Excitation energy transfer: Study with non-Markovian dynamics

In this paper, we investigate the non-Markovian dynamics of a model to mimic the excitation energy transfer (EET) between chromophores in photosynthesis systems. The numerical path integral method is used. This method includes the non-Markovian effects of the environmental affects and it does not need the perturbation approximation in solving the dynamics of systems of interest. It implies that the coherence helps the EET between chromophores through lasting the transfer time rather than enhances the transfer rate of the EET. In particular, the non-Markovian environment greatly increase the efficiency of the EET in the photosynthesis systems.Comment: 5 pages, 5 figure

Probing quantum-mechanical level repulsion in disordered systems by means of time-resolved selectively-excited resonance fluorescence

We argue that the time-resolved spectrum of selectively-excited resonance fluorescence at low temperature provides a tool for probing the quantum-mechanical level repulsion in the Lifshits tail of the electronic density of states in a wide variety of disordered materials. The technique, based on detecting the fast growth of a fluorescence peak that is red-shifted relative to the excitation frequency, is demonstrated explicitly by simulations on linear Frenkel exciton chains.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let

Cold trapped atoms detected with evanescent waves

We demonstrate the in situ detection of cold 87 Rb atoms near a dielectric surface using the absorption of a weak, resonant evanescent wave. We have used this technique in time of flight experiments determining the density of atoms falling on the surface. A quantitative understanding of the measured curve was obtained using a detailed calculation of the evanescent intensity distribution. We have also used it to detect atoms trapped near the surface in a standing-wave optical dipole potential. This trap was loaded by inelastic bouncing on a strong, repulsive evanescent potential. We estimate that we trap 1.5 x 10 4 atoms at a density 100 times higher than the falling atoms.Comment: 5 pages, 3 figure

Molecular Mechanics Simulations and Improved Tight-binding Hamiltonians for Artificial Light Harvesting Systems: Predicting Geometric Distributions, Disorder, and Spectroscopy of Chromophores in a Protein Environment

We present molecular mechanics {and spectroscopic} calculations on prototype artificial light harvesting systems consisting of chromophores attached to a tobacco mosaic virus (TMV) protein scaffold. These systems have been synthesized and characterized spectroscopically, but information about the microscopic configurations and geometry of these TMV-templated chromophore assemblies is largely unknown. We use a Monte Carlo conformational search algorithm to determine the preferred positions and orientations of two chromophores, Coumarin 343 together with its linker, and Oregon Green 488, when these are attached at two different sites (104 and 123) on the TMV protein. The resulting geometric information shows that the extent of disorder and aggregation properties, and therefore the optical properties of the TMV-templated chromophore assembly, are highly dependent on the choice of chromophores and protein site to which they are bound. We used the results of the conformational search as geometric parameters together with an improved tight-binding Hamiltonian to simulate the linear absorption spectra and compare with experimental spectral measurements. The ideal dipole approximation to the Hamiltonian is not valid since the distance between chromophores can be very small. We found that using the geometries from the conformational search is necessary to reproduce the features of the experimental spectral peaks

Picosecond fluorescence of intact and dissolved PSI-LHCI crystals

Over the last years many crystal structures of photosynthetic pigment-protein complexes have been determined, and used extensively to model spectroscopic results obtained on the same proteins in solution. However, the crystal structure is not necessarily identical to the structure of the protein in solution. Here we studied picosecond fluorescence of Photosystem I-Light Harvesting Complex I (PSI-LHCI), a multisubunit pigment protein complex that catalyzes the first steps of photosynthesis. The ultrafast fluorescence of PSI-LHCI crystals is identical to that of dissolved crystals, but differs considerably from most kinetics presented in literature. In contrast to most studies, the present data can be modeled quantitatively with only 2 compartments: PSI core and LHCI. This yields the rate of charge separation from an equilibrated core (22.5+/-2.5 ps) and rates of excitation energy transfer from LHCI to core (kLC) and vice versa (kCL). The ratio R=kCL/kLC between these rates appears to be wavelength-dependent and scales with the ratio of the absorption spectra of LHCI and core, indicating the validity of a detailed balance relation between both compartments. kLC depends slightly but non systematically on detection wavelength, averaging (9.4+/-4.9 ps)(-1). R ranges from 0.5 (below 690 nm) to around 1.3 above 720 nm

