Search results for " density"

Human neuroglobin: crystals and preliminary X-ray diffraction analysis

2002

Neuroglobin, a recently discovered member of the haemoglobin superfamily, is primarily expressed in the brain of humans and other vertebrates, where it has been proposed to enhance O(2) supply in response to hypoxia or ischaemia, protecting the neuron from hypoxic injury. Neuroglobin is the first example of a vertebrate haemoglobin in which a hexacoordinate haem geometry has been detected. A triple mutant (replacing three Cys residues) of human neuroglobin (151 amino acids) has been expressed in Escherichia coli, purified and crystallized in two crystal forms, the best of which diffracts to 1.95 A resolution using synchrotron radiation. The crystals belong to space group P2(1), with unit-ce…

Small-Angle Scattering Techniques for Analyzing Conformational Transitions in Hemocyanins

2004

Publisher Summary The precise delivery of oxygen from respiratory surfaces to the tissues is mediated by cooperative and allosterically regulated carrier proteins, such as hemoglobin or hemocyanin. To establish cooperativity, these proteins must be able to adopt different conformations. These conformations are characterized by different ligand affinities, which have their basis in different structures as is the case for the deoxy and oxy states of human hemoglobin. To understand the cooperative interaction of these molecules at the molecular level, the structures of these conformations must be resolved and the transitions between them must be monitored. Because of the nature of sample prepa…

A Study of the Effects of the Lewis Acid Catalysts on Oxa‐Diels‐Alder Reactions through Molecular Electron Density Theory

2020

Direct evaluation of the electron density correlation function of partially crystalline polymers

1980

A discussion of the general properties of the one-dimensional electron density correlation function K(z) of a partially crystalline polymer with lamellar structure shows that application of a graphical extrapolation procedure permits direct determination of the crystallinity, the specific inner surface, and the electron density difference ηc − ηa. The procedure is based upon the occurrence of a straight section in the “self-correlation” range of K(z). Curved and nonparallel lamellae do not invalidate the concept. In the case of heterogeneous samples composed of partially crystalline and totally amorphous regions, some of the parameters of the experimentally obtained correlation function, as…

A new model for C–C bond formation processes derived from the Molecular Electron Density Theory in the study of the mechanism of [3+2] cycloaddition …

2016

Abstract The [3+2] cycloaddition (32CA) reactions of the nitrile ylide (NY) with ethylene and with dicyanoethylene (DCE) have been studied using the Molecular Electron Density Theory through DFT calculations at the MPWB1K/6-31G(d) level. The analysis of the electronic structure of NY indicates that it presents a carbenoid structure with an sp 2 lone pair at the carbon atom. While the 32CA reaction with ethylene presents a low activation energy, 6.1 kcal mol −1 , the transition state structure associated with the 32CA reaction of NY with DCE is located 7.5 kcal mol −1 below the reagents, the reaction being completely regioselective. The topological analysis of the Electron Localisation Funct…

An MEDT study of the carbenoid-type [3 + 2] cycloaddition reactions of nitrile ylides with electron-deficient chiral oxazolidinones.

2016

The molecular mechanism of the carbenoid-type [3 + 2] cycloaddition (32CA) reactions of a nitrile ylide (NY) with a non-chiral and a chiral oxazolidinone has been studied within Molecular Electron Density Theory (MEDT) at the MPWB1K/6-31G(d) computational level. Topological analysis of the Electron Localisation Function (ELF) of the NY shows that it presents a carbenoid structure. The high nucleophilic character of the NY together with the electrophilic character of the non-chiral oxazolidinone favour a polar 32CA reaction with a very low activation energy, the reaction being completely meta/endo selective. A Bonding Evolution Theory (BET) study of the molecular mechanism makes it possible …

[3+2] Cycloaddition Reaction of C -Phenyl-N -methyl Nitrone to Acyclic-Olefin-Bearing Electron-Donating Substituent: A Molecular Electron Density The…

2018

A new C–C bond formation model based on the quantum chemical topology of electron density

2014

ELF topological analyses of bonding changes in non-polar, polar and ionic organic reactions involving the participation of CC(X) double bonds make it possible to establish a unified model for C–C bond formation. This model is characterised by a C-to-C coupling of two pseudoradical centers generated at the most significant atoms of the reacting molecules. The global electron density transfer process that takes place along polar and ionic reactions favours the creation of these pseudoradical centers at the most nucleophilic/electrophilic centers of the reacting molecules, decreasing activation energies. The proposed reactivity model based on the topological analysis of the changes in electron…

ChemInform Abstract: A New C-C Bond Formation Model Based on the Quantum Chemical Topology of Electron Density

2014

ELF topological analyses of bonding changes in non-polar, polar and ionic organic reactions involving the participation of CC(X) double bonds make it possible to establish a unified model for C–C bond formation. This model is characterised by a C-to-C coupling of two pseudoradical centers generated at the most significant atoms of the reacting molecules. The global electron density transfer process that takes place along polar and ionic reactions favours the creation of these pseudoradical centers at the most nucleophilic/electrophilic centers of the reacting molecules, decreasing activation energies. The proposed reactivity model based on the topological analysis of the changes in electron…

Approaching an experimental electron density model of the biologically active trans ‐epoxysuccinyl amide group—Substituent effects vs. crystal packing

2017

The trans-epoxysuccinyl amide group as a biologically active moiety in cysteine protease inhibitors such as loxistatin acid E64c has been used as a benchmark system for theoretical studies of environmental effects on the electron density of small active ingredients in relation to their biological activity. Here, the synthesis and the electronic properties of the smallest possible active site model compound are reported to close the gap between the unknown experimental electron density of trans-epoxysuccinyl amides and the well-known function of related drugs. Intramolecular substituent effects are separated from intermolecular crystal packing effects on the electron density, which allows us…