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  • 04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks  (3)
  • Conformational analysis  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Gas phase and water solution conformational analysis of the herbicide diuron (DCMU): an ab initio study (1998)
    Springer
    Theoretical chemistry accounts 99 (1998), S. 301-311 
    Details
    ISSN: 1432-2234
    Keywords: Key words: DCMU ; Diuron ; Conformational analysis ; Solvent effect ; Ab initio calculation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. In the present work, the conformational equilibrium for the herbicide diuron (DCMU) has been investigated using high level ab initio calculations. The solvent effect was included through two different continuum models: (1) the real cavity IPCM method and (2) the standard dipole Onsager model SCRF. The effect due to solute-solvent hydrogen-bond interactions was analyzed considering a hybrid discreet-continuum model. At the Hartree-Fock level, the gas phase results showed that only the trans forms (A and B) are present in the equilibrium mixture, with the relative concentrations found to be 33% (A) and 67% (B) (HF/6-311+G**//6-31G**). When the electronic correlation effect is included (MP2/6-31G*//HF/6-31G*), a relative stabilization of the cis forms was observed, with the conformational distribution calculated as 38% (A), 50% (B), 6% (C) and 6% (D). The trans conformations were found to be completely planar, which has been considered to be a prerequisite for the herbicide binding. In water solution, the trans conformation A should be the most abundant conformer, the IPCM and SCRF values being ca. 100% and ca. 85% respectively. The IPCM calculations with the isodensity level set to 0.0005 present a conformational distribution close to that obtained from the hybrid model [92% (A) and 8% (B)], which has been considered our best solvent approach. Regarding the biological action of urea-type herbicides, the results presented here are important, because some QSAR studies have suggested that the partition coefficient is related to the herbicide activity, so the conformational equilibrium may play a role in the biological action.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002140050340
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Conformational analysis of the antimalarial agent quinine (1997)
    Springer
    Structural chemistry 8 (1997), S. 95-107 
    Details
    ISSN: 1572-9001
    Keywords: Conformational analysis ; antimalarial ; quinine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Quinine, an active antimalarial compound, is one of the most abundant constituents extracted from the bark ofCinchona trees. The activity differences among structurally related molecules appear to depend on the absolute stereochemistry of some functional groups, a result that has stimulated a detailed conformational analysis of these molecules of biological interest. In the present study the potential energy surface (PES) for the antimalarial agent quinine (C20H24O2N2) has been comprehensively investigated using the molecular mechanics (MM) and quantum mechanical semiem-pirical AM1 and PM3 methods. Six distinct minimum-energy structures are located on the multidimensional PES and also characterized as true minima through harmonic frequency analysis. The relative stabilities and thermodynamic properties are reported. The coexistence of different conformers is discussed for the first time in the literature based on the calculated transition-state (TS) structures connecting the six minima located on the PES for the quinine molecule. The theoretical results reported in the present study are in agreement with the experimental proposal, based on NMR data, that there are two possible forms for the quinine molecule in solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02262845
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  • 3
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    Tephrostratigraphy and tephrochronology of lakes Ohrid and Prespa, Balkans (2011)
    EGU Biogeosciences Discussions
    Details
    Publication Date: 2020-12-21
    Description: Four cores from Balkans lakes Ohrid and Prespa were studied for recognition of tephra layers and cryptotephras, and the results presented along with the review of data from other two already published cores from Lake Ohrid. The six cores provide a previously unrealised tephrostratigraphic framework of the two lakes, and supply the first detailed tephrochronologic profile (composite) for the Balkans, which spans from the end of the Middle Pleistocene to the end of the Ancient Age (AD 472). A total of 12 tephra layers and cryptotephras were recognised in the cores. One is of Middle Pleistocene age (131 ky) and correlated to the marine tephra layer P-11 from Pantelleria Island. Eight volcanic layers are Upper Pleistocene in age, and encompass the period between ca. 107 ky and ca. 31 ky. This interval contains some of the main regional volcanic markers of the Central Mediterranean area, including X-6, X-5, Y-5 and Y-3 tephra layers. The other layers of this interval have been related to the marine tephra layers C20, Y-6 and C10, while one was for the first time recognised in distal areas and correlated to the Taurano eruption of probable Vesuvian origin. Three cryptotephras were of Holocene age. Two of which have been correlated to Mercato and AD 472 eruptions of Somma-Vesuvius, while the third has been correlated to the FL eruption from Mount Etna. These recognitions provide a link of the Ohrid and Prespa lacustrine successions to other archives of the Central Mediterranean area, like South Adriatic, Ionian, and South Tyrrhenian Seas, lakes of Southern Italy (Lago Grande di Monticchio, Pantano di San Gregorio Magno and Lago di Pergusa) and Balkans (Lake Shkodra).
