Electron transfer rates and Franck–Condon factors: an application to the early electron transfer steps in photosynthetic reaction centers

Mechanistic aspects of some of the early electron transfer steps occurring in photosynthetic reaction centers are discussed. Starting from the normal modes of the redox cofactors involved in the electron transfer processes, we show how a series of quantities which regulate electron transfer rates, such as (i) the electron transfer active modes, (ii) the intramolecular reorganization energy, and (iii) the mutual couplings between the vibronic states of the donor and the acceptor, can be obtained and used to draw qualitative conclusions on ET rates.     

Structure and spectroscopic characterization of the quaternary system: Copper(II), pyridine-2,6-dicarboxylate, 2,2′-bipyridyl and pyridine

Conclusion The x-ray structural results confirm what has been ascertained by thermodynamic and spectroscopic data in aqueous solution(7). It is evident that the addition of pyridine to the solution containing the mixed species [Cu(bipy)(pydca)(H₂O)] leads to the substitution of a water molecule directly bound to copper(II) ion by a pyridine molecule. This experiment also demonstrated the presence of a water molecule in the equatorial plane of the complex.The subsequent diffractometric study on single crystals derived from the copper(II)/bipy/pydca system revealed the existence in the solid state of [Cu₂(bipy)₂(pydca)₂] · 4H₂O. Thus the pydca dianion, instead of forming the statistically favoured mixed complex [Cu(bipy)(pydca)], gives rise to crystals containing two different copper(II) environments: [Cu(pydca)₂]^2– and [Cu(bipy)₂]²⁺, linked by O-carboxylate bridges. The facility with which [Cu(bipy)(pydca)(py)] can be obtained shows that the addition of pyridine prevents the formation of polynuclear species.     

The influence of the solvent on the conformational energy differences due to the anomeric effect

The solvation free energy ΔG^sol of molecules exhibiting the anomeric effect is computed in an approach that considers a continuous distribution for the solvent. A partition of ΔG^sol into separately evaluated contributions confirms that changes in the energy of the systems due to changes in conformation of the solute are ruled by the electrostatic contribution. A comparison with the “exact” values indicates that the approximate expression for the electrostatic contribution to ΔG^sol are not accurate enough to permit a proper modeling of the solvent influence on the anomeric effect. The systems are composed of methanediol, methoxymethanol, dimethoxymethane, and 2-methoxytetrahydropyran in carbon tetrachloride, chloroform, acetone, and water. The calculations have been performed at the SCF level with the STO -3G and 4–31G basis sets.     

A novel oxidative side-chain transformation of α-amino acids and peptides by methyltrioxorhenium/H2O2 system

N-Boc derivatives of Met, Cys, and Trp, the properties of which resemble those of the respective amino acid residues present in proteins, are efficiently oxidized by methyltrioxorhenium and H₂O₂. A high regioselectivity for the oxidation of these residues when embedded into peptides was also found.     

A novel chemo-multienzymatic synthesis of bioactive cyclophellitol and epi-cyclophellitol in both enantiopure forms

A new route to synthesize cyclophellitol and epi-cyclophellitol from racemic starting materials in enantiopure forms has been developed. The synthesis involves a multi-enzymatic biotransformation pathway of the novel cyano-cyclitol (1R,4S,5R,6R)/(1S,4R,5S,6S)-4,5,6-trihydroxycyclohex-2-enecarbonitrile by a cooperative use of lipase, nitrile hydratase, and amidase.     

The occurrence of electron transfer in aromatic nitration: dynamical aspects

Electron transfer (ET) from toluene to the nitronium ion in the region of van der Waals intermolecular distances has been investigated by a quantum dynamical analysis performed on potential-energy surfaces computed at the ab initio multireference configuration interaction level. The results show that ET is very fast, occurring on a timescale of a few picoseconds. This has important implications for the mechanism of aromatic nitration: the ET path can compete efficiently with the direct attack of the nitronium ion to the aromatic substrate to yield the Wheland intermediate and that could explain some unsettled points in the mechanism of aromatic nitration. Received: 16 September 1999 / Accepted: 3 February 2000 / Published online: 12 May 2000     

Stimuli‐Responsive Lipidic Cubic Phase: Triggered Release and Sequestration of Guest Molecules

New stimuli‐responsive nanomaterials, made up of host–guest lipidic cubic phases (LCPs) are presented. These biocompatible, stable, transparent and water‐insoluble LCPs are composed of monoolein (MO) as a neutral host, and small amounts of one of three judiciously designed and synthesized designer lipids as guest that preserve the structure and stability of LCPs, but render them specific functionalities. Efficient pH‐ and light‐induced binding, release and sequestration of hydrophilic dyes are demonstrated. Significantly, these processes can be performed sequentially, thereby achieving both temporal and dosage control, opening up the possibility of using such LCPs as effective carriers to be used in drug delivery applications. Specifically, because of the inherent optical transparency and molecular isotropy of LCPs they can be envisaged as light‐induced drug carriers in ophthalmology. The results presented here demonstrate the potential of molecular design in creating new functional materials with predicted operating mode.     

Concentrated suspensions of Brownian beads in water: dynamic heterogeneities through a simple experimental technique

Concentrated suspensions of Brownian hard-spheres in water are an epitome for understanding the glassy dynamics of both soft materials and supercooled molecular liquids. From an experimental point of view, such systems are especially suited to perform particle tracking easily, and, therefore, are a benchmark for novel optical techniques, applicable when primary particles cannot be resolved. Differential variance analysis(DVA) is one such novel technique that simplifies significantly the characterization of structural relaxation processes of soft glassy materials, since it is directly applicable to digital image sequences of the sample. DVA succeeds in monitoring not only the average dynamics, but also its spatio-temporal fluctuations, known as dynamic heterogeneities. In this work, we study the dynamics of dense suspensions of Brownian beads in water, imaged through digital video-microscopy, by using both DVA and single-particle tracking. We focus on two commonly used signatures of dynamic heterogeneities: the dynamic susceptibility, χ_4, and the non-Gaussian parameter, α_2. By direct comparison of these two quantities,we are able to highlight similarities and differences. We do confirm that χ_4 and α_2 provide qualitatively similar information, but we find quantitative discrepancies in the scalings of characteristic time and length scale on approaching the glass transition.     

Degradation kinetic (D100) of lycopene during the thermal treatment of concentrated tomato paste

Heat treatments can cause degradation in tomatoes of lycopene which has important antioxidant effects. No information about decimal reduction time (D₁₀₀) of lycopene is available. D-value is the time required at a given temperature to reduce 90% of the molecule. This study for the first time determine the kinetic of lycopene thermal degradation. The content was measured at regular intervals of pasteurization using canned tomato paste to determinate D value. Microbiological analysis was carried out to verify product stability after packaging. Yeasts, molds and lactic acid bacteria were determined. The pasteurization time allowed to observe a loss of the red color. Lycopene content, after an increase at 8 min, decreased at 32 min of pasteurization. D₁₀₀ value was calculated at 75 min; a diminution of 90% in lycopene content in the concentrated tomato paste was observed. Microbiological analysis confirmed the stability of products after 8 min of pasteurization.