An efficient route to the site-selective reactivity of electronically excited states of multicentered molecules is discussed. In the first stage the migration of the electronic excitation occurs. This can operate over an extensive range without extensive draining of energy into the nuclear frame. Only in a second stage, once the optimal site has been reached, does the excess energy become available for bond breaking or isomerization at the new, optimal, site. This two-stage mechanism, where electronic excitation (or the charge) is the scout, avoids the pitfall of conventional large molecule kinetics. (In that view, known as the quasi equilibrium theory, the electronic excitation is first converted to nuclear modes. But then there are so many available vibrational states that the probability for the excitation energy to become localized at the necessary site, is too small and the resulting reaction rate is too slow.) By confining the site search to the electronic manifold, it becomes a highly efficient process. The recent novel experiments of Weinkauf et al. on (positive) charge migration and dissociation of peptide ions are suggested as an example of the considerations above where there is a facile migration of the positive charge followed by reactivity at the selected site. The peptide is modeled as beads on a chain. Interbead and intrabead coupling are discussed in terms of adiabatic and diabatic states. We find a multistep mechanism (unlike superexchange): a charge-directed reactivity (CDR) model. Such efficient ranging could also take place in other chain structures and suggests that there will be examples where electronic processes set the time scale for the chemical change.
We report measurements of the frequency dependence of anomalous second-order electric susceptibilities of polymer films containing hyperpolarizable chromophore guest molecules and poled using coplanar electrodes. The frequency dependence, polarization dependence, and magnitude of the observed second-order susceptibilities are radically different from those of conventional corona-field poling experiments and cannot be explained by isolated chromophore models. These observations and the results of sum-over-states perturbative electronic structure calculations argue for the importance of positively charged chromophore aggregates.
A Drosophila cell-free system was used to characterize proteins that are required for targeting vesicles to chromatin and for fusion of vesicles to form nuclear envelopes. Treatment of vesicles with 1 M NaCl abolished their ability to bind to chromatin. Binding of salt-treated vesicles to chromatin could be restored by adding the dialyzed salt extract. Lamin Dm is one of the peripheral proteins whose activity was required, since supplying interphase lamin isoforms Dm1, and Dm2 to the assembly extract restored binding. As opposed to the findings in Xenopus, okadaic acid had no effect on vesicle binding. Trypsin digestion of the salt-stripped vesicles eliminated their association with chromatin even in the presence of the dialyzed salt extract. One vesicles attached to chromatin surface, fusion events took place were found to be sensitive to guanosine 5'-[gamma-thio]triphosphate (GTP gamma S). These chromatin-attached vesicles contained lamin Dm and otefin but not gp210. Thus, these results show that in Drosophila there are two populations of nuclear vesicles. The population that interacts first with chromatin contains lamin and otefin and requires both peripheral and integral membrane proteins, whereas fusion of vesicles requires GTPase activity.
J.Yellin. 1997. “The Obsidian from Netiv Hagdud: Chemical Characterization and Origin.” In An Early Neolithic Village in the Jordan Valley Part I: The Archaeology of Netiv Hagdud, edited by Ofer Bar-Yosef and Avi Gopher. Cambridge: American School of Prehistoric Research Bulletin 43, Peabody Museum of Archaeology and Ethnology, Harvard University.
J.Gunneweg and J.Yellin. 1997. “The Origin of Some Plain Ware Pottery from Tel Anafa.” In Tel Anafa II, I, The Hellenistic and Roman Pottery: The Plain Wares by Andrea Berlin, The Fine Wares by Kathleen Warner Slane, edited by Sharn C. Herbert, Pp. 236-244. Ann Arbor: Journal of Roman Archaeology and Kelsey Museum.
Some non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the production or actions of oxygen radicals generated by polymorphonuclear leucocytes (PMNs); this mechanism may contribute towards their anti-inflammatory activity. In the present study, the effects of a new enolcarboxamide NSAID, meloxicam, on oxyradical production by human PMNs exposed to various stimuli in vitro were compared with those of other standard NSAIDs. The various stimuli employed were intended to mimic the likely synergies which occur with cytokines and bacterial production (e.g. f-met-leu-phe (fMLP) peptide) in inflamed tissues and to give an insight into the site and mechanism of action of meloxicam and related drugs on the cellular processes involved in oxyradical generation.
