PubMed Health⌕ Search

Biomedical subjects

B Engels

Publications and source records attributed to B Engels.

At least 19 recordsLinked to original sources
PubMed2004

S=N versus S+-N-: an experimental and theoretical charge density study.

To elucidate the bonding situation in the widely discussed hypervalent sulfur nitrogen species, the charge density distributions rho(r) and related properties of four representative compounds, methyl(diimido)sulfinic acid H(NtBu)(2)SMe (1), methylene-bis(triimido)sulfonic acid H(2)C[S(NtBu)(2) (NHtBu)](2) (2), sulfurdiimide S(NtBu)(2) (3), and sulfurtriimide S(NtBu)(3) (4), were determined experimentally by high-resolution low-temperature X-ray diffraction experiments (T = 100 K). This set of molecules represents an ideal frame of reference for the comparison of SN bonding modes, because they contain short formal S=N double bonds as well as long S-N single bonds, some of them influenced by inter- or intramolecular hydrogen bonds. For comparison, the gas-phase ab initio calculations of the four model compounds, H(NMe)(2)SMe, H(2)C[S(NMe)(2)(NHMe)](2), S(NMe)(2), and S(NMe)(3), were performed. The topological features were found to be not particularly sensitive with respect to different substituents R (R = H, Me, tBu). In this paper, it is documented that theory and experiment differ in the eigenvalues of the Hessian matrix because of systematically differing positions of the bond critical points but agree very well concerning the spatial Laplacian distribution and the distinct polarization of all investigated sulfur-nitrogen bonds. Both recommend the S(+)-N(-) formulation of sulfur nitrogen bonds in 1 and 2 since all nitrogen atoms are found to be sp(3) hybridized. The planar SNx (x = 2, 3) units in the diimide 3 and the triimide 4 reveal characteristics of m-center-n-electron systems. For none of the investigated S-N bonds, a classical double bond formulation can be supported. This is further substantiated by the NBO/NRT approach. Valence expansion to more than eight electrons at the sulfur atom can definitely be excluded to explain the bonding.

Journal Article↗
PubMed2003

Metal partitioning in a sulfidic canal sediment: metal solubility as a function of pH combined with EDTA extraction in anoxic conditions.

The chemical forms of low concentrations of metals (Zn, Pb, Cu, Cd, Co and Mn) and main components (Fe and Ca) were determined under the original reducing conditions of a sulfide-rich sediment from the Gent-Terneuzen Canal (Belgium). Therefore, dissolution experiments as a function of pH were made in a salt solution mimicking the canal water. The centrifugates remaining after the dissolution vs. pH experiments were subsequently further extracted with 0.1 M EDTA/0.5 M NaAc (pH 4.65) to determine eventually readsorbed and/or reprecipitated metal ions. The experimental dissolution vs. pH edge of calcium, iron, manganese and cobalt had a lower slope than theoretically expected on the basis of the solubility of, respectively, calcium carbonate, iron sulfide/iron carbonate, manganese sulfide/manganese carbonate and cobalt sulfide and was explained by the combination of (a) the solubilities of the various minerals and (b) metal readsorption onto clay minerals and organic matter. Higher metal recoveries were measured in the 0.1 M EDTA/0.5 M NaAc mixture and proved that in addition coprecipitate formation with iron sulfide/iron carbonate minerals may occur. The solubility of zinc, lead, cadmium and copper was very low in the mimicking salt solution even at very low pH values (up to pH 1) in agreement with the theoretical solubility of their discrete metal sulfides. However, by using an additional 0.1 M EDTA/0.5 M NaAc extraction on the centrifugates remaining after the dissolution vs. pH experiments, it was qualitatively shown that zinc and lead were partly associated with iron sulfide/iron carbonate phases in the real sediment (in addition to their presence in discrete metal sulfides). Cadmium was present solely in discrete cadmium sulfide phases. It was not possible to verify whether copper was present in discrete copper sulfide phases and/or in mixed coprecipitates with iron sulfide/iron carbonate minerals.

