiani ◽  
C. J. L. Lock ◽  
R. A. McLeod
Keyword(s):  
Crystal Structure ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Side Chain ◽  
Oxazole Ring ◽  
X Ray ◽  
X Ray Crystallography ◽  
Cell Dimensions ◽  
Ring Nitrogen ◽  
E And Z Isomers

A series of E and Z isomers of substituted 4-(1-ethoxyethylidene)-5-oxazolones and thiazolones have been prepared and their 1H and 13C spectra recorded. The vinylic methyl 1H chemical shifts showed minimal differences between E and Z isomers whereas the vinylic OCH21H signals differed by 0.15–0.43 ppm, with the Z isomer being consistently the more deshielded. Both vinylic methyl and OCH2 groups showed different 13C resonances for each isomer, with the Z isomers being the more deshielded. The Z geometry was conclusively defined for one isomer of 4-(1-ethoxyethylidene)-2-phenyl-5-oxazolone, 5, by X-ray crystallography and this was sufficient to assign the geometry of the remaining pairs of E and Z isomers. Oxazolone 5 has the space group P21/n and cell dimensions a = 9.219(3), b = 19.899(5), c = 7.459(1) Å, β = 118.01(2)°, and has four formula units in the unit cell. Intensities were measured with use of MoKα radiation and a Nicolet P3 diffractometer. The crystal structure was determined by standard methods and refined to R1 = 0.0709, R2 = 0.0696 based on 1419 independent reflections. The molecule is essentially planar and most bond lengths and angles are normal. Exceptions are the very short C(olefin)—O(ether) bond (1.339(4) Å) and the large ether C—O—C angle (122.1(3)°) caused by extreme delocalization in the O(ether)CCCO(carbonyl) system. The planarity causes a number of strong intramolecular repulsive interactions, causing an exceptionally small external olefin angle, O(ether)CC(methyl), of 108.1(4)°. The ethoxyl side chain of 5 adopts a conformation in the solid state which places the methylene of the OCH2 group adjacent to the oxazole ring nitrogen. This conformation is proposed to persist in solution phases and is consistent with the observed 13C chemical shifts and known γ and δ substituent effects.

scholarly journals Substituent effects in phosphine ligands

2011 ◽  
Vol 30 (1) ◽  
Author(s):  
C. Alicia Renison ◽  
D. Bradley G. Williams ◽  
Alfred J. Muller
Keyword(s):  
Nmr Spectroscopy ◽  
High Resolution ◽  
Molecular Modelling ◽  
31P Nmr ◽  
Substituent Effects ◽  
Phosphine Ligands ◽  
31P Nmr Spectroscopy ◽  
Electronic Effects ◽  
X Ray ◽  
X Ray Crystallography

The study illustrates the use of the P-atom to evaluate steric and electronic effects in P-containing organic compounds. The work involves the synthesis of substituted triarylphosphines and their corresponding Rh Vaska complexes. High resolution X-ray crystallography, molecular modelling, 31P NMR spectroscopy and IR will be used to quantify substituent effects.

Some Substituent Effects in the Formation of Binuclear Metalladienes and 1,2-Dimetallacycloheptadienones From Reactions Between (η-C5H5)2Rh2(μ-CO)(CF3C2CF3) and Alkynes

1986 ◽  
Vol 39 (8) ◽  
pp. 1187 ◽  
Author(s):  
CW Baimbridge ◽  
RS Dickson ◽  
GD Fallon ◽  
I Grayson ◽  
RJ Nesbit ◽  
...  
Keyword(s):  
Heavy Atom ◽  
Substituent Effects ◽  
Unit Cell ◽  
The Other ◽  
Orthorhombic Space Group ◽  
Methyl Group ◽  
Space Group ◽  
X Ray ◽  
Alternative Pathways ◽  
X Ray Crystallography

The reaction between (η-C5H5)2Rh2(CO)(CF3C2CF3)(1) and alkynes, RC=CR′, can proceed by two alternative pathways. One gives the complexes (η- C5H5)2Rh2{C4(CF3)2RR′CO} (2) in which a pentadienone unit bridges the Rh - Rh bond. The other produces the binuclear metalladiene complexes (η-C5H5)2Rh2{C4(CF3)2RR?} (3) plus the dicarbonyl complex (η-C5H5)2Rh2(CO)2(CF3C2CF3) (4). The formation of (2) is strongly favoured with the dialkylacetylenes BuC≡CBu and MeC =CR (R = Et, Pri, But). With the unsymmetrical acetylenes, two isomers of (2) are isolated. The tendency to form (3) + (4) increases when there are electron-withdrawing substituents such as CN on the alkyne, and becomes dominant when the alkyne is polar (e.g. MeC≡CR with R = Ph, CF3 or CO2Me). The reactions with MeC≡CR are highly regioselective, with only one of the two possible regioisomers being isolated in each case. This has the methyl group in the 3-position of the metalladiene ring. Unexpected products formed in the reactions with MeC≡Cet and MeC≡CPri have been characterized by X-ray crystallography. One is a 1,2- dimetallacycloheptadienone complex, (η-C5H5)2Rh2{C4(CF3)2Me(CMe2OH)CO}. The compound crystallizes with 16 molecules in the orthorhombic space group Fdd2 in a unit cell of dimensions a 31.728(15), b 27.923(14), c 9.275(5) Ǻ. The structure was solved by heavy atom methods and refined to R 0.037 based on 2644 observed reflections. The CMe2(OH) group is adjacent to the carbonyl in the bridging group. The other is a binuclear metalladiene complex (η-C5H5)2Rh2{C4(CF3)2Me( COMe )}. It crystallizes with eight molecules in the orthorhombic space group P bca in a unit cell of dimensions a 17.349(8), b 15.819(8), c 13.720(7) Ǻ. The structure was solved by heavy atom methods and refined to R 0.053 based on 3045 observed reflections above background. The acyl group is adjacent to the metal in the metalladiene ring. These two complexes are formed from (1) and impurities present in the alkynes.

