69 search hits
-
Facultative butterfly-ant interactions - the role of variation in composition of nectar secretions
(2004)
-
Holger Daniels
- The significance of variation in nectar secretions of facultatively ant-associated lycaenid butterfly larvae was investigated. The strongly myrmecophilous European Polyommatus coridon, and two moderately myrmecophilous species, the Palaearctic P. icarus and the subtropical Zizeeria knysna were used. Both Polyommatus species are closely related, Z. knysna is a far more distant relative. To obtain high numbers of caterpillars a new method for breaking the egg diapause of the univoltine P. coridon was established, resulting in 65% subitaneously hatching larvae. Based on observations of ants tending caterpillars a new “artificial ant” was assembled, allowing further studies on the ants’ stimulatory antennation pattern. Nectar-harvesting with micro-capillaries from caterpillars attended by ants was optimised and allowed determination of individual secretion droplet size based on large samples. Mean droplet size was 3.7nl in P. coridon, 2nl in P. icarus and 1.4nl in Z. knysna, in the latter two species 65-79% smaller than previously reported. Comparative chemical analyses (HPLC) revealed sucrose as main sugar component in nectar of all three species. In P. coridon it was accompanied by glucose and rarely by further sugars, but never by melezitose. In P. icarus and Z. knysna melezitose was the second most-important component, followed by fructose and glucose. Total sugar contents were 43.6±14.8g/l for P. coridon, 74.2g/l for P. icarus and 68.3±22.6g/l for Z. knysna. P. coridon nectar contained up to 14 amino acids. Major component was always leucine (50% of total), further important were tyrosine, proline, arginine, and phenylalanine. P. icarus nectar comprised up to six amino acids, dominated by tyrosine and phenylalanine. Z. knysna nectar contained only alanine and proline. Total amino acid contents were 9.7±3.4g/l for P. coridon, 1.2g/l for P. icarus and 0.3±0.2g/l for Z. knysna. Nectar composition was considerably different from hemolymph composition. Larval food had minor influence on P. coridon nectar composition. Caterpillars fed with semi-synthetic diet secreted more sucrose, with a trend towards higher total sugar content, and produced nectar with a more homogeneous amino acid pattern than larvae reared on natural host plants. Bioassays with ants from three different subfamilies (Lasius niger (Formicinae), Myrmica rubra (Myrmicinae), Tapinoma melanocephalum (Dolichoderinae)) demonstrated a preference for sucrose (standard concentration 0.1mol/l, as in P. coridon nectar) over monosaccharides. Melezitose in nectar concentration (P. icarus) was not preferred to sucrose. Some single amino acids in sucrose solutions were preferred over pure sucrose, e.g. leucine by L. niger, or phenylalanine and tyrosine by M. rubra. In general, raising amino acid concentration did not enhance preferences and even reduced them in some cases. Mixtures of four or eleven amino acids in sucrose and complete nectar analoga were preferred to sucrose. L. niger preferred a balanced mixture over an energetically similar, less balanced mixture. Due to the tremendous variability in gustatory preferences exhibited by the broad range of largely unpredictable ant visitors, lycaenid caterpillars should either decide on a balanced mixture, containing possible amino acid ‘key compounds’ in moderate concentrations, or if this investment does not pay, should secrete sugar-rich nectars. Using the data from the HPLC analyses and the droplet size measurements combined with published studies on secretion rates allowed estimations of the lifetime energetic value of secretions. One P. coridon caterpillar would deliver 5.5-24.4J, one P. icarus 1.9-14.2J and one Z. knysna 0.7-2.8J. Dissection and gravimetric analysis allowed estimation of the caloric equivalent of larval biomass. One early P. coridon third instar (onset of nectar secretion) would yield 17.6J, and prepupae 656J. Z. knysna second instars would yield 0.95J, third instars 3.2J, and prepupae 83.1J. Thus preying on the caterpillars rather than harvesting their secretions would be of greater energetic benefit. Comparisons of this model with further reports show the rather low benefit accruing to ants from tending facultatively myrmecophilous lycaenids, underpinning that manipulation of ants (by means of still unknown chemicals) must also be involved. The model data also suggest that the mutualistic nature of facultative caterpillar-ant associations will not always be granted, and be strongly conditional. Nectar composition data support the view that in myrmecophilous lycaenids secretions rich in amino acids are related to intimate, often obligate ant-associations, whereas facultative and unspecific myrmecophiles rely more on sugars. Degree of myrmecophily seems to be a better predictor of secretion content than taxonomic relatedness. The low investment costs of the caterpillars into nectar secretions well explain the enormous taxonomic and geographical distribution of facultative myrmecophily.