Journal Article↗
PubMed2001

Multiple transcripts generated by the DCAMKL gene are expressed in the rat hippocampus.

We have recently cloned a novel Doublecortin CaMK-like kinase (rDCAMKL) cDNA, and a related cDNA called CaMK-related peptide (CARP) from the rat hippocampus. These genes are structurally highly similar to the human DCAMKL-1 gene and doublecortin, a gene associated with X-linked lissencephaly and subcortical band heterotopia. Here we report on the genomic organization of the murine DCAMKL gene and its products. Our results show that DCAMKL and CARP are alternative splice products of the same gene. The DCAMKL gene also generates three alternatively-spliced rDCAMKL transcripts of which we have cloned the corresponding cDNAs and which potentially generate different DCAMKL proteins. In situ hybridization experiments show that the different rDCAMKL transcripts are all expressed in the adult rat hippocampus. We conclude that alternative splicing of the DCAMKL gene may generate different but similar proteins in the adult rat hippocampus thereby regulating different but overlapping aspects of DCAMKL controlled neuronal plasticity.

Age Factors↗
PubMed2001

An experimental and computational study on the reactivity and regioselectivity for the nitrosoarene ene reaction: comparison with triazolinedione and singlet oxygen.

The regioselectivities and the reactivities (relative rates) for the ene reaction of the enophile 4-nitronitrosobenzene (ArNO) with an extensive set of regiochemically defined acyclic and cyclic olefins have been determined. These experimental data establish that the ArNO enophile attacks the olefinic substrate along the novel skew trajectory, with preferred hydrogen abstraction at the corner (twix regioselectivity). This is in contrast to the isoelectronic species singlet oxygen ((1)O(2)), which abstracts at the higher substituted side of the double-bond (cis effect), and triazolindione (TAD), which undergoes the ene reaction at the more crowded end (gem effect). Ab initio computations (B3LYP/6-31+g) for the ene reaction of the ArNO with 2-methyl-2-butene reveal that the steric effects between the aryl group of the enophile and the substituents of the olefin dictate the skew trajectory. These computations identify the aziridine N-oxide (AI) as a bona fide intermediate in this ene reaction, whose formation is usually rate-determining and, thus, irreversible along the skew trajectory (twix selectivity). The reversible generation of the AI becomes feasible when conformational constraints outweigh steric effects, as manifested by enhanced twin regioselectivity.

Journal Article↗
PubMed2001

The importance of the ene reaction for the C(2)-C(6) cyclization of enyne-allenes.

The present study establishes the ene reaction as a competing reaction mechanism to the diradical mechanism for the thermal C(2)-C(6) cyclization of enyne-allenes which possess bulky substituents at the alkyne terminus. Both reaction routes are found to possess nearly equal free energies of activation. As shown by our computations, primary H/D isotope effects could be used for a definite decision about the mechanism. Concerning the regioselectivity of the cyclization reactions of enyne-allenes our study resolves a long-standing deviation between theoretical results and experimental findings.

Journal Article↗
PubMed2001

Individually selecting multi-reference CI and its application to biradicalic cyclizations.

In the present review a new algorithm to perform individually selecting MR-CI calculations is discussed. The new algorithm exploits the advantages arising from the subdivision of the molecular orbitals (MOs) into an internal and an external part and avoids the recalculation of representation matrices by the use of a specially designed cache. With the new algorithms we are able to perform calculations including more than 10 million selected configurations state functions (CSFs) as a matter of routine. As an example for the possibilities of the new approach the regioselectivity of thermal biradical cyclizations (C2-C7 vs. C2-C6 cyclization) is investigated in the second part of the present work. After studying the accuracy of various quantum chemical methods (MR-CI, density functional theory, and coupled cluster approaches) using (Z)-1,2,4 Heptatriene-6-yne as a model system the influence of substituents (R = H, Ph, t-Bu, NH2) on the regioselectivity is investigated. Our results rationalize the switch between the two biradical cyclizations on the basis of mainly steric (t-Bu) or electronic (Ph) substituent effects. Moreover, the results for R = NH2 predict that the activation energy of the C2-C6 cyclization can be lowered even more. However, a change in the reaction mechanism is found.

