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  • 1
    Electronic Resource
    Electronic Resource
    A comparison of molecular orbital and crystal field calculations of ferric heme compounds (1970)
    Springer
    Theoretical chemistry accounts 17 (1970), S. 18-34 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung In dieser Arbeit werden MO- und Kristallfeldrechnungen einiger Eigenschaften von fünffach koordinierten Fe-Häm-Proteinen verglichen. Insbesondere haben wir am Beispiel des Hämins folgende Größen verglichen: Energien von einem einzigen Orbital, Elektronenabstoßungsenergien, Energieterme, Elektronenbesetzung der Eisen-d-Orbitale, atomare Nettoladungen und den elektrischen Feldgradient am Eisenkern, der auf Grund zweier Modelle berechnet wurde. Kleinere Energietermintervalle zwischen den niedrig liegenden Sextett-, Dublett- und Quartettzuständen sowie ein elektrischer Feldgradient mit entgegengesetzten Vorzeichen und dreifachem Betrag scheinen durch die MO-Rechnung vorausgesagt zu werden. Diese und andere Resultate werden eingehender diskutiert.
    Abstract: Résumé Comparaison de calculs d'orbitales moléculaires et de champ cristallin pour quelques propriétés de protéines à hème ferrique pentacoordiné. En particulier, l'hémine étant prise comme exemple, nous avons comparé les quantités suivantes: énergies orbitales, énergies de répulsion électronique, termes énergétiques, population électronique des orbitales d de Fe, charges atomiques nettes et gradient du champ électrique au noyau Fe, calculés selon les deux modèles. Les calculs en orbitales moléculaires prédisent des termes d'intervalle d'énergie plus faibles entre les plus bas états sextet, doublet et quartet, ainsi qu'un gradient du champ électrique trois fois trop grand et de signe opposé. Ces résultats et certains autres sont discutés en détail.
    Notes: Abstract In this paper molecular orbital and crystal field calculations of some properties of five coordinated ferric heme proteins are compared. In particular, choosing hemin as an example, we have compared the following quantities: single orbital energies, electron repulsion energies, term energies, electron population of the Fe d orbitals, net atomic charges and the electric field gradient at the Fe nucleus calculated from the two models. Smaller term energy intervals between low lying sextet, doublet and quartet states, and an electric field gradient of opposite sign and three times the magnitude appear to be predicted from the MO calculation. These and other results are discussed in some detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526761
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  • 2
    Electronic Resource
    Electronic Resource
    Energy-conformation studies of hydrogen cyanide tetramer: A prebiotic precursor (1972)
    Springer
    Theoretical chemistry accounts 27 (1972), S. 273-280 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Mit Hilfe der iterativen erweiterten Hückel Methode (IEHT) haben wir Energie-Konformations Untersuchungen an den beiden Isomeren der HCN-Tetrameren, das als Purinvorläufer in vorbiotischen Synthesen bekannt ist, durchgeführt. Unsere Resultate zeigen, daß dascis-Isomere stabiler als dastrans-Isomere und daß für beide die planere Konformation bevorzugt ist. Diese Ergebnisse weichen stark von der mit Hilfe der Röntgenstrukturanalyse bestimmten Kristallstruktur ab. Außerdem wurden Ionisationsenergie, Dipolmoment,π-Elektronenverteilung und optische Spektren des Tetrameren berechnet. Für alle wurde gute Übereinstimmung mit experimentellen Ergebnissen gefunden.