-
Verbesserte Schätzungen von CO2- und N2O-Flussraten von Böden mitteleuropäischer Ökosysteme - Entwicklung konzeptioneller Neuerungen von Bodenemissionsmodellen
(2004)
-
Sascha Reth
- Im Zuge dreier Intensivmessperioden innerhalb des Projektverbundes VERTIKO (Vertikaltransporte von Energie und Spurenstoffen an Ankerstationen und ihre räumliche/zeitliche Extrapolation unter komplexen natürlichen Bedingungen) wurden in Melpitz (24.09.-12.10.2001), Lindenberg (03.06.-06.07.2002) und Tharandt (18.05.-23.05. und 08.06.-14.06.2003) simultane Messungen der N2O- und CO2-Emissionen aus Wiesen-, Acker- und Waldböden durchgeführt. Die Emissionsdaten wurden mit einem photoakustischen Infrarot-Monitor in geschlossenen Kammern erhoben. Die entwickelte automatische Steuereinrichtung erlaubte es 5 Kammern simultan als räumliche Replikate zu messen und ermöglichte damit den Einsatz unter geringem Personalaufwand. In Rahmen dieser Arbeit wurde an einer Kalibrierungskampagne in Hyytiälä / Finnland teilgenommen, in der sowohl das automatisierte Messsystem gegen einen Referenzgasfluss, als auch gegen 19 Kammersysteme anderer Forschungseinrichtungen verglichen wurde. Die Anwendbarkeit der Methode wurde durch den Vergleich bestätigt und im weiteren wurde für jedes System ein Kalibrierungsfaktor ermittelt, der einen Vergleich mit anderen Ergebnissen erlaubt. Die vorliegende Arbeit konnte weiterhin zeigen, dass verschiedene Messmethoden, Bodenkammermessungen und Eddy Kovarianz, auf einer Brache vergleichbare Resultate erzielen. Ein gewichtetes Footprintmodell ermöglichte den direkten Vergleich von CO2-Flüssen, die hauptsächlich aus dem Quellgebiet Brache emittiert wurden und daher wenig durch Senkenterme kontaminiert schienen. Zudem wurde die Einfluss der Nettoflüsse aus der benachbarten Wiesenfläche untersucht. Die Emissionshöhen von CO2 und N2O variierten stark zwischen verschiedenen Landnutzungstypen. Generell konnten für beide Gase auf Wiesen und landwirtschaftlich genutzten Böden höhere Emissionen als auf Waldböden gefunden werden. Auch innerhalb der Standorte war die zeitliche und räumliche Heterogenität hoch. Diese Arbeit entwickelte ein nichtlineares Regressionsmodell auf der Basis der gemessenen Flussraten zur Berechnung von CO2-Emissionen für verschiedene Landnutzungstypen, welches eine regionale Anwendung erlaubt. Neben Bodentemperatur und -wassergehalt als Parameter wurden der pH-Wert und die Wurzelmasse berücksichtigt. Als zusätzlicher und unerwarteter Parameter wurde der zeitlicher Abstand zu einem Regenereignis als Steuergröße identifiziert. Auch für N2O wurde ein nichtlineares Regressionsmodell (DenNit) für regionale Anwendungen entwickelt, das eine Abschätzung der N2O-Emissionen von Brachen, Wiesen und Wald mit lediglich 6 Faktoren (Bodentemperatur, Bodenwassergehalt, pH-Wert, Nitrat- und Ammonium-verfügbarkeit, zeitlicher Abstand zu einem Regenereignis) erlaubt. Im Anschluss wurden die entwickelten Modelle für den Zeitraum dreier Intensivmesskampagnen für die verschiedenen Landnutzungstypen angewendet und zeigten gute Übereinstimmungen mit publizierten Emissionsdaten anderer Arbeiten. Die Vorhersagbarkeit von Bodenatmungsraten und Lachgasemissionen mit den entwickelten Modellen konnte deutlich gesteigert werden.