    Description: Published
    Description: 3931-3967
    Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi
    Description: 3.7. Dinamica del clima e dell'oceano
    Description: JCR Journal
    Description: open
    Keywords: isotopic geochemistry ; lake sediments ; paleoclimate proxies ; 03. Hydrosphere::03.01. General::03.01.03. Global climate models ; 04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 4
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    A paleoclimate record with tephrochronological age control for the last glacial-interglacial cycle from Lake Ohrid, Albania and Macedonia (2011)
    Springer
    Details
    Publication Date: 2017-04-04
    Description: Lake Ohrid is probably of Pliocene age, and the oldest extant lake in Europe. In this study climatic and environmental changes during the last glacial-interglacial cycle are reconstructed using lithological, sedimentological, geochemical and physical proxy analysis of a 15-m-long sediment succession from Lake Ohrid. A chronological framework is derived from tephrochronology and radiocarbon dating, which yields a basal age of ca. 136 ka. The succession is not continuous, however, with a hiatus between ca. 97.6 and 81.7 ka. Sediment accumulation in course of the last climatic cycle is controlled by the complex interaction of a variety of climate-controlled parameters and their impact on catchment dynamics, limnology, and hydrology of the lake. Warm interglacial and cold glacial climate conditions can be clearly distinguished from organic matter, calcite, clastic detritus and lithostratigraphic data. During interglacial periods, short-term fluctuations are recorded by abrupt variations in organic matter and calcite content, indicating climatically-induced changes in lake productivity and hydrology. During glacial periods, high variability in the contents of coarse silt to fine sand sized clastic matter is probably a function of climatically-induced changes in catchment dynamics and wind activity. In some instances tephra layers provide potential stratigraphic markers for short-lived climate perturbations. Given their widespread distribution in sites across the region, tephra analysis has the potential to provide insight into variation in the impact of climate and environmental change across the Mediterranean.
    Description: Published
    Description: 295-310
    Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi
    Description: 3.7. Dinamica del clima e dell'oceano
    Description: JCR Journal
    Description: reserved
    Keywords: Lake Ohrid ; Mediterranean ; Tephrochronology ; Paleolimnology ; Last glacial-interglacial cycle ; 03. Hydrosphere::03.01. General::03.01.03. Global climate models ; 04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 5
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    Environmental change within the Balkan region during the past ca. 50 ka recorded in the sediments from lakes Prespa and Ohrid (2011)
    EGU Biogeosciences Discussions
    Details
    Publication Date: 2017-04-04
    Description: Lakes Prespa and Ohrid, in the Balkan region, are considered to be amongst the oldest lakes in Europe. Both lakes are hydraulically connected via karst aquifers. From Lake Ohrid, several sediment cores up to 15 m long have been studied over the last few years. Here, we document the first long sediment record from nearby Lake Prespa to clarify the influence of Lake Prespa on Lake Ohrid and the environmental history of the region. Radiocarbon dating and dated tephra layers provide robust age control and indicate that the 10.5 m long sediment record from Lake Prespa reaches back to 48 ka. Glacial sedimentation is characterized by low organic matter content and absence of carbonates in the sediments, which indicate oligotrophic conditions in both lakes. Holocene sedimentation is characterized by particularly high carbonate content in Lake Ohrid and by particularly high organic matter content in Lake Prespa, which indicates a shift towards more mesotrophic conditions in the latter. Long-term environmental change and short-term events, such as related to the Heinrich events during the Pleistocene or the 8.2 ka cooling event during the Holocene, are well recorded in both lakes, but are only evident in certain proxies. The comparison of the sediment cores from both lakes indicates that environmental change affects particularly the trophic state of Lake Prespa due to its lower volume and water depth.
    Description: Published
    Description: 3365-3392
    Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi
    Description: 3.7. Dinamica del clima e dell'oceano
    Description: JCR Journal
    Description: open
    Keywords: geocehemistry ; lake sediments ; paleoclimate proxies ; 03. Hydrosphere::03.01. General::03.01.03. Global climate models ; 04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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