More memory under evolutionary learning may lead to chaos

We show that an increase of memory of past strategy performance in a simple agent-based innovation model, with agents switching between costly innovation and cheap imitation, can be quantitatively stabilising while at the same time qualitatively destabilising. As memory in the fitness measure increases, the amplitude of price fluctuations decreases, but at the same time a bifurcation route to chaos may arise. The core mechanism leading to the chaotic behaviour in this model with strategy switching is that the map obtained for the system with memory is a convex combination of an increasing linear function and a decreasing non-linear function.     

Simple RF design for human functional and morphological cardiac imaging at 7 tesla

Morphological and functional cardiac MRI can potentially benefit greatly from the recent advent of commercial high-field (7 tesla and above) MRI systems. However, conventional hardware configurations at lower field using a body-coil for homogeneous transmission are not available at these field strengths. Sophisticated multiple-transmit-channel systems have been shown to be able to image the human heart at 7 tesla but such systems are currently not widely available. In this paper, we empirically optimize the design of a simple quadrature coil for cardiac imaging at 7 tesla. The size, geometry, and position have been chosen to produce a B₁ field with no tissue-induced signal voids within the heart. Standard navigator echoes for gating were adapted for operation at the heart/lung interface, directly along the head–foot direction. Using this setup, conventional and high-resolution cine functional imaging have been successfully performed, as has morphological imaging of the right coronary artery.     

Luminescence and energy transfer in lu(3)al(5)o(12) scintillators co-doped with ce(3+) and tb(3+)

Lu(3)Al(5)O(12) (LuAG) doped with Ce(3+) is a promising scintillator material with a high density and a fast response time. The light output under X-ray or γ-ray excitation is, however, well below the theoretical limit. In this paper the influence of codoping with Tb(3+) is investigated with the aim to increase the light output. High resolution spectra of singly doped LuAG (with Ce(3+) or Tb(3+)) are reported and provide insight into the energy level structure of the two ions in LuAG. For Ce(3+) zero-phonon lines and vibronic structure are observed for the two lowest energy 5d bands and the Stokes' shift (2 350 cm(-1)) and Huang-Rhys coupling parameter (S = 9) have been determined. Tb(3+) 4f-5d transitions to the high spin (HS) and low spin (LS) states are observed (including a zero-phonon line and vibrational structure for the high spin state). The HS-LS splitting of 5400 cm(-1) is smaller than usually observed and is explained by a reduction of the 5d-4f exchange coupling parameter J by covalency. Upon replacing the smaller Lu(3+) ion with the larger Tb(3+) ion, the crystal field splitting for the lowest 5d states increases, causing the lowest 5d state to shift below the (5)D(4) state of Tb(3+) and allowing for efficient energy transfer from Tb(3+) to Ce(3+) down to the lowest temperatures. Luminescence decay measurements confirm efficient energy transfer from Tb(3+) to Ce(3+) and provide a qualitative understanding of the energy transfer process. Co-doping with Tb(3+) does not result in the desired increase in light output, and an explanation based on electron trapping in defects is discussed.     

High-speed imaging of an ultrasound-driven bubble in contact with a wall: “Narcissus” effect and resolved acoustic streaming

We report microscopic observations of the primary flow oscillation of an acoustically driven bubble in contact with a wall, captured with the ultra high-speed camera Brandaris 128 (Chin et al. 2003). The driving frequency is up to 200 kHz, and the imaging frequency is up to 25 MHz. The details of the bubble motion during an ultrasound cycle are thus resolved, showing a combination of two modes of oscillations: a radius oscillation and a translation oscillation, perpendicular to the wall. This motion is interpreted using the theory of acoustic images to account for the presence of the wall. We conclude that the bubble is subjected to a periodic succession of attractive and repulsive forces, exerted by its own image. Fast-framing recordings of a tracer particle embedded in the liquid around the particle are performed. They fully resolve the acoustic streaming flow induced by the bubble oscillations. This non-linear secondary flow appears as a tiny drift of the particle position cycle after cycle, on top of the primary back and forth oscillation. The high oscillation frequency accounts for a fast average particle velocity, with characteristic timescales in the millisecond range at the lengthscale of the bubble. The features of the bubble motion being resolved, we can apply the acoustic streaming theory near a wall, which provides predictions in agreement with the observed streaming velocity.     

Natural Abundance 13C-NMR. of Reduced Isoalloxazine

When an isoalloxazine derivative ( 1 ) was reduced ( 2 ) its ¹³C-NMR. resonances were all shifted upfield and the largest shifts were observed for C (4a), C(6) and C(8) but downfield shifts for C(2), C(4) and C(10a). The results and their biochemical implications are shortly discussed.     

Substitution Effects on the Photophysical Characteristics of the Salicylic Anion

Spectral and photophysical properties of substituted salicylic anions are investigated by steady-state and time-resolved fluorescence spectroscopy for a variety of electron donating substituents at the positions para to the hydroxyl and carboxylic groups. Next to the usual excited-state intramolecular proton transfer in aqueous solution, an excited-state intermolecular proton transfer is found to be responsible for the dual emission observed in the case of 5-aminosalicylic anions.     

Microbubble spectroscopy of ultrasound contrast agents

A new optical characterization of the behavior of single ultrasound contrast bubbles is presented. The method consists of insonifying individual bubbles several times successively sweeping the applied frequency, and to record movies of the bubble response up to 25 million frames/s with an ultrahigh speed camera operated in a segmented mode. The method, termed microbubble spectroscopy, enables to reconstruct a resonance curve in a single run. The data is analyzed through a linearized model for coated bubbles. The results confirm the significant influence of the shell on the bubble dynamics: shell elasticity increases the resonance frequency by about 50%, and shell viscosity is responsible for about 70% of the total damping. The obtained value for shell elasticity is in quantative agreement with previously reported values. The shell viscosity increases significantly with the radius, revealing a new nonlinear behavior of the phospholipid coating.     

Continuous-flow protease assay based on fluorescence resonance energy transfer

A homogeneous continuous-flow assay using fluorescence resonance energy transfer (FRET) for detection was developed to measure the hydrolysis of HIV Protease Substrate 1 (to which two choromophores, EDANS and DABCYL are covalently attached) by a protease (e.g. Subtilisin Carlsberg) and the influence of inhibitors. In the continuous-flow assay, an inhibitor solution and an enzyme solution were first eluted into the system and allowed to react with each other in a reaction coil. Subsequently, the substrate solution was added to an enzyme-inhibitor mixture in a second reaction coil and incubated for 1 min. Finally, the fluorescence intensity was monitored.The system was also utilized to measure the inhibition of the protease by two weak acidity inhibitors which are 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF) and ethylenediaminetetraacetic acid (EDTA). Using the obtained optimum conditions for AEBSF, a detection limit of 0.3 mmol/l was achieved and the relative standard deviation was below 3.7% in the 2.5-7.5 mmol/l range. For EDTA, which required a 20 times higher substrate concentration than AEBSF, a detection limit of 0.2 mmol/l was obtained and the relative standard deviation was below 9.6% in the 0.5-7.5 mmol/l range.The optimization of pH, substrate concentration, enzyme concentration, reaction time and temperature are described. Organic modifier effects were also investigated. Methanol, acetonitrile and DMSO could be tolerated up to 30%.