Aniruddha Deb

Zinc stabilization of prefibrillar oligomers of human islet amyloid polypeptide

The aggregation of human islet amyloid polypeptide (hIAPP) has been linked to beta-cell death in type II diabetes. Zinc present in secretory granules has been shown to affect this aggregation. A combination of EXAFS, NMR, and AFM experiments shows that the influence of zinc is most likely due to the stabilization of prefibrillar aggregates of hIAPP.

Understanding Spin Structure in Metallacrown Single-Molecule Magnets using Magnetic Compton Scattering

The 3d-4f mixed metallacrowns frequently show single-molecule magnetic behavior. We have used magnetic Compton scattering to characterize the spin structure and orbital interactions in three isostructural metallacrowns: Gd2Mn4, Dy2Mn4, and Y2Mn4. These data allow the direct determination of the spin only contribution to the overall magnetic moment. We find that the lanthanide 4f spin in Gd2Mn4 and Dy2Mn4 is aligned parallel to the Mn 3d spin. For Y2Mn4 (manganese-only spin) we find evidence for spin delocalization into the O 2p orbitals. Comparing the magnetic Compton scattering data with SQUID studies that measure the total magnetic moment suggests that Gd2Mn4 and Y2Mn4 have only a small orbital contribution to the moment. In contrast, the total magnetic moment for Dy2Mn4 MCs is much larger than the spin-only moment, demonstrating a significant orbital contribution to the overall magnetic moment. Overall, these data provide direct insight into the correlation of molecular design with molecular magnetic properties.

A de novo designed metalloenzyme for the hydration of CO2.

Protein design will ultimately allow for the creation of artificial enzymes with novel functions and unprecedented stability. To test our current mastery of natures approach to catalysis, a Zn(II) metalloenzyme was prepared using de novo design. α3DH3 folds into a stable single-stranded three-helix bundle and binds Zn(II) with high affinity using His3 O coordination. The resulting metalloenzyme catalyzes the hydration of CO2 better than any small molecule model of carbonic anhydrase and with an efficiency within 1400-fold of the fastest carbonic anhydrase isoform, CAII, and 11-fold of CAIII.

In Situ X-Ray Absorption Spectroscopic Study of Li1.05Ni0.35Co0.25Mn0.4O2 Cathode Material Coated with LiCoO2

After the recent upsurge of interest in the layered LiNi 1/3 Co 1/3 Mn 1/3 O 2 system for use as a cathode material for rechargeable lithium batteries, we have studied the charge compensation mechanism and structural perturbations occurring during cycling of the novel layered system of Li 1.05 Ni 0.35 Co 0.25 Mn 0.4 O 2 coated with nano-crystallized LiCoO 2 , which effectively improved the rate capability. In addition, the LiCoO 2 evidently suppressed any structural change of the Li 1.05 Ni 0.35 Co 0.25 Mn 0.4 O 2 . In situ X-ray absorption spectroscopy (XAS) measurements were performed utilizing a novel in situ electrochemical cell, specifically designed for long-term X-ray experiments. The cell was cycled at a moderate rate through a typical Li-ion battery operating voltage range (2.8-4.8 V). The electrode contained 1.9 mg of Li 1.05 Ni 0.35 Co 0.25 Mn 0.4 O 2 on a 25 μm Al foil, and had an area of 0.79 cm 2 . XAS measurements were performed at different states of charge (SOC) during cycling, at the Ni, Co, and the Mn edges, revealing details about the response of the cathode to Li insertion and extraction processes. Extended X-ray absorption fine structure region of the spectra revealed the changes of bond distance and coordination number of Ni, Co, and Mn absorbers as a function of the SOC of the material. We found that the oxidation states of the transition metals in the system are Ni 2+ , Co 3+ , and Mn 4+ in the fully discharged condition. During charging the N1 2+ is oxidized to Ni 4+ through an intermediate stage of Ni 3+ , Co 3+ is oxidized almost to Co 4+ . Utilizing a combination of Faradays calculation and XAS results, the Co was found to be at Co 3.89+ at the end of the charge, whereas Mn was found to be electrochemically inactive and remains as Mn 4+ . These measurements on this cathode material confirmed that the material retains its symmetry and good structural short-range order leading to superior cycling.

Electronic structure of the Cu2MnAl Heusler alloy

The electronic structure and chemical bonding mechanism of the Heusler alloy Cu2 MnAl are studied on the basis of band-structure calculations, using the full-potential linearized augmented-plane-wave (FLAPW) method. The calculations are performed within both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA). The calculated band structures and the densities of states within the GGA are compared with the earlier-reported experimental results of ultraviolet photoemission spectroscopy. The comparisons support the physical picture proposed by Kubler and co-workers, in which the occupied d states of Mn are delocalized by their strong interaction with the d states of Cu. The magnetic Compton profiles (MCPs) are also calculated and compared with the earlier-reported experimental results. The comparisons show that the GGA is better able to reproduce the experimental profiles than the LSDA. The spin moment on the Mn site is calculated to be 3.20 µB , which is in agreement with the value deduced from the experimental profiles.

