Biomedical subjects

Anton Plech

Publications and source records attributed to Anton Plech.

6 recordsLinked to original sources

PubMed2006

Spatiotemporal reaction kinetics of an ultrafast photoreaction pathway visualized by time-resolved liquid x-ray diffraction.

We have studied the reaction dynamics for HgI(2) in methanol by using time-resolved x-ray diffraction (TRXD). Although numerous time-resolved spectroscopic studies have provided ample information about the early dynamics of HgI(2), a comprehensive reaction mechanism in the solution phase spanning from picoseconds up to microseconds has been lacking. Here we show that TRXD can provide this information directly and quantitatively. Picosecond optical pulses triggered the dissociation of HgI(2), and 100-ps-long x-ray pulses from a synchrotron probed the evolving structures over a wide temporal range. To theoretically explain the diffracted intensities, the structural signal from the solute, the local structure around the solute, and the hydrodynamics of bulk solvents were considered in the analysis. The results in this work demonstrate that the determination of transient states in solution is strongly correlated with solvent energetics, and TRXD can be used as an ultrafast calorimeter. It also is shown that a manifold of structural channels can be resolved at the same time if the measurements are accurate enough and that global analysis is applied. The rate coefficients for the reactions were obtained by fitting our model against the experimental data in one global fit including all q-values and time delays. The comparison between all putative reaction channels confirms that two-body dissociation is the dominant dissociation pathway. After this primary bond breakage, two parallel channels proceed. Transient HgI associates nongeminately with an iodine atom to form HgI(2), and I(2) is formed by nongeminate association of two iodine atoms.

Journal Article↗

PubMed2004

Visualizing chemical reactions in solution by picosecond x-ray diffraction.

We present a time-resolved x-ray diffraction study to monitor the recombination of laser-dissociated iodine molecules dissolved in CCl4. The change in structure of iodine is followed during the whole recombination process. The deexcitation of solute molecules produces a heating of the solvent and induces tiny changes in its structure. The variations in the distance between pairs of chlorine atoms in adjacent CCl4 molecules are probed on the mA length scale. However, the most striking outcome of the present work is the experimental determination of temporally varying atom-atom pair distribution functions. Variations of the mean density of the solution during thermal expansion are also followed in real time. One concludes that not only time-resolved optical spectroscopy but also time-resolved x-ray diffraction can be used to monitor atomic motions in liquids.

Carbon Tetrachloride↗

PubMed2003

The realization of sub-nanosecond pump and probe experiments at the ESRF. European Synchrotron Radiation Facility.

We present beamline ID09B that is designed for pump and probe experiments to 50 ps time-resolution. The beamline has been refurbished with a narrow-bandwidth undulator for Laue diffraction and diffraction from liquids. The new undulator has 235 poles, a 17 mm magnetic period and is operated at 6.5 mm gap. It produces a spectral flux of 2.0 x 10(8) photon/0.1% bw/pulse (10 mA) at the fundamental at 15.5 keV and an integral flux of 1.1 x 10(10) photon pulse(-1) in a 2.5% bandwidth. The optics has been renewed with a high-precision toroidal mirror and a cryogenic monochromator. The X-ray chopper used for single pulse selection is also described together with the femtosecond laser. Finally the diffraction from excited iodine molecules in CCl4 is investigated on the nanosecond time-scale. It turns out that the high-angle scattering is insensitive to the thermal chock from the laser: these oscillations are therefore readily used for structure determination. Conversely, the low-angle scattering probes the hydrodynamics of the liquid over longer length scales and the oscillations are believed to originate from thermal stress and expansion of the solvent.

Journal Article↗

PubMed2003

Nanosecond time-resolved crystallography of photo-induced species: case study and instrument development for high-resolution excited-state single-crystal structure determination.

This work describes one of the first stages in the development of time-resolved photo-induced small-molecule single-crystal diffraction, whereby transient electron density perturbations, with lifetimes down to the nanosecond level, can be resolved at the atomic level. Knowledge of such ephemeral electronic effects is likely to yield key information regarding the origins of certain important physical properties, e.g. luminescent and non-linear optical effects, since it will allow the dynamics of electron density to be identified and quantified, and it is this that underpins such phenomena in a given molecule. The experimental methodology employs phase-locking pump-probe techniques such that the inherent time-structure of a synchrotron X-ray beam (nanoseconds) is harnessed and time-gated in-phase with a femtosecond laser. The resultant beams, made coincident on the crystal in a periodic manner, and a diffraction pattern are recorded as a function of the Bragg angle, theta. Such technology is based upon the pioneering work carried out in sub-nanosecond time-resolved crystallography of macromolecular biological moieties (non-atomic resolution) at the ESRF, although one crucial difference here is the use of monochromatic irradiation and oscillatory motion rather than Laue 'snapshot' methodology, so that atomic resolution is possible. The experimental details of a case study conducted on ID9 at the ESRF, France, are described, whereby the feasibility of the excited-state structure determination of a luminescent rhenium carbene complex, [HNCH2CH2NHCRe(2,2'-bipyridine)(CO)3]Br, is realised. Key experimental parameters that are required for the success of such an experiment are discussed in the light of this study, together with other feasibility work conducted at the SRS, UK, and in the laboratory. Plans, designs and tests for the implementation of this technique in the UK, first at the SRS, and then at DIAMOND, the forthcoming UK synchrotron, are described, in particular with reference to the world-leading potential that DIAMOND could lend toward the development of this technique.

Journal Article↗

PubMed2002

Diffuse scattering from liquid solutions with white-beam undulator radiation for photoexcitation studies.

Scattering from molecules in solution is a natural way to study fast reactions in solution with X-ray probes. With the availability of reliable femtosecond laser systems and pulsed synchrotron sources with high brilliance, it has become possible to study picosecond time-resolved photoexcitation in condensed matter. Owing to the low scattering cross section and the high background from the non-excited solvent, high flux and long exposure times are required to obtain information about isolated molecules in the conventional monochromatic scattering scheme. It is proposed that the full spectrum of a single-line undulator be used to obtain the diffuse scattering distribution. The bandwidth of 2-5% of the first harmonic, which is easily achievable in current insertion devices, is sufficient to allow the derivation of molecular form factors even in diluted systems. The relaxed bandwidth augments the usable flux drastically.

Journal Article↗

PubMed2002

Solid-liquid interface of a 2-propanol-perfluoromethylcyclohexane mixture: from adsorption to wetting.

The liquid-solid interface between a silicon substrate and the binary mixture perfluoromethylcyclohexane (PFMC) and 2-propanol (IP) is examined by x-ray specular reflectivity and diffuse scattering under grazing angles. The wetting films between the PFMC-rich phase and the substrate are characterized with respect to the density profile and lateral fluctuations. We find that the liquid-liquid interface of the film is anomalously broadened as compared to capillary wave theory. This broadening is caused by a locally slow variation of the density between the liquid phases and marks an adsorption profile that does not reflect the bulk properties of the film phase. Essentially the same behavior is present for a fused silica substrate.

Journal Article↗