Yang-Yang thermodynamics on an atom chip

We investigate the behavior of a weakly interacting nearly one-dimensional (1D) trapped Bose gas at finite temperature. We perform in situ measurements of spatial density profiles and show that they are very well described by a model based on exact solutions obtained using the Yang-Yang thermodynamic formalism, in a regime where other, approximate theoretical approaches fail. We use Bose-gas focusing [Shvarchuck etal., Phys. Rev. Lett. 89, 270404 (2002)] to probe the axial momentum distribution of the gas, and find good agreement with the in situ results.Comment: extended introduction and conclusions, and minor changes throughout; accepted for publication in Phys. Rev. Let

Box traps on an atom chip for one-dimensional quantum gases

We present the implementation of tailored trapping potentials for ultracold gases on an atom chip. We realize highly elongated traps with box-like confinement along the long, axial direction combined with conventional harmonic confinement along the two radial directions. The design, fabrication and characterization of the atom chip and the box traps is described. We load ultracold ($\lesssim1 \mu$K) clouds of $^{87}$Rb in a box trap, and demonstrate Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily shaped potentials. Our results show that box-like axial potentials on atom chips are very promising for studies of one-dimensional quantum gases.Comment: 9 pages 4 figure

Three-dimensional character of atom-chip-based rf-dressed potentials

We experimentally investigate the properties of radio-frequency-dressed potentials for Bose-Einstein condensates on atom chips. The three-dimensional potential forms a connected pair of parallel waveguides. We show that rf-dressed potentials are robust against the effect of small magnetic-field variations on the trap potential. Long-lived dipole oscillations of condensates induced in the rf-dressed potentials can be tuned to a remarkably low damping rate. We study a beam-splitter for Bose-Einstein condensates and show that a propagating condensate can be dynamically split in two vertically separated parts and guided along two paths. The effect of gravity on the potential can be tuned and compensated for using a rf-field gradient.Comment: 9 pages, 7 figure

Ultrafast polarized fluorescence measurements on monomeric and self-associated melittin

The anisotropic and magic-angle fluorescence decay of the single tryptophan (Trp) residue of melittin, a bee venom peptide, was investigated by time-resolved fluorescence anisotropy using a streak camera setup. The peptide was dissolved either in distilled water or in Hepes/NaOH buffer containing low (10 mM) or high (2 M) concentrations of NaCl, the latter resulting in tetramerized melittin. For melittin in distilled water and low NaCl concentration, two anisotropy decay times were found in the order of similar to50 and similar to800 picoseconds, reflecting, local and overall peptide dynamics, respectively, and for tetramerized melittin, two anisotropy decay times of similar to200 and similar to5500 picoseconds were found. Decay-associated spectra of the isotropic fluorescence decay show three time components in the range of similar to20 picoseconds, similar to500 picoseconds, and similar to3500 picoseconds, respectively. The relative amplitudes of the latter two change upon the self-association of melittin. This change can be explained by the existence of different rotamers of Trp in melittin, of which one is more favored in the melittin tetramer than in the melittin monomer

The expression pattern of N-acetyltransferase 1 in healthy human skin

Background N-acetyltransferase 1 (NAT1) is an enzyme expressed among others in keratinocytes in human skin. NAT1 is important in the biotransformation of aromatic amines, an important example being p-phenylenediamine (PPD), a hair dye molecule. Unoxidized PPD penetrates the skin and is N-acetylated by NAT1. Objectives To investigate in detail the expression pattern of NAT1 in human skin. Materials and Methods Cryosections obtained from healthy human skin were stained for NAT1 and expression patterns were observed. NAT1 double stainings were performed with antibodies against different cellular organelles to determine expression patterns. Result A speckled, granular expression of NAT1 was seen predominantly in the stratum basale. NAT1 was expressed in a cytoplasmic pattern, perinuclear, and in the nucleus. No co-localisation was seen with the selected cellular organelles. Local differences in NAT1 expression patterns were observed between donors and between different biopsies obtained from the same donor. Conclusions NAT1 is expressed predominantly in the stratum basale and can be found in the cytoplasm, nucleus, and perinuclear in human skin. Further studies should be performed to investigate expression of NAT1 in a larger sample size