    Notes: Abstract Using the Iterative Extended Huckel Method (IEHT), we have made energy-conformation studies of the two isomers of hydrogen cyanide tetramer, a known purine precursor in prebiotic syntheses. Our results show that the cis isomer is more stable than thetrans and that, for both, a totally planar conformation is most favorable, differing appreciably from the crystal structure determined by X-ray analysis. The ionization potential, dipole moment,π electron distribution and optical spectra for the tetramer have also been calculated. For all of these properties we find good agreement with experimental results Thus encouraged by the reliability of the description obtained, we plan to use these results for further studies of the chemical reactivity of the tetramer relevant to its postulated role in prebiotic syntheses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00868853
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  • 3
    Electronic Resource
    Electronic Resource
    Calculated properties of the active site complex of oxidized rubredoxins (1974)
    Springer
    Theoretical chemistry accounts 33 (1974), S. 125-136 
    Details
    ISSN: 1432-2234
    Keywords: Electronic structure of oxidized rubredoxins ; Rubredoxin (oxidized) ; Iron-sulfur Proteins
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract In rubredoxins, the simplest class of iron-sulfur, electron-transport proteins, the center of biological activity is a single Fe atom which is surrounded by a distorted tetrahedral array of four cysteine sulfur atoms. In addition to its electron-transport characteristics, a number of physical properties of these proteins have been determined, all of which depend crucially on the conformation and electron distribution and energy of the active site complex. Thus, in this initial study, using a semi-empirical MO program called Iterative Extended Hückel Theory (IEHT), we have calculated the variation in electron distribution and energies of the Fe-S active site complex as a function of its conformation. Ten conformers were chosen which span a symmetry range from that determined by X-Ray crystal structure analysis to an idealized regular tetrahedral symmetry. The differences in electron distribution and electronic energies obtained for these 10 conformers are reflected in all observable properties of the protein calculation of these properties, using the results reported here, are the subject of our subsequent papers in this series.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526618
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular orbital calculations of the active site complex of two iron ferredoxins (1972)
    Springer
    Theoretical chemistry accounts 26 (1972), S. 107-130 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung In der vorliegenden Arbeit werden weitere Resultate der Untersuchung eines Modells des an der aktiven Stelle gebildeten Komplexes von zwei Eisen-ferredoxinen mitgeteilt, die mit Hilfe der iterativen erweiterten Hückel-Methode gewonnen wurden. In der ersten Veröffentlichung dieser Reihe wurden hauptsächlich die Ergebnisse der Berechnungen von Konformationsenergien angegeben, die dazu beitrugen, die wichtigsten chemischen Eigenschaften sowie die Konformationen solcher Komplexe mit der niedrigsten Energie zu ermitteln. Hier werden nun die Eigenschaften der 70 niedrigsten Molekülorbitale des Modells, eine Berechnung des magnetischen Moments bei Raumtemperatur und des elektrischen Feldgradienten am Fe-Kern sowie die Zuordnung des optischen Absorptionsspektrums in der Einelektronennäherung für die Konformation niedrigster Energie des oxydierten Zustandes dieser Proteine veröffentlicht. Die Ergebnisse werden soweit möglich mit dem Experiment und früheren Interpretationen verglichen. Zusätzlich wird auf die Empfindlichkeit dieser Eigenschaften bezüglich einer Variation der Konformation in der Nähe des Fe-Atoms hingewiesen.
    Notes: Abstract In this report we present further results of molecular orbital calculations for a model of the active site complex of two iron ferredoxins, using the Iterative Extended Hückel Method for our calculations. In the first paper of this series we dealt mainly with energy-conformation calculations which helped establish the salient chemical and conformational features of the lowest energy forms for such a complex. Presented here are the nature and energy ordering of the 70 molecular orbitals in the active site model, a calculation of room temperature magnetic moment, of the electric field gradient at the Fe nucleus and an assignment of the optical absorption spectra in the one electron approximation for the lowest energy conformer of the oxidized state of these proteins. For each property, we compare our calculated results with experiment whenever possible, and with the results of previous correlations. In addition, we indicate the sensitivity of these properties to variations in conformation about the Fe atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526539
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  • 5
    Electronic Resource
    Electronic Resource
    Calculated properties of the active site complex of oxidized rubredoxins (1974)
    Springer
    Theoretical chemistry accounts 33 (1974), S. 137-145 
    Details
    ISSN: 1432-2234
