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531.
Cong H Niedzwiedzki DM Gibson GN Frank HA 《The journal of physical chemistry. B》2008,112(11):3558-3567
Many of the spectroscopic features and photophysical properties of xanthophylls and their role in energy transfer to chlorophyll can be accounted for on the basis of a three-state model. The characteristically strong visible absorption of xanthophylls is associated with a transition from the ground state S0 (1(1)Ag-) to the S2 (1(1)Bu+) excited state. The lowest lying singlet state denoted S1 (2(1)Ag-), is a state into which absorption from the ground state is symmetry forbidden. Ultrafast optical spectroscopic studies and quantum computations have suggested the presence of additional excited singlet states in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+). One of these is denoted S* and has been suggested in previous work to be associated with a twisted molecular conformation of the molecule in the S1 (2(1)Ag-) state. In this work, we present the results of a spectroscopic investigation of three major xanthophylls from higher plants: violaxanthin, lutein, and zeaxanthin. These molecules have systematically increasing extents of pi-electron conjugation from nine to eleven conjugated carbon-carbon double bonds. All-trans isomers of the molecules were purified by high-performance liquid chromatography (HPLC) and studied by steady-state and ultrafast time-resolved optical spectroscopy at 77 K. Analysis of the data using global fitting techniques has revealed the inherent spectral properties and ultrafast dynamics of the excited singlet states of each of the molecules. Five different global fitting models were tested, and it was found that the data are best explained using a kinetic model whereby photoexcitation results in the promotion of the molecule into the S2 (1(1)Bu+) state that subsequently undergoes decay to a vibrationally hot S1 (1(1)Ag-) state and with the exception of violaxanthin also to the S* state. The vibrationally hot S1 (1(1)Ag-) state then cools to a vibrationally relaxed S1 (2(1)Ag-) state in less than a picosecond. It was also found that a portion of the S* population is converted into S1 (2(1)Ag-) during deactivation, but this process and the relative yield of S* was found to depend on temperature, consistent with it being associated with a twisted conformation of the xanthophyll. The results of the global fitting suggest that subpopulations of twisted conformers of xanthophylls already exist in the ground state prior to photoexcitation. 相似文献
532.
Dagmara Jacewicz Dariusz Wyrzykowski Krzysztof ?amoj? Diana Czerwińska Paulina Czaja Lech Chmurzyński 《Structural chemistry》2012,23(2):333-340
The thermal decomposition of trans-K[Cr(C2O4)2(OH2)2]·3H2O and cis-K[Cr(C2O4)2(OH2)2] has been studied using the TG–MS technique. The measurements were carried out in an argon atmosphere over the temperature
range of 293–873 K. The influence of the complex structures and configurational geometry on the stability of the transition
products and the pathways of thermal transformations has been discussed. Furthermore, the kinetics of the isomerization reactions
of the [Cr(C2O4)2(OH2)2]− complex ion catalyzed by five different metal ions: Be2+, Mg2+, Ca2+, Sr2+ and Ba2+ have been studied. The isomerization reactions were studied in aqueous solution at five various temperatures (283–303 K),
at constant concentration of metal ions (C = 0.1 M) and the constant ionic strength of solution (Na+, NO3
−) I = 2.4 M. The rates of the isomerization reaction were determined spectrophotometrically by monitoring of absorbance changes
at 410 nm. 相似文献
533.
Control over generation and dynamics of excited electronic states is fundamental to their utilization in all areas of technology. We present the first example of multichromophoric systems in which emissive triplet states are generated via a pathway involving photoinduced electron transfer (ET), as opposed to local intrachromophoric processes. In model dyads, PtP-Ph(n)-pRhB(+) (1-3, n = 1-3), comprising platinum(II) meso-tetraarylporphyrin (PtP) and Rhodamine B piperazine derivative (pRhB(+)), linked by oligo-p-phenylene bridges (Ph(n)), upon selective excitation of pRhB(+) at a frequency below that of the lowest allowed transition of PtP, room-temperature T(1)→S(0) phosphorescence of PtP was observed. The pathway leading to the emissive PtP triplet state includes excitation of pRhB(+), ET with formation of the singlet radical pair, intersystem crossing within that pair, and subsequent radical recombination. Because of the close proximity of the triplet energy levels of PtP and pRhB(+), reversible triplet-triplet (TT) energy transfer between these states was observed in dyads 1 and 2. As a result, the phosphorescence of PtP was extended in time by the long decay of the pRhB(+) triplet. Observation of ET and TT in the same series of molecules enabled direct comparison of the distance attenuation factors β between these two closely related processes. 相似文献
534.
