Laser Driven Neutron Sources: Characteristics, Applications and Prospects

The basics of laser driven neutron sources, properties and possible applications are discussed. We describe the laser driven nuclear processes which trigger neutron generation, namely, nuclear reactions induced by laser driven ion beam (ion n), thermonuclear fusion by implosion and photo-induced nuclear (gamma n) reactions. Based on their main properties, i.e. point source (<100 μm) and short durations (< ns), different applications are described, such as radiography, time-resolved spectroscopy and pump-probe experiments. Prospects on the development of laser technology suggest that, as higher intensities and higher repetition rate lasers become available (for example, using DPSSL technology), laser driven methodologies may provide neutron fluxes comparable to that achieved by accelerator driven neutron sources in the near future.

A proposed two-stage two-tether scientific mission at Jupiter

A two-stage mission to place a spacecraft (SC) below the Jovian radiation belts, using a spinning bare tether with plasma contactors at both ends to provide propulsion and power,is proposed. Capture by Lorentz drag on the tether, at the periapsis of a barely hyperbolic equatorial orbit, is followed by a sequence of orbits at near-constant periapsis, drag finally bringing the SC down to a circular orbit below the halo ring. Although increasing both tether heating and bowing, retrograde motion can substantially reduce accumulated dose as compared with prograde motion, at equal tether-to-SC mass ratio. In the second stage,the tether is cut to a segment one order of magnitude smaller, with a single plasma contactor, making the SC to slowly spiral inward over severalmonths while generating large onboard power, which would allow multiple scientific applications, including in situ study of Jovian grains, auroral sounding of upper atmosphere, and space- and time-resolved observations of surface and subsurface.

Detection of internal quality in kiwi with a new optical technique (TDRS)

A compact system based on time-resolved diffuse reflectance spectroscopy (TDRS) has been developed to measure internal fruit quality parameters and has been applied to the non-destructive estimation of firmness, sugar content and acidity of kiwifruits. This new optical technique, developed in medical applications and related areas, provides a complete optical characterisation of a diffusive sample as it estimates at the same time and independently the light absorption inside the tissues and the scattering across them. The working principle of the technique is the analysis of the attenuation and broadening of the time-distribution of the remitted light, and the correct interpretation with a proper theoretical model. This main advantage compared to conventional optical techniques (which are only able to register the global attenuation spectrum) added to the compact, portable prototype developed along a three-year work opens the possibilities of this new measurement method in the food industry.

Web based interactive educational software introducing semiconductor laser dynamics: Sound Of Lasers (SOL)

In this work, educational software for intuitive understanding of the basic dynamic processes of semiconductor lasers is presented. The proposed tool is addressed to the students of optical communication courses, encouraging self consolidation of the subjects learned in lectures. The semiconductor laser model is based on the well known rate equations for the carrier density, photon density and optical phase. The direct modulation of the laser is considered with input parameters which can be selected by the user. Different options for the waveform, amplitude and frequency of thpoint. Simulation results are plotted for carrier density and output power versus time. Instantaneous frequency variations of the laser output are numerically shifted to the audible frequency range and sent to the computer loudspeakers. This results in an intuitive description of the “chirp” phenomenon due to amplitude-phase coupling, typical of directly modulated semiconductor lasers. In this way, the student can actually listen to the time resolved spectral content of the laser output. By changing the laser parameters and/or the modulation parameters,consequent variation of the laser output can be appreciated in intuitive manner. The proposed educational tool has been previously implemented by the same authors with locally executable software. In the present manuscript, we extend our previous work to a web based platform, offering improved distribution and allowing its use to the wide audience of the web.

trans/cis (Z/E) photoisomerization of the chromophore of photoactive yellow protein is not a prerequisite for the initiation of the photocycle of this photoreceptor protein