-
Funktionale Analyse des ATPase- und des Zn2+-Bindungs-Motivs der Bacillus subtilis Protease FtsH und deren Einfluss auf die Sporulation
(2004)
-
Matthias Kotschwar
- Die membrangebundene, ATP- und Zn2+- abhängige Metalloprotease FtsH ist in E. coli bereits gut untersucht. Es handelt sich um ein multifunktionales Protein mit ATPase- und Protease-Aktivität. Dort wurden eine Vielzahl von E. coli ftsH-Mutanten mit pleiotropen Phänotyp charakterisiert und die Auswirkung von gezielten Mutationen in vivo und in vitro analysiert (Akiyama et al., 1994; Akiyama et al., 1996). Über FtsH aus B. subtilis ist hingegen noch wenig bekannt. Bisher wurden erst die Phänotypen einer ftsH-Nullmutante und einer Insertionsmutante beschrieben, die zu einem pleiotropen Phänotyp führen. Die Rolle bestimmter Bereiche oder Aminosäuren von FtsH wurden hier nicht untersucht (Deuerling et al., 1995; Deuerling et al., 1997; Lysenko et al., 1997). In ihrer Diplomarbeit konstruierte Eva Harfst 1999 vier B. subtilis ftsH-Allele, indem sie gezielt Aminosäuren in der Walker-A-Box und im Zinkbindungsmotiv austauschte, wie sie bereits für E. coli und Hefe FtsH beschrieben wurden. Diese mutierten ftsH-Gene und ein wildtypisches zur Kontrolle brachte sie in den pX-Vector ein, wodurch sie unter die Kotrolle eines Xylose-induzierbaren Promotors gestellt wurden. In der vorliegenden Arbeit wurden die mutierten ftsH-Allele in das B. subtilis-Chromosom integriert und in einem zweiten Schritt das wildtypische ftsH deletiert. Die Phänotypen der so erzeugten Mutanten (sowohl die aus dem ersten Schritt, die noch das wildtypische ftsH tragen, als auch die aus dem zweiten Schritt, die nur noch das mutierte Allel tragen) wurden hinsichtlich des filamentösen Wachstums, der Sporulation, der Sensitivität gegenüber Hitzeschock und hyperosmotischen Bedingungen analysiert. Dabei stellte sich heraus, das die mutierten Allele keinen Einfluss auf den Phänotyp der Stämme haben, die das wildtypische ftsH noch tragen, und in den Knockout-Stämmen das wildtypische ftsH nicht ersetzen können, also den Phänotyp einer ftsH- Nullmutante zeigen. Des weiteren sind die mutierten ftsH-Allele ebenfalls nicht in der Lage, die Defizienz in der Subtilisin-Sekretion zu kompensieren. ATPase-Tests zeigten, dass die eine Mutation im Walker-A-Motiv die ATPase-Aktivität der beiden betroffenen Proteine ausschaltete. Protease-Tests ergaben, dass keines der mutierten FtsH-Proteine noch eine Protease-Aktivität aufweist. Das bedeutet, dass einerseits die Basenaustausche im Zinkbindungsmotiv die Protease-Aktivität ausschalten, andererseits aber auch keine Protease-Aktivität ohne ATPase- Aktivität möglich ist. Daraus folgt weiterhin, dass die ATPase-Aktivität des FtsH-Proteins allein nicht ausreicht, um einen der untersuchten pleiotropen Phänotypen einer ftsH--Mutante zu kompensieren. Die Funktionalität sowohl der ATPase- als auch der Protease-Domäne ist also für FtsH und somit für B. subtilis hinsichtlich des filamentösen Wachstums, der Sporulation, der Sensitivität gegenüber Hitzeschock und hyperosmotischen Bedingungen essentiell. Anhand von Northern-Blot-Hybridisierungsexperimenten konnte gezeigt werden, dass FtsH eine essentielle Rolle bei der Phosphorylierung von Spo0A haben muss, da ohne FtsH kein aktives Spo0A gebildet wird. Die daraus resultierende Sporulationsdefizienz kann aber durch eine Spo0A-Mutante, die ohne Phosphorylierung aktiv ist, fast vollständig kompensiert werden. FtsH ist also während der Sporulation nur für die Phosphorylierung von Spo0A essentiell.
-
Charakterisierung der reduktiven Aktivitäten der CO-Dehydrogenasen aus Oligotropha carboxidovorans und Streptomyces thermoautotrophicus
(2004)
-
Antonius Sarjiya
- Es wurde in dieser Arbeit entdeckt, dass die aerobe CO-Dehydrogenase in der Lage ist, alternative Substrate wie Azid, Nitrit und Nitrat zu reduzieren. CO-Dehydrogenase aus Oligotropha carboxidovorans und Streptomyces thermoautotrophicus führen die Reduktionsreaktion wie folgt durch: Azid wird zu Ammonium und molekularem Distickstoff reduziert. N3- + 2 e- + 3 H+ -> NH3 + N2 Nitrit wird zu Ammonium reduziert. NO2- + 6 e- + 7 H+ -> NH3 + 2H2O Nitrat wird zu Nitrit reduziert. NO3- + 2H+ + 2e- -> NO2- + H2O Das katalytische [CuSMoO2]-Zentrum der CO-Dehydrogenase ist verantwortlich für die Reduktion von Azid und Nitrit. Ohne einem aktiven katalytischen-Zentrum findet die Reduktion von Azid und Nitrit nicht statt. Die CO-Dehydrogenase katalysiert gleichzeitig die H2-Oxidation und die Azid-Reduktion. Die während der H2-Oxidation entstehenden Elektronen werden für die Azid-Reduktion verwendet. H2 + N3- + H+ -> NH3 + N2 Oxidation und Reduktion finden mit größter Wahrscheinlichkeit am katalytisch aktiven [CuSMoO2]-Cluster der CO-Dehydrogenase statt. Die gleichzeitige Anwesenheit von CO und Azid hemmt die CO-Oxidations- und Azid-Reduktionsaktivität. Die Kristallstruktur zeigt, dass in Gegenwart von CO und Azid aus dem [CuSMoO2]-Cluster das Cu-Atom und der Sulfidoligand entfernt sind. Die Nitrat-Reduktion der CO-Dehydrogenase ist unabhängig vom katalytisch aktiven [CuSMoO2]-Cluster. Die CO-Dehydrogenase aus S. thermoautotrophicus zeigt ähnliche Oxidations- und Reduktionsaktivitäten. Die CO-Dehydrogenase aus dem thermophilen, Gram-positiven Bakterium S. thermoautotrophicus reagiert sehr ähnlich wie die CO-Dehydrogenase aus dem mesophilen, Gram-neagtiven Bakterium O. carboxidovorans.