    Keywords: Rubredoxins, oxidized ; Iron-sulfur proteins ; Mössbauer resonance ; Hyperfine interactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The active site of rubredoxins, the simplest class of iron-sulfur, electron-transfer proteins consists of a single Fe atom surrounded by a distorted tetrahedral array of four cysteine sulfur atoms. With the aid of a newly formulated computer program, we have calculated the electric field gradient at the Fe57 nucleus, the resultant quadrupole splitting (ΔE Q) and the isotropic and anisotropic parts of the hyperfine interaction between the Fe nuclear spin and the net electron spin in the sextet ground state for ten conformational variations of the active site complex. For each conformer we used an electron distribution obtained from an Iterated Extended Huckel Theory (IEHT) molecular orbital calculation. Comparison of calculated and experimental results of (ΔE Q) obtained from Mossbauer resonance spectra indicated that a number of conformers have field gradient values in excellent agreement with experiment. Good agreement with experiment was also found for the calculated hyperfine coupling constant. In this interaction, the isotropic contribution is dominant while the anisotropic contribution is more symmetry dependent. Since a single average experimental value is observed, hyperfine interaction in this system is not a very sensitive probe of active site conformation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526619
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  • 6
    Electronic Resource
    Electronic Resource
    Calculated properties of the active site of oxidized rubredoxins (1974)
    Springer
    Theoretical chemistry accounts 33 (1974), S. 147-156 
    Details
    ISSN: 1432-2234
    Keywords: Zero field splittings ; Magnetic moments ; Electron spin resonance spectra ; Rubredoxins (oxidized)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract For a molecular model of the Fe-S active site complex in oxidized rubredoxin, we have calculated the spin-orbit coupling between the ground sextet state and excited quartet and doublet states which gives rise to the observed zero field splitting of the sextet ground state into three spin-mixed Kramers doublets. Additionally, we have used the six spin-mixed sextet state components to calculate effective magnetic moments, magnetic field energies and nine g values corresponding to transitions between the three pairs of Kramers doublets in applied magnetic fields along three perpendicular axes. We have calculated these properties for eight conformational variations of the ligands around the Fe at the active site. The results of these calculations clearly show the origin of the observed g=4.3 signal previously described only in terms of the phenomenological spin-Hamiltonian formalism. For the eight conformations considered, five have this characteristic signal. Zero field splitting comparable to the observed values could be obtained for all symmetries studied. In addition, the calculated values of magnetic moment in all symmetries correspond to that of high spin ferric ion and do not vary appreciably with temperature above 77° K, in agreement with experimental results. From comparison of all our calculated results with experiment, it appears that the active site in oxidized rubredoxins could have small conformational variations in different rubredoxins and under the various experimental conditions used.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526620
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  • 7
    Electronic Resource
    Electronic Resource
    A theoretical study of the electromagnetic properties of ferrocene and the ferrocenium ion (1976)
    Springer
    Theoretical chemistry accounts 41 (1976), S. 1-6 
    Details
    ISSN: 1432-2234
    Keywords: Ferrocene ; Ferrocenium ion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An iterative extended Hückel molecular orbital calculation was used to obtain wave functions for the ground state of ferrocene and several low lying states of the ferrocenium ion. Photoemission spectra in terms of relative ionization potentials and the electric field gradient at the iron nucleus were calculated for ferrocene andg values and the electric field gradient were calculated for the ferrocenium ion. These values are in good agreement with experiment. The nature of the molecular orbitals was compared with results of previous semi-empirical andab initio calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00558019
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  • 8
    Electronic Resource
    Electronic Resource
    Energy-conformation studies of HCN, HNC and CN−: A comparison of results from EH-SCC and SCF molecular orbital calculations (1971)
    Springer
    Theoretical chemistry accounts 20 (1971), S. 203-215 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Rechnungen nach der erweiterten Hückeltheorie werden für HCN, HNC und CN− durchgeführt und mit ab initio Resultaten verglichen. Im einzelnen wurden Gesamtenergie, Bindungsenergie in Abhängigkeit von der geometrischen Struktur, Ionisierungspotential und Dipolmoment von HCN berechnet und außerdem die Energie für zwei doppelt angeregte Konfigurationen in Abhängigkeit vom Bindungswinkel bestimmt. Darüber hinaus sind MO-Rechnungen für HNC und CN− gemacht worden.