The chromophore of photoactive yellow protein (PYP) (i.e., 4-hydroxycinnamic acid) has been replaced by an analogue with a triple bond, rather than a double bond (by using 4-hydroxyphenylpropiolic acid in the reconstitution, yielding hybrid I) and by a “locked” chromophore (through reconstitution with 7-hydroxycoumarin-3-carboxylic acid, in which a covalent bridge is present across the vinyl bond, resulting in hybrid II). These hybrids absorb maximally at 464 and 443 nm, respectively, which indicates that in both hybrids the deprotonated chromophore does fit into the chromophore-binding pocket. Because the triple bond cannot undergo cis/trans (or E/Z) photoisomerization and because of the presence of the lock across the vinyl double bond in hybrid II, it was predicted that these two hybrids would not be able to photocycle. Surprisingly, both are able. We have demonstrated this ability by making use of transient absorption, low-temperature absorption, and Fourier-transform infrared (FTIR) spectroscopy. Both hybrids, upon photoexcitation, display authentic photocycle signals in terms of a red-shifted intermediate; hybrid I, in addition, goes through a blue-shifted-like intermediate state, with very slow kinetics. We interpret these results as further evidence that rotation of the carbonyl group of the thioester-linked chromophore of PYP, proposed in a previous FTIR study and visualized in recent time-resolved x-ray diffraction experiments, is of critical importance for photoactivation of PYP.

Mobilization of the A-kinase N-myristate through an isoform-specific intermolecular switch

Although the catalytic (C) subunit of cAMP-dependent protein kinase is N-myristylated, it is a soluble protein, and no physiological role has been identified for its myristyl moiety. To determine whether the interaction of the two regulatory (R) subunit isoforms (RI and RII) with the N-myristylated C subunit affects its ability to target membranes, the effect of N-myristylation and the RI and RII subunit isoforms on C subunit binding to phosphatidylcholine/phosphatidylserine liposomes was examined. Only the combination of N-myristylation and RII subunit interaction produced a dramatic increase in the rate of liposomal binding. To assess whether the RII subunit also increased the conformational flexibility of the C subunit N terminus, the effect of N-myristylation and the RI and RII subunits on the rotational freedom of the C subunit N terminus was measured. Specifically, fluorescein maleimide was conjugated to Cys-16 in the N-terminal domain of a K16C mutant of the C subunit, and the time-resolved emission anisotropy was determined. The interaction of the RII subunit, but not the RI subunit, significantly increased the backbone flexibility around the site of mutation and labeling, strongly suggesting that RII subunit binding to the myristylated C subunit induced a unique conformation of the C subunit that is associated with an increase in both the N-terminal flexibility and the exposure of the N-myristate. RII subunit thus appears to serve as an intermolecular switch that disrupts of the link between the N-terminal and core catalytic domains of the C subunit to expose the N-myristate and poise the holoenzyme for interaction with membranes.

Crossover isomer bias is the primary sequence-dependent property of immobilized Holliday junctions

Recombination of genes is essential to the evolution of genetic diversity, the segregation of chromosomes during cell division, and certain DNA repair processes. The Holliday junction, a four-arm, four-strand branched DNA crossover structure, is formed as a transient intermediate during genetic recombination and repair processes in the cell. The recognition and subsequent resolution of Holliday junctions into parental or recombined products appear to be critically dependent on their three-dimensional structure. Complementary NMR and time-resolved fluorescence resonance energy transfer experiments on immobilized four-arm DNA junctions reported here indicate that the Holliday junction cannot be viewed as a static structure but rather as an equilibrium mixture of two conformational isomers. Furthermore, the distribution between the two possible crossover isomers was found to depend on the sequence in a manner that was not anticipated on the basis of previous low-resolution experiments.

NMR of laser-polarized xenon in human blood

By means of optical pumping with laser light it is possible to enhance the nuclear spin polarization of gaseous xenon by four to five orders of magnitude. The enhanced polarization has allowed advances in nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), including polarization transfer to molecules and imaging of lungs and other void spaces. A critical issue for such applications is the delivery of xenon to the sample while maintaining the polarization. Described herein is an efficient method for the introduction of laser-polarized xenon into systems of biological and medical interest for the purpose of obtaining highly enhanced NMR/MRI signals. Using this method, we have made the first observation of the time-resolved process of xenon penetrating the red blood cells in fresh human blood—the xenon residence time constant in the red blood cells was measured to be 20.4 ± 2 ms. The potential of certain biologically compatible solvents for delivery of laser-polarized xenon to tissues for NMR/MRI is discussed in light of their respective relaxation and partitioning properties.