-
Optically induced orientational transitions in nematic liquid crystals
(2004)
-
Dmitry Krimer
- I have presented in this thesis a theoretical study of some dynamical phenomena and orientational transitions induced by intense light in homeotropically oriented nematic layers. A large number of experiments has been performed in such systems and various interesting dynamical regimes have been identified. However, systematical theories capable of describing the observed phenomena have been derived for some cases only. In other cases oversimplified models exist with limited applicability. In Chapter 2 I considered the case of a circularly polarized plane light wave incident perpendicularly on the layer. I have constructed a theory that is capable of describing the observed regimes of director motion and the transitions between them in detail. The first instability is the Freedericksz transition from the homeotropic state to a small-amplitude reoriented state with uniform director precession around the layer normal. With increasing light intensity, this state destabilizes via a supercritical Hopf bifurcation and a new frequency in the time Fourier spectra of the dynamical variables appears. This regime is quasiperiodic and corresponds to a precession and nutation of the director. As the intensity increases further, this state disappears at a certain critical value where the period of nutation becomes infinite. There a strongly hysteretic transition to a state with large reorientation occurs via a homoclinic bifurcation. The homoclinic orbit involved is of the simplest type where a limit cycle collides with a saddle point having one unstable direction. The new state corresponds to a uniform precession of the director, however, with very large period and with large reorientation. I have also investigated the influence of an additional static electric field on the dynamical scenario described above. In Chapter 3 the treatment is generalized to the case of elliptically polarized light. The complete bifurcation diagram with the light intensity and the ellipticity as control parameters has been calculated in the region where rotating states exist. I have shown that for a fairly narrow region of ellipticities close to circular polarization the first periodic rotating state loses its stability in a supercritical Hopf bifurcation. I have found that with increasing light intensity at different ellipticities different sequences of transitions all finally lead to a state with large director distortion as the intensity is increased. The nature of this largely distorted state, as well as intermediate regimes vary with ellipticity. Some of the regimes that appear at lower intensities were studied previously, both experimentally and theoretically, but a complete picture up to the largely distorted regime was missing. In the theoretical treatments developed in the Chapters above, as in all other treatments, the velocity field induced by the director motion (backflow) has been neglected. In Chapter 4 I have investigated the influence of backflow on the dynamical scenario described in Chapter 2 and have shown that the backflow leads to substantial quantitative changes. It turns out that the regime of nonuniform precession shifts to higher light intensities and exists in a larger interval. I have also found unanticipated spatial oscillations of the backflow across the layer for the state with large director distortion. This is a signature of the interference pattern of the light within the layer. Actually, in the theory presented, for the first time, a light-induced dynamical phenomenon has been derived from the full nematodynamic equations. Thus, for the first time, full quantitative comparison with experiments using a transversally extended laser light could be done. Also, in all previous theoretical treatments involving plane wave incident light, it was assumed that the director distortion does not depend on the coordinates in the plane of the layer, i.e. one dealt with a one dimensional situation. In Chapter 5 I have studied the instabilities induced by a linearly polarized ordinary light wave incident at a small oblique angle allowing for spatial variations of the director in the plane of the layer and including the case of a dye-doped nematic. It was previously known that for sufficiently small angles of incidence the homeotropic state looses stability in a stationary, homogeneous pitchfork bifurcation. I have shown that the resulting stationary distorted state looses stability via a secondary Hopf bifurcation to spatially inhomogeneous state (nonzero critical wavenumber) that leads to the formation of travelling waves in the plane of the layer. The wavelength of these waves depend on the angle of incidence and the ratios of the elastic constants. It is typically several times larger than the thickness of the layer.