Structural and Electrochemical Investigation of Li ( Ni0.4Co0.15Al0.05Mn0.4 ) O2 Cathode Material

LiNi0.4Co0.15Al0.05Mn0.4O2 was investigated to understand the effect of replacement of the cobalt by aluminum on the structural and electrochemical properties. In situ X-ray absorption spectroscopy XAS was performed, utilizing a novel in situ electrochemical cell, specifically designed for long-term X-ray experiments. The cell was cycled at a moderate rate through a typical Li-ion battery operating voltage range. 1.0‐4.7 V XAS measurements were performed at different states of charge SOC during cycling, at the Ni, Co, and the Mn edges, revealing details about the response of the cathode to Li insertion and extraction processes. The extended X-ray absorption fine structure EXAFS region of the spectra revealed the changes of bond distance and coordination number of Ni, Co, and Mn absorbers as a function of the SOC of the material. The oxidation states of the transition metals in the system are Ni 2+ ,C o 3+ , and Mn 4+ in the as-made material fully discharged, while during charging the Ni 2+ is oxidized to Ni 4+ through an intermediate stage of Ni 3+ ,C o 3+ is oxidized toward Co 4+ , and Mn was found to be electrochemically inactive and remained as Mn 4+ . The EXAFS results during cycling show that the Ni‐O changes the most, followed by Co‐O, and Mn‐O varies the least. These measurements on this cathode material confirmed that the material retains its symmetry and good structural short-range order leading to the superior cycling reported earlier.

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B12

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Development of a single-cell X-ray fluorescence flow cytometer.

An X-ray fluorescence flow cytometer that can determine the total metal content of single cells has been developed. Capillary action or pressure was used to load cells into hydrophilic or hydrophobic capillaries, respectively. Once loaded, the cells were transported at a fixed vertical velocity past a focused X-ray beam. X-ray fluorescence was then used to determine the mass of metal in each cell. By making single-cell measurements, the population heterogeneity for metals in the µM to mM concentration range on fL sample volumes can be directly measured, a measurement that is difficult using most analytical methods. This approach has been used to determine the metal composition of 936 individual bovine red blood cells (bRBC), 31 individual 3T3 mouse fibroblasts (NIH3T3) and 18 Saccharomyces cerevisiae (yeast) cells with an average measurement frequency of ∼4 cells min(-1). These data show evidence for surprisingly broad metal distributions. Details of the device design, data analysis and opportunities for further sensitivity improvement are described.

Abnormal metal levels in the primary visual pathway of the DBA/2J mouse model of glaucoma

The purpose of this study was to determine metal ion levels in central visual system structures of the DBA/2J mouse model of glaucoma. We used inductively coupled plasma mass spectrometry (ICP-MS) to measure levels of iron (Fe), copper (Cu), zinc (Zn), magnesium (Mg), manganese (Mn), and calcium (Ca) in the retina and retinal projection of 5-month (pre-glaucomatous) and 10-month (glaucomatous) old DBA/2J mice and age-matched C57BL/6J controls. We used microbeam X-ray fluorescence (μ-XRF) spectrometry to determine the spatial distribution of Fe, Zn, and Cu in the superior colliculus (SC), which is the major retinal target in rodents and one of the earliest sites of pathology in the DBA/2J mouse. Our ICP-MS experiments showed that glaucomatous DBA/2J had lower retinal Fe concentrations than pre-glaucomatous DBA/2J and age-matched C57BL/6J mice. Pre-glaucomatous DBA/2J retina had greater Mg, Ca, and Zn concentrations than glaucomatous DBA/2J and greater Mg and Ca than age-matched controls. Retinal Mn levels were significantly deficient in glaucomatous DBA/2J mice compared to aged-matched C57BL/6J and pre-glaucomatous DBA/2J mice. Regardless of age, the SC of C57BL/6J mice contained greater Fe, Mg, Mn, and Zn concentrations than the SC of DBA/2J mice. Greater Fe concentrations were measured by μ-XRF in both the superficial and deep SC of C57BL/6J mice than in DBA/2J mice. For the first time, we show direct measurement of metal concentrations in central visual system structures affected in glaucoma and present evidence for strain-related differences in metal content that may be specific to glaucomatous pathology.

Further insights into the metal ion binding abilities and the metalation pathway of a plant metallothionein from Musa acuminata

The superfamily of metallothioneins (MTs) combines a diverse group of metalloproteins, sharing the characteristics of rather low molecular weight and high cysteine content. The latter provides MTs with the capability to coordinate thiophilic metal ions, in particular those with a d10 electron configuration. The sub-family of plant MT3 proteins is only poorly characterized and there is a complete lack of three-dimensional structure information. Building upon our previous results on the Musa acuminata MT3 (musMT3) protein, the focus of the present work is to understand the metal cluster formation process, the role of the single histidine residue present in musMT3, and the metal ion binding affinity. We concentrate our efforts on the coordination of ZnII and CdII ions, using CoII as a spectroscopic probe for ZnII binding. The overall protein-fold is analysed with a combination of limited proteolytic digestion, mass spectrometry, and dynamic light scattering. Histidine coordination of metal ions is probed with extended X-ray absorption fine structure spectroscopy and CoII titration experiments. Initial experiments with isothermal titration calorimetry provide insights into the thermodynamics of metal ion binding.