Journal Article↗
PubMed2001

Valproate in very young children: an open case series with a brief follow-up.

We report nine cases of juvenile mania, of which six began in the preschool years. We provide validation by clinical description, family history of bipolar disorder, worsening on stimulants, and considerable mood stabilization with divalproex. This is a relatively new area of clinical observation, and systematic studies are needed to firmly establish this diagnostic category in very young children. Our case series enriches the existing scant literature and provide the rationale for the use of mood stabilizers rather than stimulants in this juvenile population. However, no controlled studies exist on the efficacy and safety of valproate in this age group; lithium that has received greater clinical attention, has not been subjected to controlled studies either. Our clinical observations with divalproex are preliminary but encouraging.

Affect↗
PubMed2000

New dyes based on amino-substituted acridizinium salts--synthesis and exceptional photochemical properties

The novel amino-substituted acridizinium salts 5a and 5b represent a new class of cyanine dyes which exhibit intense color along with efficient fluorescence properties. These dyes show moderate solvatochromism in their absorption and emission spectra. The absorption and emission shifts of the two chromophores display a reasonable correlation with solvent parameters such as donor and acceptor number. It was found that the dyes 5a and 5b interact with DNA, with quenching of the band intensies accompanied with a red shift of the absorption and emission bands. Moreover, irradiation of salts 5a and 5b in the presence of DNA results in DNA damage. Solution photolysis of acridizinium salt 5a gave the head-to-tail dimers as the major products (>95%), whereas the acridizinium salt 5b afforded the anti-head-to-tail dimer along with both head-to-head isomers. The latter are thermally labile and rapidly revert to the monomer.

Journal Article↗
PubMed1999

Kainate-elicited seizures induce mRNA encoding a CaMK-related peptide: a putative modulator of kinase activity in rat hippocampus.

By means of differential display techniques, we have previously identified an mRNA transcript whose expression is highly induced in the rat hippocampus by kainate-elicited seizures. Here, we report the cloning of a corresponding cDNA encoding a 55-amino-acid, serine-rich peptide which contains four predicted phosphorylation sites. The peptide was designated CaMK-related peptide (CARP) as it shares significant amino acid sequence identity with part of a novel putative calcium/calmodulin-dependent kinase (CaMK-VI) that was also cloned in this study. It appears that CARP and CaMK-VI are derived from the same gene through differential splicing. Intriguingly, CARP also exhibits 64% amino acid sequence identity with the C-terminal part of human doublecortin, encoded by a recently identified gene which is mutated in patients with X-linked lissencephaly and the double-cortex syndrome. In addition, the structure of CARP resembles the autoinhibitory, serine-rich N-terminal domain of CaMK-IV, suggesting a possible modulatory role of CARP with respect to CaMK activity. Northern blot analysis and in situ hybridization experiments showed that CARP mRNA is specifically induced by kainate-elicited seizures in the dentate gyrus and in the pyramidal layers CA1 and CA2, but not in CA3. In contrast, kainate-induced seizures did not change the level of expression of the CaMK-VI gene. We propose that CARP induction leads to the modulation of kinase activity in specific subregions of the rat hippocampus, providing a negative feedback mechanism for seizure-induced kinases.

Amino Acid Sequence↗
PubMed1998

Microvascular surgery utilizing the endoscope as the sole source of visual assistance.

With the introduction of endoscopy to surgery, it has become apparent that magnification similar to the magnification provided by the surgical microscope can be achieved with its use. Endoscopic techniques provide both magnification and the ability to operate at a distance, potentially increasing the applications of microsurgery. An endoscopic unit is significantly less expensive than the operating microscope. Furthermore, it enables invasive techniques utilizing smaller incisions. The purpose of this project was to investigate whether the visual assistance provided by the endoscope is sufficient to perform a microvascular anastomosis. An initial experiment with six rats is presented. The right femoral artery was isolated, divided, and reconstructed by standard microanastomosis with the visual assistance provided by a 4-mm endoscope. All anastomoses were patent at 7 days by microangiography and histology. The magnification provided by the endoscope is sufficient for the creation of a microvascular anastomosis.