-
Magnetooptische Untersuchungen an Halbleitersystemen mit reduzierter Dimensionalität
(2004)
-
Jens Fürst
- Diese Arbeit beschäftigt sich mit der magnetooptischen Untersuchung von niedrigdimensionalen Halbleiterstrukturen, bestehend aus III-V und IV-VI Halbleiternsystemen. Als Vertreter der III-V Halbleiter stehen InAs Quantenpunktproben zur Verfügung. Die elektronischen Eigenschaften der InAs Quantenpunkte werden mit Hilfe von Photolumineszenz- und Hanle-Effekt-Messungen analysiert. Dabei wird der Einfluss einer Vielzahl von Probenparametern wie Dotierung und Zusammensetzung der Quantenpunkte sowie des Matrixmaterials auf die Elektronenspinlebenszeit untersucht. Grundlegende Analysen werden an dem ternären IV-VI Materialsystem PbSeTe durchgeführt. Bei der Entstehung von PbSe Quantenpunkten in PbEuTe Matrix ist es nicht auszuschließen, dass es an der Grenzfläche der Quantenpunkte und des Matrixmaterials zu Durchmischungen dieser beiden Materialien kommt und sich dadurch das System PbSeTe bildet. Deshalb werden die Bandparameter dieses Mischkristalls in einer Probenserie von reinem PbSe zu reinem PbTe mit Hilfe von Magnetotransmissionsmessungen und kohärenten anti-Stokes Ramanstreuung bestimmt. Zur Untersuchung der Emission von PbSe Quantenpunkten sind diese in einen vertikalen Resonator eingebracht. Das von diesen nulldimensionalen Systemen emittierte Licht wird mit der emittierten Strahlung von Resonatoren mit zweidimensionalen und dreidimensionalen aktiven Zonen verglichen. In weiteren Experimenten werden vertikal emittierende Laserstrukturen in einem Spektralbereich nahe 8 Mikrometer bei kontinuierlicher und gepulster Anregung untersucht. Dabei zeigen sich sehr schmale Emissionslinien. In einem longitudinalen Magnetfeld konnte aufgrund der kleinen Halbwertsbreite des emittierten Lichtes eine Aufspaltung beobachtet werden.
-
Synthesis, Molecular Structure and Reactivity of Di(1-cyclohepta-2,4,6-trienyl) thioether, S(C7H7)2 and Tri(1-cyclohepta-2,4,6-trienyl) amine, N(C7H7)3
(2004)
-
Jinnan Liu
- This thesis describes the synthesis, characterization and coordination chemistry of di(1-cyclohepta-2,4,6-trienyl) thioether, S(C7H7)2 (1), tri(1-cyclohepta-2,4,6-trienyl) amine, N(C7H7)3 (24), and di(1-cyclohepta-2,4,6-trienyl) amine, NH(C7H7)2 (25). The reaction of tropylium bromide, C7H7Br, with the gas hydrogen sulfide (H2S) leads to the di(1-cyclohepta-2,4,6-trienyl) thioether, S(C7H7)2 (1), which can undergo Diels-Alder reactions with maleic anhydride, maleimide and N-phenyl maleimide to give the adducts S(C7H7)(C7H7C4H2O3) (4a), S(C7H7)(C7H7C4H3NO2) (4b) and S(C7H7)(C7H7C10H7NO2) (4c), respectively. In these three compounds, one cyclohepta-2,4,6-trienyl ring has remained intact, whereas the other seven-membered ring was involved in the Diels-Alder reaction. The advantage of the potential ligand S(C7H7)2 (1) in comparison to other simple sulfanes is that it can act as a chelate ligand, using one of the non-planar seven-membered rings. In general, the central double bond of one of the seven-membered rings becomes coordinated to the metal. The disadvantage of the sulfane 1 is its sensitivity towards oxidation and its low thermal stability. The reactions of either Cr(CO)5(thf) or Cr(CO)4(eta4-C7H8) with one equivalent of 1 lead to the monosubstituted derivative Cr(CO)5[PhCH2S(C7H7)] (7), in which one seven-membered ring has remained unchanged and the other seven-membered ring has been transformed into a benzyl substituent. From the reaction of 