The results show that meloxicam is a potent inhibitor of oxyradical production at drug concentrations comparable with those encountered during therapy. Its mechanism of action appears similar to that of other enolcarboxamides and, while relatively complex, involves effects which are stimulus dependent and myeloperoxidase sensitive. They probably do not involve inhibition of fMLP-Gi protein receptor activation but may involve tumour necrosis factor-⇌ post-receptor activation. Enolcarboxamides have variable effects on phorbol myristate acetate-protein kinase C3-mediated oxyradical production.
The effect of diethyldithiocarbamate (DDC) and sodium nitroprusside (SNP) on the killing of endothelial cells and on the release of arachidonate by mixtures of oxidants and membrane-damaging agents was studied in a tissue culture model employing bovine aortic endothelial cells labeled either with 51Chromium or 3arachidonic acid. While exposure to low, subtoxic concentrations of oxidants (reagent H2O2, glucose-oxidase generated peroxide, xanthine xanthine oxidase, AAPH-generated peroxyl radical, menadione-generated oxidants) did not result either in cell death or in the loss of membrane-associated arachidonic acid, the addition of subtoxic amounts of a variety of membrane-damaging agents (streptolysin S, PLA2, histone, taurocholate, wheatgerm agglutinin) resulted in a synergistic cell death. However, no significant amounts of arachidonate were released unless proteinases were also present. The addition to these reaction mixtures of subtoxic amounts of DDC (an SOD inhibitor and a copper chelator) not only very markedly enhanced cell death but also resulted in the release of large amounts of arachidonate (in the complete absence of added proteinases). Furthermore, the inclusion in DDC-containing reaction mixtures of subtoxic amounts of SNP, a generator of NO, further enhanced, in a synergistic manner, both cell killing and the release of arachidonate. Cell killing and the release of arachidonate induced by the DDC and SNP-containing mixtures of agonists were strongly inhibited by catalase, glutathione, N-acetyl cysteine, vitamin A, and by a nonpenetrating PLA2 inhibitor as well as by tetracyclines. A partial inhibition of cell killing was also obtained by 1,10-phenanthroline and by antimycin. It is suggested that DDC might amplify cell damage by forming intracellular, loosely-bound complexes with copper and probably also by depleting antioxidant thiols. It is also suggested that "cocktails" containing oxidants, membrane-damaging agents, DDC, and SNP might be beneficial for killing of tumor cells in vivo and for the assessment of the toxicity of xenobiotics in vitro.
We present experimental evidence indicating the formation of dipolar holographic gratings in potassium lithium tantalate niobate at the paraelectric phase slightly above the phase transition, These gratings are formed in the dark, following the writing of space-charge-based photorefractive gratings, under an external electric field. The dipolar gratings create a spatial modulation of the low-frequency dielectric constant, and the latter induces a spatially correlated modulation of the polarization, which is transformed into birefringence gratings through the quadratic electro-optic effect. (C) 1997 Optical Society of America.
The photodissociation of HCl in solid Ar is studied by non-adiabatic Molecular Dynamics simulations, based on a surface-hopping treatment of transitions between different electronic slates. The relevant 12 potential energy surfaces and the non-adiabatic interactions between them were generated by a Diatomics-in-Molecules (DIM) approach, which incorporated also spin-orbit coupling. The focus of the study is on the non-adiabatic transitions, and on their role both in the cage-exit of the H atom, and in the recombination process. It is found that non-adiabatic transitions occur very frequently. In some of the trajectories, all the 12 electronic states are visited during the timescale studied. At least one non-adiabatic transition was found to occur even in the fastest cage-exit events. The other main results are: (1) The total yields for photofragment separation (by cage exit of the H atom) and for H+Cl recombination onto the ground state are roughly equal in the conditions used. (2) The cage exit events take place in the time-window between similar to 70 fs and similar to 550 fs after the excitation pulse, and are thus all ar least somewhat delayed. The recombination events span a much broader time-window, from almost immediately after excitation, and up to similar to 1100 fs and beyond. (3) The electronic energy relaxation events during the process depend significantly on symmetry and interactions of the states involved, and not only on the energy gaps between them. (4) Different electronic stales reached in the course of the process exhibit; different propensities with regard to the recombination versus cage exit outcome. (5) Spin-orbit interactions, and spin-forbidden transitions play an important role in the process, especially for recombination events. (C) 1997 American Institute of Physics.
