Alzheimer’s disease (AD), the most prevalent neurodegenerative disease, is characterized in part by disruptions in axonal transport. Axonal transport is a process by which motor proteins carry organelles and other cargo made in the neuronal cell body along microtubule tracks to distal regions of the axon. The microtubule-associated protein (MAP) Tau plays a crucial role in regulating axonal transport, and is implicated in the development of AD and other types of dementia collectively known as Tauopathies. Tau is a neuronal-specific MAP that has six isoforms alternatively spliced from a single gene. These isoforms differ by the presence of zero, one, or two N-terminal acidic inserts and three or four C-terminal microtubule binding repeats. Tau is also known to be an intrinsically disordered protein that undergoes a dynamic equilibrium between static and diffusive states on the microtubule surface. The dynamics of Tau are important in the regulation of motor protein mediated axonal transport in neurons. Isoform-specific differences in the dynamic behavior of Tau on the microtubule surface, however, are not yet fully understood. Diffusive Tau is thought to be stabilized by electrostatic interactions between its N- and C-termini while static Tau is proposed to be extended with its C-terminal repeats contacting the microtubule and the N-terminus projected away from the microtubule surface. Thus, the N-terminal inserts may help regulate Tau’s dynamic behavior and function during axonal transport. In this study, the dynamics of two different isoforms of Tau, both with three-microtubule binding repeats but a different number of N-terminal acidic inserts, were assessed using single molecule imaging techniques and novel data analysis methods.

In this study, Amsterdam Density Functional software is used to model bimetallic and ligated aluminum clusters. The stability of the bimetallic clusters is well described by the Jellium model, and the nature of bonding between dopants and aluminum in the bimetallic clusters is analyzed using different criteria. We find that sodium tends to bind ionically, while the bonding of magnesium is not so obvious. We also determine that examining the Mulliken population is the most useful parameter in differentiating bonding character. Calculations on ligated aluminum clusters reveal it behaves fundamentally different than the bimetallic clusters studied in the first part. The ligated clusters contained a high HOMO-LUMO gap regardless of size and the aluminum showed a high 3p Mulliken population. These results show ligated aluminum clusters behave according to Wade-Mingos counting rules.

Total cross sections for charge transfer and electron detachment for collisions of a variety of negative ions with atomic hydrogen have been separately determined for laboratory collision energies ranging from about 5 eV to 500 eV. The experiments are performed with an apparatus that utilizes a crossed-beam configuration with a radio-frequency discharge as the source of atomic hydrogen.;For collisions of H{dollar}\sp-{dollar}(D{dollar}\sp-{dollar}) with H the charge transfer cross sections increase monotonically with decreasing energy and display an isotope effect. at the lowest collision energies, the electron detachment cross sections are about one order of magnitude smaller than those for charge transfer; for the two projectiles the detachment cross sections are identical when compared at the same relative collision energy.;Total electron detachment cross sections have also been measured for collisions of Halogen anions with atomic hydrogen. For F{dollar}\sp-{dollar}, Cl{dollar}\sp-{dollar}, and Br{dollar}\sp-{dollar} projectiles the measured detachment cross sections increase with decreasing collision energy, and no energetic threshold is indicated; no charge transfer is observed. For I{dollar}\sp-{dollar} + H, however, the detachment cross sections are small at low collision energies, and increase rapidly with increasing energy. HI{dollar}\sp-{dollar} is known to form a stable molecular anion, and a small charge transfer cross section is measured to be less than 1 A{dollar}\sp2{dollar} at the highest collision energy.;For collisions of O{dollar}\sp-{dollar} and S{dollar}\sp-{dollar} with atomic hydrogen, electron detachment is also found to be the dominant electron loss mechanism, and the measured total detachment cross sections are found to increase with decreasing collision energy. For both projectiles, charge transfer cross sections are measured to be small and energetic thresholds are indicated.;The experimental results are compared with several calculations and previous measurements that overlap the present results at the highest energies, and are discussed, where possible, in terms of various intermolecular potentials which have been calculated previously.

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In this thesis a general theory of electron detachment in slow collisions of negative ions with atoms is presented. The theory is based upon a semiclassical close-coupling framework, following the work of Taylor and Delos. The Schrodinger equation is reduced, under certain assumptions, to a non-denumerably infinite set of coupled equations. We develop a new method for solving these equations that is more general than the methods used by Taylor and Delos. A zero-order approximation of our solution is applied to the case of H('-)(D('-)) on Ne collisions, the results are compared with the experimental data, and we find good agreement between theory and experiment, particularly with regard to the isotope effect. A first-order approximation of the solution is proved to be very close to the exact solution, and it is applied to the case of H('-)(D('-)) on He collisions. We use quadratic and quartic approximations for the energy gap (DELTA)(t) to calculate, among other things, the survival probability and electron energy spectrum. There are some interesting results for the electron energy spectrum which have not yet been observed in experiments.