Dariusz Szychowski Barbara Pacewska Grzegorz Makomaski Janusz Zieliński Wiesława Ciesińska Tatiana Brzozowska 《Journal of Thermal Analysis and Calorimetry》2012,107(3):893-900
The purpose of this study was to determine the possibility of producing hydrophobic mesoporous mineral–carbon sorbents from
aluminum hydroxide and compositions of coal tar pitch–polymers on carbonization at 600 °C in a nitrogen atmosphere. Blends
of the products of co-precipitation of aluminum hydroxide in the carbonaceous substances medium were subjected to carbonization
process. The extent of porous structure development was evaluated using low temperature nitrogen adsorption, adsorption of
benzene vapors, and adsorption of iodine from aqueous solution. The highest value of BET surface area of about 370 m2/g was achieved for the carbonization product obtained from co-precipitated raw components with 10 wt% compositions coal tar
pitch–polymer. These materials demonstrated high capacity to reduce organic pollutions from sewage. Pitch–polymer composition
containing poly(ethylene terephthalate) or phenol–formaldehyde resin was studied by the means of DSC method in order to determine
the high-temperature transformations taking place under the conditions of carbonization. DSC method enables to determine i.a.
the decomposition temperatures of carbonizates produced from pitch–polymer compositions and the evaluation of their sorption
abilities. The additive of poly(ethylene terephthalate) and phenol–formaldehyde resin caused the increase of thermal resistance
of the pitch expressed by higher decomposition temperatures. 相似文献
535.
Attila Borics Katarzyna Gach Jakub Fichna Dariusz Sobolewski Géza Toth Anna Janecka 《Central European Journal of Chemistry》2012,10(1):172-179
In the present study, the effect of a conformational constraint introduced into the endomorphin-2 (Tyr-Pro-Phe-Phe-NH2, EM-2) structure was studied using computational analysis and radioligand binding assay. EM-2 was modified by connecting
nitrogen atoms of both phenylalanine residues by a methylene bridge. The obtained analog did not bind to the μ- or δ-opioid
receptors in the in vitro studies. The computational analysis of this analog showed twisted, type IV turns and the absence of canonical β-turns typical
for the EM-2 structure, which can be explained by the lack of hydrogen bonds involving Phe4. Our results show that the introduction of chemical constraint in the EM-2 structure has a significant effect on opioid receptor
affinity and in vitro bioactivity. 相似文献
536.
Springer JW Parkes-Loach PS Reddy KR Krayer M Jiao J Lee GM Niedzwiedzki DM Harris MA Kirmaier C Bocian DF Lindsey JS Holten D Loach PA 《Journal of the American Chemical Society》2012,134(10):4589-4599
Biohybrid antenna systems have been constructed that contain synthetic chromophores attached to 31mer analogues of the bacterial photosynthetic core light-harvesting (LH1) β-polypeptide. The peptides are engineered with a Cys site for bioconjugation with maleimide-terminated chromophores, which include synthetic bacteriochlorins (BC1, BC2) with strong near-infrared absorption and commercial dyes Oregon green (OGR) and rhodamine red (RR) with strong absorption in the blue-green to yellow-orange regions. The peptides place the Cys 14 (or 6) residues before a native His site that binds bacteriochlorophyll a (BChl-a) and, like the native LH proteins, have high helical content as probed by single-reflection IR spectroscopy. The His residue associates with BChl-a as in the native LH1 β-polypeptide to form dimeric ββ-subunit complexes [31mer(-14Cys)X/BChl](2), where X is one of the synthetic chromophores. The native-like BChl-a dimer has Q(y) absorption at 820 nm and serves as the acceptor for energy from light absorbed by the appended synthetic chromophore. The energy-transfer characteristics of biohybrid complexes have been characterized by steady-state and time-resolved fluorescence and absorption measurements. The quantum yields of energy transfer from a synthetic chromophore located 14 residues from the BChl-coordinating His site are as follows: OGR (0.30) < RR (0.60) < BC2 (0.90). Oligomeric assemblies of the subunit complexes [31mer(-14Cys)X/BChl](n) are accompanied by a bathochromic shift of the Q(y) absorption of the BChl-a oligomer as far as the 850-nm position found in cyclic native photosynthetic LH2 complexes. Room-temperature stabilized oligomeric biohybrids have energy-transfer quantum yields comparable to those of the dimeric subunit complexes as follows: OGR (0.20) < RR (0.80) < BC1 (0.90). Thus, the new biohybrid antennas retain the energy-transfer and self-assembly characteristics of the native antenna complexes, offer enhanced coverage of the solar spectrum, and illustrate a versatile paradigm for the construction of artificial LH systems. 相似文献
537.
538.
Catalytic cross-metathesis of commercial divinyl sulfone allowed direct access to novel (E)-alkenylvinyl sulfones and (E,E)-dialkenyl sulfones with excellent stereoselectivity. These compounds are useful building blocks, e.g., in the synthesis of substituted thiomorpholine 1,1-dioxide derivatives. [reaction: see text] 相似文献
539.
We prove an Ekeland’s type vector variational principle for monotonically semicontinuous mappings with perturbations given
by a convex bounded subset of directions multiplied by the distance function. This generalizes the existing results where
directions of perturbations are singletons. 相似文献
540.
The affine group of a homogeneous tree is the group of all its isometries fixing an end of its boundary. We consider a random
walk with law μ on this group and the associated random processes on the tree and its boundary. In the drift-free case there exists on the
boundary of the tree a unique μ-invariant Radon measure. In this paper we describe its behaviour at infinity. 相似文献