Interaction between HLA-DM and HLA-DR involves regions that undergo conformational changes at lysosomal pH

Antigenic peptide loading of major histocompatibility complex class II molecules is enhanced by lysosomal pH and catalyzed by the HLA-DM molecule. The physical mechanism behind the catalytic activity of DM was investigated by using time-resolved fluorescence anisotropy (TRFA) and fluorescence binding studies with the dye 8-anilino-1-naphthalenesulfonic acid (ANS). We demonstrate that the conformations of both HLA-DM and HLA-DR3, irrespective of the composition of bound peptide, are pH sensitive. Both complexes reversibly expose more nonpolar regions upon protonation. Interaction of DM with DR shields these hydrophobic domains from the aqueous environment, leading to stabilization of the DM and DR conformations. At lysosomal pH, the uncovering of additional hydrophobic patches leads to a more extensive DM–DR association. We propose that DM catalyzes class II peptide loading by stabilizing the low-pH conformation of DR, favoring peptide exchange. The DM–DR association involves a larger hydrophobic surface area with DR/class II-associated invariant chain peptides (CLIP) than with stable DR/peptide complexes, explaining the preferred association of DM with the former. The data support a release mechanism of DM from the DM–DR complex through reduction of the interactive surface, upon binding of class II molecules with antigenic peptide or upon neutralization of the DM–DR complex at the cell surface.

Reducing the flexibility of retinal restores a wild-type-like photocycle in bacteriorhodopsin mutants defective in protein–retinal coupling

The thermal re-isomerization of retinal from the 13-cis to the all-trans state is a key step in the final stages of the photocycle of the light-driven proton pump, bacteriorhodopsin. This step is greatly slowed upon replacement of Leu-93, a residue in van der Waals contact with retinal. The most likely role of this key interaction is that it restricts the flexibility of retinal. To test this hypothesis, we have exchanged native retinal in Leu-93 mutants with bridged retinal analogs that render retinal less flexible by restricting free rotation around either the C10—C11 (9,11-bridged retinal) or C12—C13 (11,13-bridged retinal) single bonds. The effect of the analogs on the photocycle was then determined spectroscopically by taking advantage of the previous finding that the decay of the O intermediate in the Leu-93 mutants provides a convenient marker for retinal re-isomerization. Time-resolved spectroscopic studies showed that both retinal analogs resulted in a dramatic acceleration of the photocycling time by increasing the rate of decay of the O intermediate. In particular, exchange of native retinal in the Leu-93 → Ala mutant with the 9,11-bridged retinal resulted in an acceleration of the decay of the O intermediate to a rate similar to that seen in wild-type bacteriorhodopsin. We conclude that the protein-induced restriction of conformational flexibility in retinal is a key structural requirement for efficient protein–retinal coupling in the bacteriorhodopsin photocycle.

Detection of cortical activation during averaged single trials of a cognitive task using functional magnetic resonance imaging

Functional neuroimaging studies in human subjects using positron emission tomography or functional magnetic resonance imaging (fMRI) are typically conducted by collecting data over extended time periods that contain many similar trials of a task. Here methods for acquiring fMRI data from single trials of a cognitive task are reported. In experiment one, whole brain fMRI was used to reliably detect single-trial responses in a prefrontal region within single subjects. In experiment two, higher temporal sampling of a more limited spatial field was used to measure temporal offsets between regions. Activation maps produced solely from the single-trial data were comparable to those produced from blocked runs. These findings suggest that single-trial paradigms will be able to exploit the high temporal resolution of fMRI. Such paradigms will provide experimental flexibility and time-resolved data for individual brain regions on a trial-by-trial basis.

Kinetic and structural analysis of mutant CD4 receptors that are defective in HIV gp120 binding

The T-cell antigen coreceptor CD4 also serves as the receptor for the envelope glycoprotein gp120 of HIV. Extensive mutational analysis of CD4 has implicated residues from a portion of the extracellular amino-terminal domain (D1) in gp120 binding. However, none of these proteins has been fully characterized biophysically, and thus the precise effects on molecular structure and binding interactions are unknown. In the present study, we produced soluble versions of three mutant CD4 molecules (F43V, G47S, and A55F) and characterized their structural properties, thermostability, and ability to bind gp120. Crystallographic and thermodynamic analysis showed minimal structural alterations in the F43V and G47S mutant proteins, which have solvent-exposed mutant side chains. In contrast, some degree of disorder appears to exist in the folded state of A55F, as a result of mutating a buried side chain. Real time kinetic measurements of the interaction of the mutant proteins with gp120 showed affinity decreases of 5-fold for G47S, 50-fold for A55F, and 200-fold for F43V. Although both rate constants for the binding reaction were affected by these mutations, the loss in affinity was mainly due to a decrease in on rates, with less drastic changes occurring in the off rates. These observations suggest the involvement of conformational adaptation in the CD4–gp120 interaction. Together, the structural and kinetic data confirm that F43V is a critical residue in gp120 recognition site, which may also include main chain interactions at residue Gly-47.

