This paper is concerned with the parameters influencing the interfacial electron transfer kinetics, and therefore the sensitizing efficiency, for different sensitizer dyes adsorbed to nanocrystalline titanium dioxide films. We consider three sensitizer dyes: Ru(2,2′-bipyridyl-4,4′-dicarboxylate) 2 -cis-(NCS) 2 (Ru(dcbpy) 2 (NCS) 2 ) and zinc and free base tetracarboxyphenyl porphyrins (ZnTCPP & H 2 TCPP). These dyes were selected as they exhibit large differences in their oxidation potentials and photophysics, while retaining similar carboxylate groups for binding to the TiO 2 surface. For example, whereas the photophysics of Ru(dcbpy) 2 (NCS) 2 in solution is dominated by ultrafast (<100 fs) relaxation processes to nonemissive excited states associated with metalto-ligand charge transfer excited states and extensive singlet/triplet mixing, both porphyrins exhibit longlived (>1 ns) π* singlet excited states and only weak singlet/triplet mixing. The ground and excited-state oxidation potentials also differ by up to 600 mV between these different dyes. Remarkably, we find that the large differences in these dyes' photophysics and redox chemistry have rather little influence upon the interfacial electron transfer kinetics observed following adsorption of these dyes to the nanocrystalline TiO 2 films. The kinetics of electron injection into the TiO 2 conduction band following pulsed optical excitation of the adsorbed sensitizer dyes are found to be indistinguishable for all three sensitizer dyes. For all three dyes, the kinetics are ultrafast and multiexponential, requiring a minimum of three time constants ranging from <100 fs to ∼10 ps. Similarly, the recombination kinetics were also found to be highly nonexponential and only weakly sensitive to the identity of the sensitizer dye. We conclude that the multiexponential nature of the injection/ recombination kinetics are not associated with properties of the sensitizer dye, but rather with heterogeneities/ trap states associated with the TiO 2 film. We further conclude that the large difference between the rate of electron injection and recombination observed for all three dyes is not associated with specific characteristics of the sensitizer dyes but rather results from electron trapping within defect/surface states of the TiO 2 film. Finally, we conclude that the higher sensitizing efficiency reported for Ru(dcbpy) 2 (NCS) 2 compared to ZnTCPP cannot be attributed to differences in the interfacial electron transfer kinetics between these dyes and discuss alternative mechanisms influencing the sensitizing efficiencies of these dyes.

We present a study of the kinetics of electron injection in ruthenium(II) cis- (2,2′-bipyridyl-4,4′-dicarboxylate) 2 -(NCS) 2 -sensitized nanocrystalline TiO 2 films as a function of electrical potential applied to the TiO 2 film and as a function of the composition of the electrolyte in which the film is immersed. At moderate applied potentials (-0.2 V vs Ag/AgCl), and in the presence of potential determining ions (0.1 M Li + ) in the electrolyte, the electron injection kinetics were found to be multiphasic, with a half time for electron injection of 500 fs. These injection kinetics were retarded by either the omission of potential determining ions or the application of more negative potentials. Omission of Li + ions from the electrolyte resulted in a 7-fold retardation of the injections kinetics. The application of -0.7 V to the TiO 2 electrode resulted in a 25-fold retardation of the injection kinetics. These observations are discussed in terms of nonadiabatic interfacial electron transfer theory. The retardation of the injection kinetics in the absence of potential determining ions is attributed to the influence of these ions upon the electronic density of states of the TiO 2 electrode. The retardation of the injection kinetics at negative applied potentials is attributed to the increased occupancy of this density of states. Fits to the potential dependence of the injection kinetics following nonadiabatic theory yield a reorganizational energy for the electron injection process of 0.25 ( 0.05 eV.

Since the linking of mutations in the Cu,Zn superoxide dismutase gene (sod1) to amyotrophic lateral sclerosis (ALS) in 1993, researchers have sought the connection between SOD1 and motor neuron death. Disease-linked mutations tend to destabilize the native dimeric structure of SOD1, and plaques containing misfolded and aggregated SOD1 have been found in the motor neurons of patients with ALS. Despite advances in understanding of ALS disease progression and SOD1 folding and stability, cytotoxic species and mechanisms remain unknown, greatly impeding the search for and design of therapeutic interventions. Here, we definitively link cytotoxicity associated with SOD1 aggregation in ALS to a nonnative trimeric SOD1 species. We develop methodology for the incorporation of low-resolution experimental data into simulations toward the structural modeling of metastable, multidomain aggregation intermediates. We apply this methodology to derive the structure of a SOD1 trimer, which we validate in vitro and in hybridized motor neurons. We show that SOD1 mutants designed to promote trimerization increase cell death. Further, we demonstrate that the cytotoxicity of the designed mutants correlates with trimer stability, providing a direct link between the presence of misfolded oligomers and neuron death. Identification of cytotoxic species is the first and critical step in elucidating the molecular etiology of ALS, and the ability to manipulate formation of these species will provide an avenue for the development of future therapeutic strategies.neurodegeneration | protein aggregation | protein misfolding | ALS | structural modeling P rotein misfolding and aggregation are linked to cell death and disease progression in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). In these diseases and others, the formation of amyloid plaques, often observed post mortem, has long been thought to play a role in neurodegeneration, but toxicity has never been confirmed (1-3). Recent research has shown that small, soluble oligomers, rather than insoluble amyloids, are likely to be the cytotoxic species causing neurodegeneration (4-14). These small, soluble oligomers undergo aberrant interactions with cell machinery and activate cell death pathways, but their exact stoichiometry is not known and their properties have yet to be characterized. Recently, metastable soluble Cu,Zn superoxide dismutase (SOD1) oligomers have been identified that contain an epitope associated with disease-linked species of SOD1, mutants of which are implicated in a subset of ALS (15-18). Size exclusion chromatography (SEC) of these oligomers revealed a size range of two to four monomers, consistent with previous findings of potentially cytotoxic SOD1 oligomers (19)(20)(21).Knowledge of the structures of these species would not only allow for definitive testing of their toxicity but could potentially lead to an understanding of disease mechanism and therapeutic strategies against diseases for which no ...

Understanding the role of coherent electronic motion is expected to resolve general questions of importance in macromolecular energy transfer. We demonstrate a novel nonlinear optical method, angle-resolved coherent wave mixing, that separates out coherently coupled electronic transitions and energy transfers in an instantaneous two-dimensional mapping. Angular resolution of the signal is achieved by using millimeter laser beam waists at the sample and by signal relay to the far field; for this we use a high energy, ultrabroadband hollow fiber laser source. We reveal quantum electronic beating with a time-ordered selection of transition energies in a photosynthetic complex.