Anastomosis, Surgical↗
PubMed1998

Changes of hemorheological and biochemical parameters after plasma perfusion using a tryptophan-polyvinyl alcohol adsorber leading to clinical improvement in patients suffering from maculopathy.

BACKGROUND: Selective adsorption is an extracorporeal treatment able to reduce high-molecular-weight proteins and lipids. We evaluated its efficacy in lowering hemorheological parameters to achieve a better microcirculation of the retina. PATIENTS AND METHODS: Ten patients suffering from maculopathies of various origin underwent a selective plasma adsorption procedure using the TR-350. Plasma and whole blood viscosity, erythrocyte aggregation and proteins and lipids were determined before and 24 h after therapy. RESULTS: Selective adsorption therapy reduced the high-molecular-weight proteins and lipids. Plasma viscosity, standardized whole blood viscosity and erythrocyte aggregation were significantly lowered to 87, 88 and 65%, respectively, of their values prior to treatment. An improvement of visual acuity was achieved in 6/10 patients. Minor side effects were noted in 2/10 patients. CONCLUSIONS: Selective adsorption using the TR-350 adsorber is a safe technique, showing a high impact on blood rheology. The changes of hemorheological parameters led to clinical improvement in 6/10 patients suffering from retinal disorders.

Blood Viscosity↗
PubMed1997

Ab Initio Study of the Electronic Spectrum of B2H2

The present paper represents the first part of an extensive theoretical study on the electronic spectrum of B2H2. The results of ab initio calculations of the vertical spectrum and the trans- and cis-bending potential curves for the low-lying triplet and singlet electronic states are reported. Special attention is paid to the study of the interaction between valence- and Rydberg-type electronic species. Copyright 1997Academic Press

Journal Article↗
PubMed1997

Ab Initio Study of the Electronic Spectrum of B2H2

The results of ab initio calculations of the potential curves for the torsional motion, symmetric H-B stretching, and B-B stretching in low-lying valence- and Rydberg-character triplet and singlet electronic states of B2H2 are reported. Particularly, the dissociation process B2H2 --> BH + BH out of various electronic states of B2H2 is studied. The nature of binding in B2H2 is discussed in terms of the composition of the electronic wavefunctions. Copyright 1997Academic Press

Journal Article↗
PubMed1995

Characterization of a Schistosoma mansoni cDNA encoding a B-like cyclophilin and its expression in Escherichia coli.

A cDNA encoding a Schistosoma mansoni cyclophilin (SmCyP) has been cloned by polymerase chain reaction amplification using degenerate oligonucleotides based on known conserved cyclophilin (CyP) sequences and by screening an expression cDNA library. The cDNA sequence encodes a 21.5-kDa protein, which shares 59% sequence identity with human CyP B. The SmCyP protein was expressed in Escherichia coli with a hexahistidine affinity tag at its amino terminus and antibodies to the purified (His6)-SmCyP fusion protein were raised in a rabbit. Fractionation of parasite material followed by immunoblot analysis revealed that schistosome CyP is a soluble protein. The N-terminus of the predicted protein contains a hydrophobic region, suggestive of a signal sequence. Accordingly, a recombinant SmCyP protein, lacking the first 23 amino acids was found to share the same gel electrophoretic mobility as the parasite-derived CyP protein, suggesting cleavage of a leader sequence. Hybridization of genomic DNA to a full-length cDNA probe indicates that the SmCyP gene is present as a single copy. Immunohistological experiments in conjunction with confocal scanning laser microscopy and immune electron microscopy show that SmCyP is present in abundance in the adult worm as well as in the schistosomula. The function of CyP in the schistosome is presently unclear, but since its ligand, cyclosporin A, has antischistosomal activity, its function is expected to be a vital one.

Amino Acid Isomerases↗