RAPID REDUCTION OF P-700 PHOTOOXIDIZED BY A FLASH AT LOW TEMPERATURE IN SPINACH CHLOROPLASTS

Abstract—Excitation of chloroplasts at low temperature (down to 4.2 K) by short laser flashes causes largely reversible absorption changes which are attributed to the photooxidation of P-700, the primary electron donor of Photosystem I. At temperatures below 100 K the dark re-reduction of P-700⁺ is biphasic. with half-times of about 122μs and 1.7 ms. The relative contribution of the two phases varies with temperature with the fast phase becoming dominant at the lowest temperatures (˜90% at 5 K). The results are interpreted in terms of tunnelling of the electron from one or two primary accepting sites back to P-700, a process which is largely dominant over the process of charge stabilisation.     

Metal-Catalyzed Haloalkynylation Reactions

Metal catalysis has revolutionized synthetic chemistry, leading to entirely new, very efficient transformations, which enable access to complex functionalized molecules. One such new transformation method is the haloalkynylation reaction, in which both a halogen atom and an alkynyl unit are transferred to an unsaturated carbon-carbon bond. This minireview summarizes the development of metal-catalyzed haloalkynylation reactions since their beginning about a decade ago. So far, arynes, alkenes and alkynes have been used as unsaturated systems and the reactivities of these systems are summarized in individual chapters of the minireview. Especially, the last few years have witnessed a rapid development due to gold-catalyzed reactions. Here, we discuss how the choice of the catalytic system influences the regio- and stereoselectivity of the addition.     

A new modeling approach for mixture fraction statistics based on dissipation elements

The probabilty density function (PDF) of the mixture fraction is of integral importance to a large number of combustion models. Here, a novel modelling approach for the PDF of the mixture fraction is proposed which employs dissipation elements. While being restricted to the commonly used mean and variance of the mixture fraction, this model approach individually considers contributions of the laminar regions as well as the turbulent core and the turbulent/non-turbulent interface region. The later region poses a highly intermittent part of the flow which is of high relevance to the non-premixed combustion of pure hydrocarbon fuels. The model assumptions are justified by means of the gradient trajectory based analysis of high fidelity direct numerical simulation (DNS) datasets of two turbulent inert configurations and a turbulent non-premixed jet flame. The new dissipation element based model is validated against the DNS datasets and a comparison with the beta PDF is presented.     

Spinodal decomposition of an ABv model alloy: Patterns at unstable surfaces

We develop mean-field kinetic equations for a lattice gas model of a binary alloy with vacancies (ABv model) in which diffusion takes place by a vacancy mechanism. These equations are applied to the study of phase separation of finite portions of an unstable mixture immersed in a stable vapor. Due to a larger mobility of surface atoms, the most unstable modes of spinodal decomposition are localized at the vapor-mixture interface. Simulations show checkerboard-like structures at the surface or surface-directed spinodal waves. We determine the growth rates of bulk and surface modes by a linear stability analysis and deduce the relation between the parameters of the model and the structure and length scale of the surface patterns. The thickness of the surface patterns is related to the concentration fluctuations in the initial state. Received 28 October 1998     

Gradient reversed-phase liquid chromatographic-electrospray ionization mass spectrometric method for the comparison of smokeless powders

A gradient reversed-phase liquid chromatographic-electrospray ionization mass spectrometric (LC-ESIMS) method was developed to determine compositional variation in the organic additives of smokeless powders. The method was optimized for the separation and detection of selected powder constituents, including diphenylamine, along with isomers of its nitroso and nitro derivatives, centralite I and II, in addition to dialkylphthalate acid esters. A series of commercially available smokeless powders was prepared by organic liquid extraction and characterized using the LC-ESIMS method. The results demonstrate the differentiation of smokeless powders by their additive profile.     

Palm-tree architectures derived from C60-terminated polystyrene

By grafting polystyryllithium (PSLi) or polyisoprenyllithium (PILi) onto the fullerene borne by the mono-adduct polystyrene (PS)-C₆₀ (≈90%) obtained upon reacting the fullerene with azide-terminated PS, fairly well-defined palm-tree like polymers PS_aC₆₀(PS_b)₅ and block copolymers PS_aC₆₀(PI)₅ could be obtained. In these architectures, 5 PS (PI) branches of equal length are connected to a PS trunk through a single fullerene molecule.     