1 with a mixture of chromium(carbonyl)(acetonitrile) complexes, Cr(CO)6-x(CH3CN)x (x = 1, 2, 3), the pentacarbonyl Cr(CO)5[S(C7H7)2] (6) could be obtained, in which both cycloheptatrienyl ring substituents are freely pending. Using the same method as for the chromium complexes, tetracarbonyl molybdenum Mo(CO)4[(eta2-C7H7)S(C7H7)] (10) was obtained from the reactions of either Mo(CO)5(thf) or Mo(CO)4(eta4-C7H8) with the sulfane 1. In complex 10 the ligand S(C7H7)2 is coordinated to the metal both through a lone pair of electrons at the sulfur atom and the central C=C double bond of a cyclohepta-2,4,6-trienyl substituent. Mo(CO)5[S(C7H7)2] (9) could be obtained from Mo(CO)6-x(CH3CN)x (x = 1, 2, 3); in analogy to 6, the sulfane ligand in 9 is coordinated to molybdenum exclusively through a lone pair of electrons at the sulfur atom. From the intermediate W(CO)5(thf), the pentacarbonyltungsten complex W(CO)5[S(C7H7)2] (13) was obtained, which could be converted thermally to the benzyl complex W(CO)5[PhCH2S(C7H7)] (14). In the reaction with the mixture of tungsten(carbonyl)(acetonitrile) complexes, W(CO)6-x(CH3CN)x (x = 1, 2, 3), the compounds W(CO)4[(eta2-C7H7)S(C7H7)] (15) and W(CO)5[S(C7H7)2] (13) were obtained; complex 15 was the main product. The reactions of either Mn2(CO)10 (under irradiation) or Mn(CO)5X (X = Cl, Br) with di(1-cyclohepta-2,4,6-trienyl) thioether (1) led to the organothiolato-bridged dimer Mn2(CO)8[S(C7H7)]2 (17) in addition to ditropyl, (C7H7)2. Subsequent substitution of two carbonyl ligands in 17 by stronger sigma-donor-pi-acceptor ligands such as tert-butyl isocyanide and trimethyl phosphite (tBuNC and P(OMe)3) gave the complexes Mn2(CO)6[tBuNC]2[S(C7H7)]2 (18) and Mn2(CO)6[P(OMe)3]2[S(C7H7)]2 (19). Both 17 and 18 contain a planar Mn2S2 core with the 1-cyclohepta-2,4,6-trienyl substituents in anti-position. In addition, some investigations with tri(1-cyclohepta-2,4,6-trienyl) amine, N(C7H7)3 (24), and di(1-cyclohepta-2,4,6-trienyl) amine, NH(C7H7)2 (25) were carried out. In analogy to the sulfane S(C7H7)2 (1), 24 and 25 can undergo Diels-Alder reactions with maleimide and N-phenyl maleimide to give the educts (C7H7)2N(C7H7C4H3O2N) (26b), (C7H7)N(C7H7C10H7NO2)2 (26c), (C7H7)NH(C7H7C4H3O2N) (27b) and (C7H7)NH(C7H7C10H7NO2) (27c), respectively. Similar to the synthesis of 24 and 25, the mixed tertiary amines di(1-cyclohepta-2,4,6-trienyl)phenylamine, N(Ph)(C7H7)2 (28), and (1-cyclohepta-2,4,6-trienyl)diphenylamine, N(Ph)2(C7H7) (29) were prepared using aniline or diphenylamine. The reaction of N(C7H7)3 (24) with [C7H7]BF4 (1:1) leads to the N-tropylidene-N-(1-cyclohepta-2,4,6-trienyl)immonium tetrafluoroborate salt, [(C7H6)N(H)(C7H7)]BF4 (30). Compound 30 has also been obtained using NH(C7H7)2 (25) as the educt. As expected, the tertiary amine N(Ph)(C7H7)2 (28) reacted with [C7H7]BF4 to give the dark-red salt [(C7H6)N(Ph)(C7H7)]BF4 (31). Attempts to use the amine N(C7H7)3 (24) as a ligand in transition metal complexes were unsuccessful. However, NH(C7H7)2 (25) reacts with the mixture of tungsten(carbonyl)(acetonitrile) derivatives to give the chelate tetracarbonyl complex W(CO)4[(C7H7)NH(eta2-C7H7)] (32), which is similar to the sulfane complexes 10 and 15. Compared with the versatile phosphane ligand tri(1-cyclohepta-2,4,6-trienyl) phosphane, P(C7H7)3, the analogous amine N(C7H7)3 (24) is unable to act as a coordination ligand, probably as a result of the steric shielding of the lone pair of electrons at the nitrogen atom.
