Studying and switching electron transfer: from the ensemble to the single molecule

We report here on the systematic investigation of photoinduced intramolecular electron transfer (IET) in a series of donor-bridge-acceptor molecules as a means of understanding electron transport through the bridge. Perylenebisimide chromophores connected by various oligophenylene bridges are studied because their electron-transfer behavior can readily be monitored by following changes in the fluorescence intensity. We find dramatic switching of the IET behavior as the solvent polarity (dielectric constant) is increased. By combining steady-state and time-resolved fluorescence spectroscopy in a variety of solvents at multiple temperatures with standard theories of electron transfer, we determine parameters governing the IET behavior of these dimers, such as the electronic coupling through the bridges. We also deploy available ab initio quantum chemical methods to calculate the through-space component of the electronic coupling matrix element. Single-molecule investigations of the electron-transfer behavior also show that IET can be switched reversibly by a similar mechanism in an isolated individual molecule.     

Modulating Electrostatic Interactions in Ion Pair Intermediates To Alter Site Selectivity in the C−O Deoxygenation of Sugars

Controlling which products one can access from the predefined biomass‐derived sugars is challenging. Changing from CH₂Cl₂ to the greener alternative toluene alters which C?O bonds in a sugar are cleaved by the tris(pentafluorophenyl)borane/HSiR₃ catalyst system. This increases the diversity of high‐value products that can be obtained through one‐step, high‐yielding, catalytic transformations of the mono‐, di‐, and oligosaccharides. Computational methods helped identify this non‐intuitive outcome in low dielectric solvents to non‐isotropic electrostatic enhancements in the key ion pair intermediates, which influence the reaction coordinate in the reactivity‐/selectivity‐determining step. Molecular‐level models for these effects have far‐reaching consequences in stereoselective ion pair catalysis.     

The reactions of isotopically labeled N15NO+ with CO,NO, O2, N2, NO2, and N2O

The reactions of labeled N¹⁵NO⁺ with CO, NO, O₂, ¹⁸O₂, N₂, NO₂, and N₂O have been investigated using a tandem ICR instrument. In each case the total rate coefficient, product distribution, and kinetic energy dependence were measured. The results indicate that very specific reaction mechanisms govern these reactions. This conclusion is suggested by the lack of isotopic scrambling in many cases and by the complete absence of energetically allowed products in almost all of the systems. The kinetic energy studies indicate that most of the reaction channels proceed through an intermediate complex at low energies and via a direct mechanism at higher kinetic energies. Such direct mechanisms include long range charge transfer and atom or ion transfer.     

Brillouin-enhanced four-wave-mixing vector phase-conjugate mirror with beam-combining capability

It is often desirable to remove both wave front and polarization aberrations from an optical beam. Scalar phase conjugation, such as ordinary stimulated Brillouin scattering, can correct only for wave-front aberrations. We have developed a new geometry for Brillouin-enhanced four-wave mixing that performs vector phase conjugation to correct for both wave-front and polarization distortions. Results show a reduction in the depolarization losses from 50% to less than 2% of the total output energy. Coherent, variable, multiple-beam combination is achieved without need of nonreciprocal devices such as Faraday rotators.     

Ab initio studies of structural features not easily amenable to experiment: Part V. Conformational analysis of keto- and enol-acetone

The structures of several conformations of keto- and enol-acetone were determined by unconstrained ab initio geometry refinements using the 4-21G basis set. The geometry of propene was also refined to compare it with enol-acetone. The structural consequences of hyperconjugation for the local geometries of the methyl groups were determined in all conformations. In the most stable form of keto-acetone, one hydrogen atom of each methyl group was found in an eclipsed arrangement with respect to the carbonyl group. The stability of this crowded structure has previously been rationalized in terms of aromatic π-electron delocalization. This result is in contrast to one of two previous gas electron diffraction studies. It is concluded that the electron diffraction data may not contain enough information to determine the exact conformational arrangement of the methyl groups in acetone. The calculated structures are found to be in excellent agreement with experiment. Uncertainties in calculated bond distances and bond angles are on the order of magnitude of 0.01–0.02 Å and 1–2°, respectively.     

Thermospray liquid chromatography-mass spectrometry for the characterisation of sulphate ester conjugates.

Formation of polar conjugates is a well documented metabolic pathway for xenobiotics containing phenolic hydroxyl groups. This paper describes the analysis of two sulphate ester conjugates by fast atom bombardment mass spectrometry and thermospray liquid chromatography-mass spectrometry. Thermospray liquid chromatography-mass spectrometry proved the more successful technique for obtaining the molecular weight of the intact conjugate, but only by removal of the buffer from the high-performance liquid chromatography eluent.     

Is it biologically relevant to measure the structures of small peptides in the gas-phase?

Recent developments in sample introduction of biologically relevant molecules have heralded a new era for gas-phase methods of structural determination. One of the biggest challenges is to relate gas-phase structures, often measured in the absence of water and counter ions, with in vivo biologically active structures. An advantage of gas-phase based techniques is that a given peptide can be analysed in a variety of different forms, for example, as a function of charge state, or with additional water molecules. Molecular modelling can provide insight into experimental findings and help elucidate the differences between structural forms. Combining experiment and theory provides a thorough interrogation of candidate conformations. Here two important naturally occurring peptide systems have been examined in detail and results are assessed in terms of their biological significance.The first of these is gonadotropin-releasing hormone (GnRH), a decapeptide which is the central regulator of the reproductive system in vertebrates. We have examined several naturally occurring variants of this peptide using Ion Mobility Mass Spectrometry and Electron Capture Dissociation (ECD) in conjunction with Fourier Transform Ion Cyclotron Mass Spectrometry (FT-ICR-MS). Candidate conformations are modelled using the AMBER force field. Single amino acid changes, for example Gly6 → Ala6, or Ala6 → D-Ala6, have observable effects on the gas phase structure of GnRH. It has been shown that evolutionary primary sequence variations are key to the biological activity of GnRH, and it is thought that this is due to different binding affinities at target receptors. This work provides strong evidence that this activity is structurally based. The second system examined is the relationship between the quaternary structure and activity of two novel β-defensins. FT-ICR mass spectrometry has been employed to characterize di-sulphide bridging and dissociation based experiments utilised to investigate their structural core. Defr1, with five cysteines, exists as a covalently bound disulphide linked dimer; Defr1 Y5C with six cysteines also is observed as a dimer, but non-covalently bound, suggesting that this defensin has a tendency to aggregate. The activity of Defr1 is 10 times higher than that of Defr1 Y5C when tested against the pathogen Pseudomonas aeruginosa. The results from these studies could inform future design of novel GnRH type ligands and anti-microbial agents, and illustrate the power of gas-phase based techniques for solving peptide structures.     

Characteristic equations for different ARROW structures

Based on radiation modes and phase relations in different ARROW structures, the characteristic equations are presented that can avoid root searching in the complex plane and find the effective index, loss and field profile easily. This simple model gives an accurate intuitive picture for low loss leaky modes and it can be used to design and optimize the low loss ARROW devices of practical interest.