Absolute total cross sections for electron detachment, reactive scattering, charge transfer and dissociative charge transfer have been measured for collisions of hydrogen and halogen negative ions with various molecular targets. The reactants investigated involve H('-), D('-), F('-), Cl('-), Br('-) and I('-) ions as projectiles and H(,2), D(,2), HD, N(,2), CO, O(,2), CO(,2), CH(,4) and Cl(,2) molecules as targets. The energy range of these experiments extended from about 1 eV to about 300 eV in the lab.;The threshold behavior of the detachment cross sections for the reactants H('-)(D('-)) + H(,2),D(,2) and HD has been determined. The thresholds for detachment for both H('-) and D('-) ions are found to be larger than the electron affinity of hydrogen and isotopic substitution reveals that the detachment cross sections scale with relative collision energy at low collision energies and with relative collision velocity at high collision energies. Upper and lower bounds on detachment-rate constants which are based upon the measurements are presented.;Studies of the reactants H('-)(D('-)) and B(,2), CO, O(,2), CO(,2) and CH(,4) reveal that electron detachment is the dominant process for all the molecular targets except O(,2) for which charge transfer dominates. Isotope effects are observed in all the cross sections. The general features of the charge-transfer cross section for the O(,2) target are in agreement with the ideas of a simple two-state collision model. The cross sections for charge transfer (or dissociative charge transfer) are found to be small for all targets except O(,2).;In the case of the collisions of F('-) and Cl('-) with H(,2), D(,2) and HD, reactive scattering is found to be the dominant inelastic channel for F('-) projectile. Electron detachment of F('-) is found to occur by two distinct mechanisms. A striking difference in the detachment and reactive cross sections is observed when Cl('-) is substituted for F('-) in that the electron detachment cross section is generally larger than that for reactive scattering. Isotope effects are observed in all the cross sections for both F('-) and Cl('-).;Charge transfer and dissociative charge transfer cross sections are found to be the dominant channels for collisions of Cl('-), Br('-) and I('-) with Cl(,2). The electron detachment cross section for I('-) + Cl(,2) is found to be anomalously low. Some energy loss spectra are reported for I('-) + Cl(,2). They exhibit substantial inelastic scattering which is consistent with the calculated potentials of Cl(,2).

Measurements of the total cross section for electron detachment, (sigma)(,e)(E), are presented for low-energy (E(,1ab) < 300 eV) collisions of Na('-), K('-), and Cs('-) with atomic and molecular targets. For many of the atomic (rare-gas) targets, the energy dependence of (sigma)(,e)(E) is striking: virtually no detachment is observed until relatively high collision energies (50 eV in the center-of-mass frame) are reached, in contradistinction to what has been observed for similar collisions involving H('-). The thresholds for alkali anion detachment are approximately equal to the thresholds for excitation observed in collisions of neutral alkali atoms with these same targets. The similarity between the dynamics of the neutral system and that of the negative ion system, together with the observation (at greater energies) of detachment accompanied by excitation of the alkali parent, suggests that electron detachment may be mediated by a two-electron process in some cases. A simple curve-crossing mechanism adequately reproduces the observed (sigma)(,e)(E) for several of these rare-gas targets.;Measurements of both (sigma)(,e)(E) and the cross section for charge transfer (sigma)(,i)(E) have also been completed for H(,2), D(,2), N(,2), O(,2), CO, CO(,2), SO(,2), N(,2)O, CH(,4), and SF(,6) targets. Electron detachment is the dominant process for all of these targets except O(,2), SO(,2), and SF(,6), with thresholds on the order of a few eV. Structure in (sigma)(,e)(E) for the CO(,2) target has been attributed to charge transfer to a metastable state of CO(,2)('-)(('2)A(,1)). Similarly, in the case of N(,2)O, both (sigma)(,e)(E) and (sigma)(,i)(E) exhibit behavior which suggests that a temporary negative ion state is formed during the collision. In the case of the O(,2), SO(,2), and SF(,6) targets, charge transfer is observed to have particularly large cross sections (>100 (ANGSTROM)('2)) at low collision energies.