Publications

Abstract During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.

Structural investigations of bare and surface-modified polystyrene microspheres (beads) have been carried out by infrared-visible sum frequency generation (SFG) vibrational spectroscopy and scanning force microscopy (SFM). Bead surfaces have been modified by either the covalent linking of immunoglobulin G (IgG) and bovine serum albumin (BSA) or the nonspecific adsorption of a Pluronic surfactant. After surface modification with protein, SFG signals in the aliphatic CH-stretch region are detected at both the buffer/bead and air/bead interfaces, indicating that some amino acid residues in proteins adopt preferred orientations. SFG results indicate that the hydrophobic poly(propylene glycol) moieties in the Pluronic order when adsorbed onto the bead, at both the buffer/bead and air/bead interfaces, whereas hydrophilic poly(ethylene glycol) groups align to a lesser extent. SFG spectra also show that the phenyl rings of bare polystyrene beads in contact with air or buffer are ordered, with a dipole component directed along the surface normal, but become less ordered after the adsorption of either proteins or the polymer. Molecular orientation and ordering at the bead surface affect its hydrophobicity and aggregation behavior. SFM results reveal the formation of nonuniform islands when bare beads with more hydrophobic character are spun-cast onto a silica substrate. In the presence of adsorbed protein, a hexagonal packing of beads, with some defects, is observed, depending on the bulk pH and the type of attached protein. Adsorbed Pluronic causes the beads to aggregate in a disordered fashion, as compared to the behavior of bare and protein-modified beads.

Motivated by the potential for new intercalation chemistries, sodium-ion batteries have received renewed interest as a possible replacement technology for lithium-ion batteries. To date, fluorophosphates, particularly Na 3 V 2 (PO 4 ) 2 F 3 , are among the most promising sodium-ion cathodes studied. Fluorophosphates exhibit redox activity near 4V due to the strong inductive effect of phosphates and fluorine, and have been shown to reversibly cycle near their single-electron capacity of 128mAh/g, yielding energy densities close to 480Wh/kg 1,2 . However, efforts to further improve the capacity of these materials have achieved very little success. This work reports on our efforts to determine the mechanism by which fluorophosphate cathodes appear to be limited to their single-electron capacity. Our method is to examine systems that decouple previously observed Na + extraction limits from redox limits. We will discuss the electrochemical characterization of novel mixed-transition metal fluorophosphate systems, from which we can conclude that fluorophosphate cathodes’ capacities are not redox-limited. We demonstrate reversible capacity exceeding the single-electron theoretical capacity during a single charge/discharge. In addition, computational examination of limiting behaviors will be elaborated on. Specifically, possible site-limiting and diffusion-limiting mechanisms in fluorophosphate cathodes will be presented. References : [1] R. A. Shakoor, D. H. Seo, H Kim, Y. Park, J. Kim, S. Kim, H. Gwon, S. Lee, and K. Kang. J. Mater. Chem. , 22 ,20535–20541 (2012). [2] K. Chihara, A. Kitajou, I. Gocheva, S. Okada, and J. Yamaki. J. Power Soources. , 227 , 80 – 85 (2013).

We have investigated the structural and electronic properties of individual ethylene molecules on the GaP(110) surface by combining low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) with density functional theory (DFT) calculations. Isolated molecules were adsorbed on in situ cleaved GaP(110) surfaces through ethylene exposures at 300 K and 15 K. DFT calculations suggest two possible stable adsorption geometries for a single ethylene molecule on GaP(110) at low temperature. High-resolution STM images, however, reveal only one adsorption geometry for this system, consistent with the site having the largest computed binding energy. Unlike adsorption of ethylene on other metallic and semiconducting surfaces, ethylene physisorbs to GaP(110) through a weak hybridization of molecular π-states with substrate surface states, leaving the frontier molecular orbitals largely unperturbed. Differential conductivity spectra acquired on single molecules are consistent with self-energy corrected DFT calculations.

Calorie restriction (CR) promotes longevity. A prevalent mechanistic hypothesis explaining this effect suggests that protein degradation, including mitochondrial autophagy, is increased with CR, removing damaged proteins and improving cellular fitness. At steady state, increased catabolism must be balanced by increasing mitochondrial biogenesis and protein synthesis, resulting in faster protein replacement rates. To test this hypothesis, we measured replacement kinetics and relative concentrations of hundreds of proteins in vivo in long-term CR and ad libitum-fed mice using metabolic (2)H(2)O-labeling combined with the Stable Isotope Labeling in Mammals protocol and LC-MS/MS analysis of mass isotopomer abundances in tryptic peptides. CR reduced absolute synthesis and breakdown rates of almost all measured hepatic proteins and prolonged the half-lives of most (≈ 80%), particularly mitochondrial proteins (but not ribosomal subunits). Proteins with related functions exhibited coordinated changes in relative concentration and replacement rates. In silico expression pathway interrogation allowed the testing of potential regulators of altered network dynamics (e.g. peroxisome proliferator-activated receptor gamma coactivator 1-alpha). In summary, our combination of dynamic and quantitative proteomics suggests that long-term CR reduces mitochondrial biogenesis and mitophagy. Our findings contradict the theory that CR increases mitochondrial protein turnover and provide compelling evidence that cellular fitness is accompanied by reduced global protein synthetic burden.