314 research outputs found
Hybrid-GewÀchs : Fallbasierte Expertensystem-Shell ESTEEM
Denken Menschen in Objekten? Oder in Regeln? Haben Ihre Wlssensstrukturen die Gestalt relationaler Datenbanken? Wer Expertenwissen auf den Rechner bringen will, sollte sich vor einfachen Antworten auf dieee Fragen hĂŒten
Evaluating the Nature of So-Called S*-State Feature in Transient Absorption of Carotenoids in Light-Harvesting Complex 2 (LH2) from Purple Photosynthetic Bacteria
Carotenoids are a class of natural pigments present in
all phototrophic organisms, mainly in their light-harvesting proteins in
which they play roles of accessory light absorbers and photoprotectors.
Extensive time-resolved spectroscopic studies of these
pigments have revealed unexpectedly complex photophysical properties,
particularly for carotenoids in light-harvesting LH2 complexes
from purple bacteria. An ambiguous, optically forbidden electronic
excited state designated as S* has been postulated to be involved in
carotenoid excitation relaxation and in an alternative carotenoid-tobacteriochlorophyll
energy transfer pathway, as well as being a
precursor of the carotenoid triplet state. However, no definitive and
satisfactory origin of the carotenoid S* state in these complexes has
been established, despite a wide-ranging series of studies. Here, we
resolve the ambiguous origin of the carotenoid S* state in LH2 complex from Rba. sphaeroides by showing that the S* feature can
be seen as a combination of ground state absorption bleaching of the carotenoid pool converted to cations and the Stark
spectrum of neighbor neutral carotenoids, induced by temporal electric field brought by the carotenoid cationâ
bacteriochlorophyll anion pair. These findings remove the need to assign an S* state, and thereby significantly simplify the
photochemistry of carotenoids in these photosynthetic antenna complexes
First record of ategmic ovules in Orchidaceae offers new insights into Mycoheterotrophic plants
The number of integuments found in angiosperm ovules is variable. In orchids, most species show bitegmic ovules, except for some mycoheterotrophic species that show ovules with only one integument. Analysis of ovules and the development of the seed coat provide important information regarding functional aspects such as dispersal and seed germination. This study aimed to analyze the origin and development of the seed coat of the mycoheterotrophic orchid Pogoniopsis schenckii and to compare this development with that of other photosynthetic species of the family. Flowers and fruits at different stages of development were collected, and the usual methodology for performing anatomical studies, scanning microscopy, and transmission microscopy following established protocols. P. schenckii have ategmic ovules, while the other species are bitegmic. No evidence of integument formation at any stage of development was found through anatomical studies. The reduction of integuments found in the ovules could facilitate fertilization in this species. The seeds of P. schenckii, Vanilla planifolia, and V. palmarum have hard seed coats, while the other species have seed coats formed by the testa alone, making them thin and transparent. P. schenckii, in contrast to the other species analyzed, has a seed coat that originates from the nucellar epidermis, while in other species, the seed coat originates from the outer integument10CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTĂFICO E TECNOLĂGICO - CNPQCOORDENAĂĂO DE APERFEIĂOAMENTO DE PESSOAL DE NĂVEL SUPERIOR - CAPESFUNDAĂĂO DE AMPARO Ă PESQUISA DO ESTADO DE SĂO PAULO - FAPESP447453/2014-9; 310184/2016-9sem informação2015/26479-6This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de NĂvel Superior - Brasil (CAPES) - Finance Code 001. Juliana Lischka Sampaio Mayer thank FAPESP (2015/26479-6), FAEPEX 0944/14, CNPq (447453/2014-9), and CNPQ (310184/2016-9) for funding suppor
Quenching Capabilities of Long-Chain Carotenoids in Light-Harvesting-2 Complexes from Rhodobacter sphaeroides with an Engineered Carotenoid Synthesis Pathway
Six light-harvesting-2 complexes (LH2) from genetically modified
strains of the purple photosynthetic bacterium Rhodobacter (Rb.) sphaeroides were
studied using static and ultrafast optical methods and resonance Raman
spectroscopy. These strains were engineered to incorporate carotenoids for
which the number of conjugated groups (N = NCC + NCO) varies from 9 to 15.
The Rb. sphaeroides strains incorporate their native carotenoids spheroidene (N =
10) and spheroidenone (N = 11), as well as longer-chain analogues including
spirilloxanthin (N = 13) and diketospirilloxantion (N = 15) normally found in
Rhodospirillum rubrum. Measurements of the properties of the carotenoid first
singlet excited state (S1) in antennas from the Rb. sphaeroides set show that
carotenoid-bacteriochlorophyll a (BChl a) interactions are similar to those in LH2 complexes from various other bacterial species
and thus are not significantly impacted by differences in polypeptide composition. Instead, variations in carotenoid-to-BChl a
energy transfer are primarily regulated by the N-determined energy of the carotenoid S1 excited state, which for long-chain (N â„
13) carotenoids is not involved in energy transfer. Furthermore, the role of the long-chain carotenoids switches from a lightharvesting
supporter (via energy transfer to BChl a) to a quencher of the BChl a S1 excited state B850*. This quenching is
manifested as a substantial (âŒ2-fold) reduction of the B850* lifetime and the B850* fluorescence quantum yield for LH2
housing the longest carotenoids
Carotenoid-to-(bacterio)chlorophyll energy transfer in LH2 antenna complexes from Rba. sphaeroides reconstituted with non-native (bacterio)chlorophylls
Six variants of the LH2 antenna complex from Rba. sphaeroides, comprising the native B800-B850, B800-free LH2 (B850) and four LH2s with various (bacterio)chlorophylls reconstituted into the B800 site, have been investigated with static and time-resolved optical spectroscopies at room temperature and at 77 K. The study particularly focused on how reconstitution of a non-native (bacterio)chlorophylls affects excitation energy transfer between the naturally bound carotenoid spheroidene and artificially substituted pigments in the B800 site. Results demonstrate there is no apparent trend in the overall energy transfer rate from spheroidene to B850 bacteriochlorophyll a; however, a trend in energy transfer rate from the spheroidene S1 state to Qy of the B800 (bacterio)chlorophylls is noticeable. These outcomes were applied to test the validity of previously proposed energy values of the spheroidene S1 state, supporting a value in the vicinity of 13,400 cmâ1 (746 nm)
New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides
Light-harvesting complex 2 (LH2) from the
semi-aerobically grown purple phototrophic bacterium
Rhodobacter sphaeroides was studied using optical (static
and time-resolved) and resonance Raman spectroscopies.