Difference Fourier Analysis of Glucose Embedded and Frozen Hydrated Purple Membrane

1982 ◽  
Vol 40 ◽  
pp. 74-75
Author(s):  
Jules S. Jaffe ◽  
Robert M. Glaeser
Keyword(s):  
Purple Membrane ◽  
Data Set ◽  
High Resolution Data ◽  
X Ray ◽  
X Ray Crystallography ◽  
Fourier Techniques ◽  
Versus Protein ◽  
The Difference ◽  
Difference Fourier ◽  
Ideal Method

Although difference Fourier techniques are standard in X-ray crystallography it has only been very recently that electron crystallographers have been able to take advantage of this method. We have combined a high resolution data set for frozen glucose embedded Purple Membrane (PM) with a data set collected from PM prepared in the frozen hydrated state in order to visualize any differences in structure due to the different methods of preparation. The increased contrast between protein-ice versus protein-glucose may prove to be an advantage of the frozen hydrated technique for visualizing those parts of bacteriorhodopsin that are embedded in glucose. In addition, surface groups of the protein may be disordered in glucose and ordered in the frozen state. The sensitivity of the difference Fourier technique to small changes in structure provides an ideal method for testing this hypothesis.

3-D reconstruction of molecular shape from microcrystal thin sections

1983 ◽  
Vol 41 ◽  
pp. 748-749
Author(s):  
S. Cusack ◽  
J.-C. Jésior
Keyword(s):  
Three Dimensional ◽  
Molecular Shape ◽  
Biological Molecules ◽  
Fourier Space ◽  
Single Particles ◽  
Thin Sections ◽  
Standard Methods ◽  
X Ray ◽  
X Ray Crystallography ◽  
Dimensional Reconstruction

Three-dimensional reconstruction techniques using electron microscopy have been principally developed for application to 2-D arrays (i.e. monolayers) of biological molecules and symmetrical single particles (e.g. helical viruses). However many biological molecules that crystallise form multilayered microcrystals which are unsuitable for study by either the standard methods of 3-D reconstruction or, because of their size, by X-ray crystallography. The grid sectioning technique enables a number of different projections of such microcrystals to be obtained in well defined directions (e.g. parallel to crystal axes) and poses the problem of how best these projections can be used to reconstruct the packing and shape of the molecules forming the microcrystal.Given sufficient projections there may be enough information to do a crystallographic reconstruction in Fourier space. We however have considered the situation where only a limited number of projections are available, as for example in the case of catalase platelets where three orthogonal and two diagonal projections have been obtained (Fig. 1).

Electron microscopy of rabbit FC crystals

1983 ◽  
Vol 41 ◽  
pp. 730-731
Author(s):  
Robert A. Grant ◽  
Laura L. Degn ◽  
Wah Chiu ◽  
John Robinson
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
High Resolution Electron Microscopy ◽  
Molecular Replacement ◽  
X Ray ◽  
X Ray Crystallography ◽  
Resolution Electron ◽  
Replacement Technique ◽  
Hydrated Crystal ◽  
Molecular Structure Analysis

Proteolytic digestion of the immunoglobulin IgG with papain cleaves the molecule into an antigen binding fragment, Fab, and a compliment binding fragment, Fc. Structures of intact immunoglobulin, Fab and Fc from various sources have been solved by X-ray crystallography. Rabbit Fc can be crystallized as thin platelets suitable for high resolution electron microscopy. The structure of rabbit Fc can be expected to be similar to the known structure of human Fc, making it an ideal specimen for comparing the X-ray and electron crystallographic techniques and for the application of the molecular replacement technique to electron crystallography. Thin protein crystals embedded in ice diffract to high resolution. A low resolution image of a frozen, hydrated crystal can be expected to have a better contrast than a glucose embedded crystal due to the larger density difference between protein and ice compared to protein and glucose. For these reasons we are using an ice embedding technique to prepare the rabbit Fc crystals for molecular structure analysis by electron microscopy.

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