-
Template-Controlled Synthesis of Magnetic/Semiconducting Nanoparticles within Amphiphilic Core-Shell Cylindrical Polymer Brushes
(2004)
-
Mingfu Zhang
- Core-shell cylindrical polymer brushes with poly(t-butyl acrylate)-b-poly(n-butyl acrylate) (PtBA-b-PnBA) diblock copolymer side chains were synthesized via the “grafting from” technique using a combination of anionic polymerization (for the synthesis of the backbone) and atom transfer radical polymerization (ATRP, for the synthesis of the side chains). The formation of well-defined brushes was confirmed by 1H-NMR and GPC. The selective hydrolysis of the PtBA block of the side chains resulted in novel amphiphilic core-shell cylindrical polymer brushes with poly(acrylic acid)-b-poly(n-butyl acrylate) (PAA-b-PnBA) side chains. The characteristic core-shell cylindrical structure of the brushes was directly visualized on mica by scanning force microscopy (SFM). Amphiphilic brushes with 1500 block copolymer side chains and a length distribution of lw/ln = 1.04 at a total length ln = 179 nm were obtained. These amphiphilic polymer brushes can be regarded as unimolecular cylindrical micelles, because of the core-shell structure and the amphiphilicity of side chains. The amphiphilic brushes can be used as single molecular templates for the synthesis of inorganic nanoparticles, because the carboxylic acid groups (or carboxylate groups, after neutralization) in the polymer core can coordinate with various metal ions. The hydrophilic core of polymer brushes, poly(acrylic acid), was neutralized by NaOH and afterward iron cations (Fe3+ and Fe2+) were loaded into the polymer core via ion exchange. The formation of the polychelates of polymer brushes and iron cations was confirmed and characterized by various techniques such as Fourier transform infrared spectroscopy (FTIR), UV/vis spectroscopy, transmission electron microscopy (TEM) and SFM. A peculiar “pearl necklace” morphology was observed for the polychelates, which is caused by the physical cross-linking of the side chains via multivalent iron cations. Formation of crystalline alpha-Fe2O3 (hematite) was observed during the He-Ne laser irradiation in the confocal Raman microscopy measurement of the polychelate containing Fe3+ ions. Magnetic nanoparticles were successfully produced from the coordinated iron cations within polymer brushes via single molecule templating technique, as confirmed by various techniques such as SFM, TEM, and UV/visible spectroscopy. Superconducting quantum interference device (SQUID) magnetization measurements show that the hybrid nanocylinders are superparamagnetic at room temperature. The polymer shell provides not only the stability of the nanoparticles but also the solubility of the hybrid nanocylinders. After the formation of the magnetic nanoparticles, the carboxylate coordination sites within the polymer brushes are liberated and ready for further coordination with more iron ions, thus it is possible to increase the amount and/or particle size of the nanoparticles by multi-cycles of iron ion loading and particle formation. The as-prepared hybrid nanocylinders combine the promising properties of polymers and superparamagnetic nanoparticles, and may find potential applications such as in ferrofluids. Similarly, using the amphiphilic core-shell cylindrical polymer brush with PAA core and PnBA shell as template, wire-like assemblies of CdS nanoparticles were successfully synthesized under mild solution conditions, as confirmed by various characterization techniques. Quantum confinement of the CdS nanoparticles was observed, indicated by the blue shift of the absorbance edge in UV/visible spectrum. The technique using a single cylindrical molecule as template for inorganic nanoparticle fabrication presented in this thesis is not restricted to magnetic/semiconductor nanoparticles, but can also be used for the preparation of a number of metal, metal oxide, and metal chalcogenide nanoparticles.
-
Biomass and Nutrient Studies of Selected Tree Species of Natural and Plantation Forests: Implications for a Sustainable Management of the Munessa-Shashemene Forest, Ethiopia
(2004)
-
Asferachew Abate
- Plantation forests with exotic tree species have been introduced to alleviate the problems of deforestation in Ethiopia. In the future, more plantation forests with fast growing species should be grown for coping with the ever-increasing demands for fuelwood and other forest products. However, it is not known whether plantation forests are sustainable or not. For the sustainability of plantation forests with exotic tree species, it is of paramount importance to thoroughly understand their ecological and social attributes through a holistic approach. For this reason, a multidisciplinary project was initiated in the Munessa-Shashemene Forest. Such an approach gives valuable information about the sustainability of plantation forests when the basic ecological features of the natural forests are compared with plantation forests. As an integral part of the multidisciplinary project, the objectives of this study are to: 1) quantify the fine roots and aboveground biomass of selected tree species in both natural and plantation forests; 2) quantify the macronutrient stocks of the fine roots and aboveground components of selected trees species in both natural and plantation forests; and 3) evaluate the implication of the changes in the biomass and macronutrient stocks for a sustainable management of forests. The study focused on four tree species, Podocarpus falcatus (Thunb.) Mirb., Podocarpaceae and Croton macrostachys Hochst. ex Del. Euphorbiaceae, were selected from a natural forest. Cupressus lusitanica Miller, Cupressaceae and Eucalyptus globulus Labill. Myrtaceae were selected from plantation forests. Root architectures