    Notes: Abstract We have made an Extended Hückel Self Consistent Charge (EH-SCC) molecular orbital calculation for hydrogen cyanide, hydrogen isocyanide and cyanide ion. The main purpose of this calculation was to compare the EH-SCC and the more accurate SCF MO calculations for HCN in order to evaluate the method we used here for future use. Specifically, we have calculated and compared the following properties of HCN: total energy, binding energy, variation of ground state energy with geometric conformation, ionization potential and dipole moment. In addition, we have extended previous calculations of HCN by also considering its energy variation with bond angle for two excited state configurations and deducing some of the characteristics of its electronic spectra. Finally we have also made an MO calculation of the isocyanide isomer HNC and CN− ion to compare with and add to the known characterization of the H, C, N, system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00528547
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  • 9
    Electronic Resource
    Electronic Resource
    Molecular orbital calculations of the active site complex of two iron ferredoxins (1971)
    Springer
    Theoretical chemistry accounts 23 (1971), S. 239-258 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Die vorliegenden Ergebnisse stellen den ersten Teil einer systematischen theoretischen Untersuchung einiger physikalischer und biochemischer Eigenschaften des Fe2-Ferredoxins mit Hilfe einer selbstkonsistenten erweiterten Hückelmethode dar. In diesem ersten Teil wird besonders die elektronische Energie in Abhängigkeit von der Molekülgeometrie und der Art der Liganden an der aktiven Stelle untersucht, um die stabilste Form des Komplexes der aktiven Stelle zu finden. Dieser Komplex enthält zwei Eisenatome, zwei saure, nicht fest gebundene Schwefelatome unbekannter anorganischer Herkunft sowie vier Schwefelatome, vermutlich aus nahegelegenen Cysteinresten. Bei den 15 untersuchten Konformationen der Schwefelliganden wurden Änderungen der Fe-Fe-Abstände, Bindungslängen und -winkel sowie chemische Variationen wie die Einwirkung der axialen Liganden, der Disulfidbindungen und zusätzlicher Protonen berücksichtigt. Es ergibt sich, daß das Eisen vierfach tetraedrisch koordiniert ist und nicht planar, selbst bei der Addition axialer Liganden. In diesem Modell ergeben sich Wechselwirkungen zwischen den beiden Eisenatomen zwanglos, da sie zum gleichen aktiven Komplex gehören. Dadurch wird das Fehlen eines Tieftemperatur-Paramagnetismus im oxydierten Zustand hinreichend erklärt. Eine vorläufige Untersuchung des reduzierten Zustandes mit einem zusätzlichen Elektron zeigt, daß dieses delokalisiert ist, genau wie es mit ESR und ENDOR beobachtet wird. seine Gegenwart destabilisiert offenbar alle MO-Energien, was in Übereinstimmung mit der Beobachtung steht, daß nur ein Elektron von diesen Proteinen addiert werden kann, ohne sie zu zerstören.
    Notes: Abstract The results presented here are the first part of a systematic theoretical study of some of the physical and biochemical properties of two iron ferredoxins obtained by the use of an Extended Hückel Self-Consistent Charge iteration method of molecular orbital calculations. In this initial study, attention is focused on the calculation of electronic energies as a function of molecular geometry and the nature of the bonding ligands at the active site in order to determine the most stable form of the active site complex. Included in the active site complex are two iron atoms, two acid labile sulfur atoms of unknown inorganic origin and four sulfur atoms presumably from nearby cysteine residues. Fifteen chemical-conformational variations of this basic active site complex were considered. Among these conformational variations of the sulfur ligands, Fe-Fe distances, bond lengths and angles and chemical variations such as the effect of axial ligands, disulfide bonds and added protons were included. Our results indicate that that with all reasonable variations of the ligands, the preferred molecular geometry about 4-coordinated Fe is tetrahedral rather than planar. The planar conformation is somewhat stabilized by the addition of axial ligands, but is still less favorable than the tetrahedral conformation. In this model, interactions between the two iron atoms occur automatically since they are both part of the same active site complex. Hence the absence of low temperature paramagnetism in the oxidized state is readily explained. Preliminary investigations of the reduced state with one additional electron indicate that the odd electron is delocalized, as observed in both ESR and ENDOR. Its presence apparently substantially destabilizes all of the molecular orbital energies in accord with the observation that only one electron can be added to these proteins without decomposing them.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00530234
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular orbital calculation of hyperfine interactions, electric field gradient and quadrupole splitting in reduced rubredoxins (1974)
    Springer
    Theoretical chemistry accounts 32 (1974), S. 217-226 
    Details
    ISSN: 1432-2234
    Keywords: Hyperfine interaction ; Electric field gradient ; Quadrupole splitting ; Rubredoxin (reduced)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The active site of rubredoxins, the simplest class of iron-sulfur, electron-transfer proteins consists of a single Fe atom surrounded by a distorted tetrahedral array of four cysteine sulfur atoms. In a previous paper on the oxidized rubredoxins, we calculated the electric field gradient at the Fe57 nucleus, the resultant quadrupole splitting and the hyperfine field due to interaction between the Fe nucleus spins and the net electron spin of the molecular complex. In this paper, we are going to present our calculated results of the same set of properties for the reduced state, using the molecular orbitals similarly obtained from an Iterated Extended Hückel calculation. Significant contribution of the anisotropy to the hyperfine field as well as the sign of the electric field gradient, and hence the quadrupole splitting allow us to pin down conformations which give the best agreement with the experimental results. In fact, the conformation (conformer A) which represents a small movement of one of the S atoms from its crystal structure position in such a way that pairs of S atoms lie in ⊥ planes yield remarkably good results for all the physical properties calculated in both oxidation states. This consistency also suggests that there is no appreciable conformational change through the oxidation process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00527477
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