Luminescent Eu(III) and Gd(III) trisbipyridine cryptates: experimental and theoretical study of the substituent effects.

Synthesis, absorption spectra and luminescebce properties of a series of lanthanide trisbipyridine cryptates Ln within R-Bpy x R-Bpy x R-Bpy, where Ln = Eu, Gd and R = H, COOH, COOCH3, CONH(CH2)2NH2 are described. Comparison of the unsubstituted parent compound with the substituted compounds shows that bipyridine substitution doesn't alter significantly the photophysical properties of the lanthanide cryptate. The absorption maximum is slightly red-shifted when three bipyridines are substituted, whereas substituting one bipyridines has a negligible effect on the absorption spectra. The experimental triplet state energy is between 21600 and 22 100 cm(-1) for the series of compounds and the luminescence lifetimes at 77 K are between 0.5 and 0.8 ms in HO2 and equal to 1.7 ms in D2O. The experimental characterizations are completed by DFT and TD-DFT calculations to assess the ability of these approaches to predict absorption maxima, triplet state energies and structural parameters of lanthanide cryptates and to characterize the electronic structure of the excited states. The calculations on the unsubstituted parent and substituted compounds show that absorption maxima and lowest 3pipi* triplet state energies can be accurately determined from density functional theory (DFT) and time-dependent (TD) DFT calculations.     

Morphology development during the polymerization of styrene within a polyurethane network: Investigations by small-angle X-ray and light scattering

Phase separation that takes place during the formation of semi-interpenetrating polymer networks based on crosslinked polyurethane and linear polystyrene was studied by small-angle X-ray scattering and light scattering. The kinetics of the chemical reactions was followed by Fourier transform infrared spectroscopy. The occurrence of broad peaks in the X-ray scattering curves was interpreted in terms of distances between the urethane crosslinks. Small modulations on these curves were assigned to sphere-like structures with a diameter of around 5 nm which might be related to the urethane crosslink regions. Small modulations on the light-scattering curves at the beginning of styrene polymerization were assigned to spheres with diameters of around 4.5 μm, which can be related to the polystyrene-rich phase. These modulations disappear with time, which might indicate an increasing polydispersity of the domain sizes. The final morphology was found to depend on the time at which polymerization of styrene is initiated with respect to the time of gelation of polyurethane.     

A Bioorthogonal Click Chemistry Toolbox for Targeted Synthesis of Branched and Well-Defined Protein–Protein Conjugates

Bioorthogonal chemistry holds great potential to generate difficult-to-access protein–protein conjugate architectures. Current applications are hampered by challenging protein expression systems, slow conjugation chemistry, use of undesirable catalysts, or often do not result in quantitative product formation. Here we present a highly efficient technology for protein functionalization with commonly used bioorthogonal motifs for Diels–Alder cycloaddition with inverse electron demand (DA_inv). With the aim of precisely generating branched protein chimeras, we systematically assessed the reactivity, stability and side product formation of various bioorthogonal chemistries directly at the protein level. We demonstrate the efficiency and versatility of our conjugation platform using different functional proteins and the therapeutic antibody trastuzumab. This technology enables fast and routine access to tailored and hitherto inaccessible protein chimeras useful for a variety of scientific disciplines. We expect our work to substantially enhance antibody applications such as immunodetection and protein toxin-based targeted cancer therapies.     

Gold(I)‐Catalyzed Haloalkynylation of Aryl Alkynes: Two Pathways,One Goal

Haloalkynylation reactions provide an efficient method for the simultaneous introduction of a halogen atom and an acetylenic unit. For the first time, we report a gold(I)‐catalyzed haloalkynylation of aryl alkynes that delivers exclusively the cis addition product. This method enables the simple synthesis of conjugated and halogenated enynes in yields of up to 90 %. Notably, quantum chemical calculations reveal an exceptional interplay between the place of the attack at the chloroacetylene: No matter which C?C bond is formed, the same enyne product is always formed. This is only possible through rearrangement of the corresponding skeleton. Hereby, one reaction pathway proceeds via a chloronium ion with a subsequent aryl shift; in the second case the corresponding vinyl cation is stabilized by a 1,3‐chlorine shift. ¹³C‐labeling experiments confirmed that the reaction proceeds through both reaction pathways.