The photodissociation of HCl embedded in argon clusters is studied by semiclassical molecular dynamics, based on a surface-hopping approach for the non-adiabatic transitions. The diatomics-in-molecules (DIM) method is used to construct the 12 electronic potential energy surfaces that are involved, and the non-adiabatic couplings. Caging effects, including recombination and electronic relaxation are investigated for Ar-12(HCl) and Ar-54(HCl), corresponding respectively to one and two complete solvation layers. The effects of the process on the cluster, e.g. fragmentation and structural deformation, are also studied. The main findings are: (1) non-adiabatic transitions play a major role in the dynamics for both clusters; (2) some recombination occurs in Ar-12(HCl), and it is much greater, about 7%, in Ar-54(HCl); (3) all 12 electronic states are visited, at least to some extent, in the process, but the distributions remain non-statistical throughout in both systems; (4) rates of spin-forbidden transitions are roughly of similar magnitudes to these of spin-allowed transitions between electronic states; (5) the energy gap law of radiationless relaxation theory does not work well for these systems. Symmetry and shape of the electronic states greatly affect the relaxation rates; (6) the clusters undergo melting and extensive evaporation in the processes.
The qualitative physical aspects and the quantitative description of time and frequency resolved absorption spectroscopy of high molecular Rydberg states are discussed. The frequency is that of the excitation laser and the time is the independently variable delay before detection. The discussion allows for the presence of a weak external electrical field. The essential new ingredient is the finite slice of Rydberg states that are detected (=are in the detection window) and the variation of this population with time due to the coupling of the Rydberg electron with the molecular core. Line shapes are provided showing the effect of the depth of the detection window and the advantages and limitations imposed by the finite width of the excitation laser. The sharpening of the spectrum as the delay time to detection is increased is also illustrated. The quantitative theory is expressed in terms of the expectation value of a detection operator; describing the range of states that can be ionized by the delayed field, taken over a wave function. This wave function is the state of the system at the time of detection. However, even just at the end of the excitation stage, due to the interseries coupling, this wave function is not identical to the state that is directly optically accessed. The time correlation function of this wave function, obtained as a Fourier transform. of the frequency resolved spectrum, is shown to provide further insight into the dynamics, the more so when the excitation laser has a narrow width in frequency. (C) 1997 American Institute of Physics.
Any correlated equilibrium of a strategic game with bounded payoffs and convex strategy sets which has a smooth concave potential, is a mixture of pure strategy profiles which maximize the potential. If moreover, the strategy sets are compact and the potential is strictly concave, then the game has a unique correlated equilibrium.
Dissipative dynamics of an adsorbate near a metal surface is formulated consistently by replacing the infinite system-bath Hamiltonian by a finite surrogate Hamiltonian. This finite representation is designed to generate the true short time dynamics of a primary system coupled to a bath. A detailed wave packet description is employed for the primary system while the bath is represented by an array of two-level systems. The number of bath modes determines the period the surrogate Hamiltonian reproduces the dynamics of the primary system. The convergence of this construction is studied for the dissipating Harmonic oscillator and the double-well tunneling problem. Converged results are obtained for a finite duration by a bath consisting of 4–11 modes. The formalism is extended to dissipation caused by electron-hole-pair excitations. The stopping power for a slow moving proton is studied showing deviations from the frictional limit at low velocities. Vibrational line shapes of hydrogen and deuterium on nickel were studied. In the bulk the line shape is mostly influenced by nonadiabatic effects. The interplay between two baths is studied for low temperature tunneling between two surface sites of hydrogen on nickel. A distinction between lattice modes that enhance the tunneling and ones that suppress it was found.
A novel approach for modeling of quantum dynamics and spectroscopy in large polyatomic systems is presented. The recently developed classical separable potential (CSP) method propagates a multidimensional separable quantum wavepacket using effective time-dependent single mode potentials guided by classical trajectories. In this way, couplings between individual degrees of freedom are taken into account in a mean-field approximation. Comparison with ``numerically exact'' calculations for small systems and with experiments on large systems show that the CSP scheme is accurate and suitable for the description of short timescale processes in the context of dynamical and spectroscopic applications. Moreover, calculations for systems having up to 10(3) coupled modes on scalar and up to 10(4) degrees of freedom on parallel computers are not computationally extremely demanding. This paper presents the program package QDYN which implements the CSP method. Copyright (C) 1998 Elsevier Science Ltd.