This antenna complex comprises bacteriochlorophyll
(BChl) a and the carotenoid spheroidenone, a ketolated
derivative of spheroidene. The results indicate that the
spheroidenone-LH2 complex contains two spectral forms
of the carotenoid: (1) a minor, ââblueââ form with an S2
(11
Bu
?) spectral origin band at 522 nm, shifted from the
position in organic media simply by the high polarizability
of the binding site, and (2) the major, ââredââ form with the
origin band at 562 nm that is associated with a pool of
pigments that more strongly interact with protein residues,
most likely via hydrogen bonding. Application of targeted
modeling of excited-state decay pathways after carotenoid
excitation suggests that the high (92%) carotenoid-to-BChl
energy transfer efficiency in this LH2 system, relative to
LH2 complexes binding carotenoids with comparable
double-bond conjugation lengths, derives mainly from
resonance energy transfer from spheroidenone S2 (11
Bu
?)
state to BChl a via the Qx state of the latter, accounting for
60% of the total transfer. The elevated S2 (11
Bu
?) ? Qx
transfer efficiency is apparently associated with substantially
decreased energy gap (increased spectral overlap)
between the virtual S2 (11
Bu
?) ? S0 (11
Ag
-) carotenoid
emission and Qx absorption of BChl a. This reduced
energetic gap is the ultimate consequence of strong carotenoidâprotein
interactions, including the inferred hydrogen
bondin
Carotenoid-to-(bacterio)chlorophyll energy transfer in LH2 antenna complexes from Rba. sphaeroides reconstituted with non-native (bacterio)chlorophylls
Six variants of the LH2 antenna complex from Rba. sphaeroides, comprising the native B800-B850, B800-free LH2 (B850) and four LH2s with various (bacterio)chlorophylls reconstituted into the B800 site, have been investigated with static and time-resolved optical spectroscopies at room temperature and at 77 K. The study particularly focused on how reconstitution of a non-native (bacterio)chlorophylls affects excitation energy transfer between the naturally bound carotenoid spheroidene and artificially substituted pigments in the B800 site. Results demonstrate there is no apparent trend in the overall energy transfer rate from spheroidene to B850 bacteriochlorophyll a; however, a trend in energy transfer rate from the spheroidene S1 state to Qy of the B800 (bacterio)chlorophylls is noticeable. These outcomes were applied to test the validity of previously proposed energy values of the spheroidene S1 state, supporting a value in the vicinity of 13,400 cmâ1 (746 nm)
A photosynthetic antenna complex foregoes unity carotenoid-to-bacteriochlorophyll energy transfer efficiency to ensure photoprotection
Carotenoids play a number of important roles in photosynthesis, primarily providing light-harvesting and photoprotective energy dissipation functions within pigmentâprotein complexes. The carbonâcarbon double bond (C=C) conjugation length of carotenoids (N), generally between 9 and 15, determines the carotenoid-to-(bacterio)chlorophyll [(B)Chl] energy transfer efficiency. Here we purified and spectroscopically characterized light-harvesting complex 2 (LH2) from Rhodobacter sphaeroides containing the N = 7 carotenoid zeta (ζ)-carotene, not previously incorporated within a natural antenna complex. Transient absorption and time-resolved fluorescence show that, relative to the lifetime of the S1 state of ζ-carotene in solvent, the lifetime decreases âŒ250-fold when ζ-carotene is incorporated within LH2, due to transfer of excitation energy to the B800 and B850 BChls a. These measurements show that energy transfer proceeds with an efficiency of âŒ100%, primarily via the S1 â Qx route because the S1 â S0 fluorescence emission of ζ-carotene overlaps almost perfectly with the Qx absorption band of the BChls. However, transient absorption measurements performed on microsecond timescales reveal that, unlike the native N â„ 9 carotenoids normally utilized in light-harvesting complexes, ζ-carotene does not quench excited triplet states of BChl a, likely due to elevation of the ζ-carotene triplet energy state above that of BChl a. These findings provide insights into the coevolution of photosynthetic pigments and pigmentâprotein complexes. We propose that the N â„ 9 carotenoids found in light-harvesting antenna complexes represent a vital compromise that retains an acceptable level of energy transfer from carotenoids to (B)Chls while allowing acquisition of a new, essential function, namely, photoprotective quenching of harmful (B)Chl triplets
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