of the study trees were studied by excavation. The live fine root biomass (<2 mm in diameter) of the dry and wet seasons was determined from samples collected at the distances of 1, 2 and 3 meters from the bole of the study trees. At each of the distances, root cores were taken at the depth intervals 0-10, 10-35, 35-60, 60-85 and 85-100 cm using a hand auger. Linear regression equations were used to estimate the aboveground biomass on the basis of the relation between DBH and dry weights of the aboveground plant components. Macronutrient concentrations were determined following a standard laboratory procedure. Studies on the root architecture revealed that C. lusitanica has a shallow root and is more susceptible to windthrow compared to E. globulus. With the exception of E. globulus, the dry season live fine root (LFR) biomass was higher for all trees studied. The seasonal variation in the fine root biomass was attributed to the changes in soil moisture of the study area. For all trees investigated, the mean annual LFR biomass was highest at the depth interval 0-10 cm at all distances. The favorable soil texture, pH and organic matter content at the depth interval 0-10 cm might be responsible for higher LFR biomass. The significantly higher LFR biomass of P. falcatus (1.34 kg m-2) coupled with its higher macronutrient stocks compared to C. macrostachys (0.32 kg m-2) suggest the importance of P. falcatus in the sustainability of the natural forest by transferring more macronutrients to the soil through its fine roots. Similarly, the significantly higher total LFR biomass of C. lusitanica (0.88 kg m-2) coupled with its higher macronutrient stock compared to E. globulus (0.27 kg m-2) indicated less depletion of soil nutrients by the former. The stand structure of the natural and plantation forests differed largely. In the natural forest, the density of C. macrostachys was much higher (143 ± 72 trees ha-1) than the density of P. falcatus (73 ± 39 trees ha-1). Generally, the structural change of the natural forest due to selective cutting of P. falcatus was found to have negative implications on the sustainability of the natural forest. The differences in the structure of C. lusitanica and E. globulus, despite their similar densities, resulted in a significantly lower understory ground cover by herbaceous and shrub species in the former. The effect of a poor understory growth on the floor litter thickness and thereby on nutrient capital of the soil may negatively affect the sustainability of C. lusitanica plantation. The harvesting of the stemwood of C. lusitanica and E. globulus removes a substantial amount of nutrients from the plantation sites. Furthermore, the current practice of collecting foliage, twigs and branches for firewood by the local people results in a higher depletion of nutrients. In order to make the plantation forests sustainable, the silvicultural practice in the future should consider on site conservation of foliage and bark. It is recommended that more studies on aboveground and belowground biomass, fine root turnover, and nutrient concentrations of the plantation forests should be carried out in a chronosequence in order to gain more insight on their sustainability.
-
Pigment-Pigment Interactions and Protein Dynamics in Light-Harvesting Complexes: a Single-Molecule Study
(2004)
-
Clemens Hofmann
- Light harvesting complexes that are involved in the first steps of photosynthesis in purple bacteria were studied with low-temperature optical single molecule spectroscopy. In one series of experiments the spectral properties of the bacteriochlorophyll a molecules within the complexes were studied in the view of model systems of molecular aggregates. It was found that the excitations in the B800 band of the light harvesting 2 complex are mainly localised on individual chromophores although evidence was found for an electronic coupling in the weak to intermediate range between individual bacteriochlorophyll a molecules. In contrast, for the B850 band of the light harvesting complex 2 as well as for the B870 band of the light harvesting complex 1 it was found that the assembly of bacteriochlorophyll molecules represents a strongly coupled system and that the excitation is coherently delocalised over a substantial part of the chromophores. By performing Monte-Carlo simulation an estimate on the amount of random and correlated energetic disorder in the site energies of the chromophores as well as on structural properties of the complexes could be given. In experiments on individual LH2-LH1-RC complexes, the energy transfer within a single photosynthetic unit was observed. In further experiments the chromophores were used as local probes to monitor conformational fluctuations of the protein residues in their binding pocket. Looking at the spectral diffusion of individual chromophores allowed to elucidate the organisation of the protein-energy landscape in tiers. In addition a clear correlation for the transition rates between those states and the energy separation of the levels involved could be uncovered. To simplify and automatise the analysis of a large number of consecutively recorded spectra a pattern recognition approach using multivariate statistical analysis proved to be a very useful tool. Apart from elucidating spectral diffusion processes detailed information about the line shape of individual chromophore absorptions could be gained from which it was inferred that the electron-phonon coupling in the B800 pigment pool is very weak. In summary, this thesis demonstrates that low-temperature single-molecule spectroscopy provides a unique method to reveal details of pigment-pigment interactions in the weak to intermediate as well as strong coupling limit that are inaccessible by other experimental methods.
