635 research outputs found
Superradiance and exciton delocalization in bacterial photosynthetic light-harvesting systems.
LH-2 complexes of Rhodobacter sphaeroides and on the isolated B820 subunit of Rhodospirillum rubrum. From these measurements the superradiance is calculated, which is related to the delocalization of excitations in these complexes. In the B820 preparation we find a radiative rate that is 30 % higher than that of monomeric bacteriochlorophyll, in agreement with a dimer model of this subunit. At room temperature both LH-1 and LH-2 are superradiant relative to monomeric Bchl-a with enhancement factors of 3.8 and 2.8, respectively. In LH-2 the radiative rate does not change significantly upon lowering the temperature to 4 K. LH-1 however exhibits a strong temperature dependence, giving rise to a 2.4 times higher radiative rate at 4 K relative to room temperature. From modeling of the superradiance using a Hamiltonian based on the LH-2 structure and including site inhomogeneity, we conclude that the ratio of inhomogeneity over the coupling betwee
Three-pulse photon echo measurements on LH1 and LH2 complexes of Rhodobacter sphaeroides: A nonlinear spectroscopic probe of energy transfer.
Three-pulse echo peak shift measurements were performed on the B875 and B850 bands of detergent-isolated LH1 and LH2 complexes at room temperature. The peak shifts are much larger and decay much faster than typically observed for dye molecules in solution. Simulations of the peak shifts based on the optical transition frequency correlation function,M(t), are presented. M(t) includes contributions from rapid protein fluctuations, vibrational motion, and energy transfer. The model reproduces the room temperature absorption spectra of B850 and B875, shows that the coupling of electronic and nuclear degrees of freedom is much weaker than for dyes in solution, and identifies contributions to the line shapes that may be important to the energy transfer processes. The implications of these results for the extent of electronic delocalization in LH1 and LH2 are also discussed. Although the role of coherence transfer still needs to be understood, the results are shown to be consistent with the use of weak-coupling excitation transfer models of B850 and B875. I
Cluster derivation of Parisi's RSB solution for disordered systems
We propose a general scheme in which disordered systems are allowed to
sacrifice energy equi-partitioning and separate into a hierarchy of ergodic
sub-systems (clusters) with different characteristic time-scales and
temperatures. The details of the break-up follow from the requirement of
stationarity of the entropy of the slower cluster, at every level in the
hierarchy. We apply our ideas to the Sherrington-Kirkpatrick model, and show
how the Parisi solution can be {\it derived} quantitatively from plausible
physical principles. Our approach gives new insight into the physics behind
Parisi's solution and its relations with other theories, numerical experiments,
and short range models.Comment: 7 pages 5 figure
Linear dichroism of CdSe nanodots: Large anisotropy of the band-gap absorption induced by ground-state dipole moments
We measured the electric field induced linear dichroism for a wide range of sizes of CdSe nanocrystals. Large ground-state dipole moments were observed, especially for the smallest crystals. In these, we found a very large anisotropy of the absorption and most of the dipole strength is along the direction of the ground-state dipole moment. For the anisotropy, we propose a mechanism for intensity borrowing from intraband transitions, induced by the field of the ground-state dipole moment. © 2008 The American Physical Society
Ultrafast Electronic and Structural Phenomena in Graphite and Graphene
Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 - August 5, 201
A Solvable Model of Secondary Structure Formation in Random Hetero-Polymers
We propose and solve a simple model describing secondary structure formation
in random hetero-polymers. It describes monomers with a combination of
one-dimensional short-range interactions (representing steric forces and
hydrogen bonds) and infinite range interactions (representing polarity forces).
We solve our model using a combination of mean field and random field
techniques, leading to phase diagrams exhibiting second-order transitions
between folded, partially folded and unfolded states, including regions where
folding depends on initial conditions. Our theoretical results, which are in
excellent agreement with numerical simulations, lead to an appealing physical
picture of the folding process: the polarity forces drive the transition to a
collapsed state, the steric forces introduce monomer specificity, and the
hydrogen bonds stabilise the conformation by damping the frustration-induced
multiplicity of states.Comment: 24 pages, 14 figure
CO2-dosering bij roos onder verschillende temperaturen en belichtingniveaus : vervolgonderzoek kwantificering van CO2-effecten op productie en kwaliteit bij roos
Uit eerder onderzoek van PPO bleek dat met een hogere CO2-dosering en meer assimilatiebelichting een hogere productie van een betere kwaliteit kon worden gerealiseerd. De gegevens uit dat onderzoek werden gebruikt om een groeimodel voor groente (ECP) te valideren. Omdat het ECP-productiemodel niet geschikt bleek te zijn voor roos, is een vervolgonderzoek gestart om een dynamisch productiemodel te ontwikkelen specifiek voor roos. Zo kan de teler de productie en kwaliteit sturen door middel van drie "knoppen": CO2, licht en temperatuur. In deze publicatie staat een beschrijving van dit vervolgonderzoek. Twee CO2-concentraties (700 en 1400 ppm), twee lichtintensiteiten (3750 en 7100 lux), drie teelttemperaturen (17, 20 en 23°C) en twee gewasleeftijden (een jonge aanplant en een twee jaar oud gewas van ‘Indian Femma’) zijn onderling gecombineerd in 6 verschillende afdelingen. Daarnaast zijn twee extra afdelingen in gebruik genomen, waarin door middel van koelen met behulp van verneveling geprobeerd is CO2-verlies door ventilatie te vermindere
Photo-induced dynamics of the heme centers in cytochrome bc 1
The ultrafast response of cytochrome bc1 is investigated for the first time, via transient absorption
spectroscopy. The distinct redox potentials of both c1- and b-hemes allow for a clear differentiation of
their respective signals. We find that while the c1-heme photo-product exhibits the characteristics of a
5-coordinated species, the b-hemes presumably undergo photo-oxidation at a remarkably high
quantum yield. The c1-heme iron–ligand recombination time is 5.4 ps, in agreement with previous
reports on homologous cytochromes. The suggested photo-oxidized state of the b-hemes has a lifetime
of 6.8 ps. From this short life-time we infer that the electron acceptor must be within van der Walls
contact with the heme, which points to the fact that the axial histidine residue is the electron acceptor.
The different heme-responses illustrate the flexibility of the c1-heme ligation in contrast to the more
rigid b-heme binding, as well as the higher electronic reactivity of the b-hemes within the bc1 complex.
This study also demonstrates the remarkable connection between the heme local environment and its
dynamics and, therefore, biological functio
The low-energy forms of photosystem I light-harvesting complexes: Spectroscopic properties and pigment-pigment interaction characteristics
In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1-4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to "normal" chlorophyll a (Chl a) bands present in light-harvesting complexes. The low-energy forms are characterized by a very large bandwidth (400-450 c
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