The photoisomerization of retinal in bacteriorhodopsin: Experimental evidence for a three-state model

The primary events in the all-trans to 13-cis photoisomerization of retinal in bacteriorhodopsin have been investigated with femtosecond time-resolved absorbance spectroscopy. Spectra measured over a broad range extending from 7000 to 22,400 cm−1 reveal features whose dynamics are inconsistent with a model proposed earlier to account for the highly efficient photoisomerization process. Emerging from this work is a new three-state model. Photoexcitation of retinal with visible light accesses a shallow well on the excited state potential energy surface. This well is bounded by a small barrier, arising from an avoided crossing that separates the Franck–Condon region from the nearby reactive region of the photoisomerization coordinate. At ambient temperatures, the reactive region is accessed with a time constant of ≈500 fs, whereupon the retinal rapidly twists and encounters a second avoided crossing region. The protein mediates the passage into the second avoided crossing region and thereby exerts control over the quantum yield for forming 13-cis retinal. The driving force for photoisomerization resides in the retinal, not in the surrounding protein. This view contrasts with an earlier model where photoexcitation was thought to access directly a reactive region of the excited-state potential and thereby drive the retinal to a twisted conformation within 100–200 fs.

Femtosecond observation of benzyne intermediates in a molecular beam: Bergman rearrangement in the isolated molecule

In this communication, we report our femtosecond real-time observation of the dynamics for the three didehydrobenzene molecules (p-, m-, and o-benzyne) generated from 1,4-, 1,3-, and 1,2-dibromobenzene, respectively, in a molecular beam, by using femtosecond time-resolved mass spectrometry. The time required for the first and the second C-Br bond breakage is less than 100 fs; the benzyne molecules are produced within 100 fs and then decay with a lifetime of 400 ps or more. Density functional theory and high-level ab initio calculations are also reported herein to elucidate the energetics along the reaction path. We discuss the dynamics and possible reaction mechanisms for the disappearance of benzyne intermediates. Our effort focuses on the isolated molecule dynamics of the three isomers on the femtosecond time scale.

Femtosecond dynamics of the forbidden carotenoid S1 state in light-harvesting complexes of purple bacteria observed after two-photon excitation

Time-resolved excited-state absorption intensities after direct two-photon excitation of the carotenoid S1 state are reported for light-harvesting complexes of purple bacteria. Direct excitation of the carotenoid S1 state enables the measurement of subsequent dynamics on a fs time scale without interference from higher excited states, such as the optically allowed S2 state or the recently discovered dark state situated between S1 and S2. The lifetimes of the carotenoid S1 states in the B800-B850 complex and B800-B820 complex of Rhodopseudomonas acidophila are 7 ± 0.5 ps and 6 ± 0.5 ps, respectively, and in the light-harvesting complex 2 of Rhodobacter sphaeroides ≈1.9 ± 0.5 ps. These results explain the differences in the carotenoid to bacteriochlorophyll energy transfer efficiency after S2 excitation. In Rps. acidophila the carotenoid S1 to bacteriochlorophyll energy transfer is found to be quite inefficient (φET1 <28%) whereas in Rb. sphaeroides this energy transfer is very efficient (φET1 ≈80%). The results are rationalized by calculations of the ensemble averaged time constants. We find that the Car S1 → B800 electronic energy transfer (EET) pathway (≈85%) dominates over Car S1 → B850 EET (≈15%) in Rb. sphaeroides, whereas in Rps. acidophila the Car S1 → B850 EET (≈60%) is more efficient than the Car S1 → B800 EET (≈40%). The individual electronic couplings for the Car S1 → BChl energy transfer are estimated to be approximately 5–26 cm−1. A major contribution to the difference between the energy transfer efficiencies can be explained by different Car S1 energy gaps in the two species.