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ABSTRACT: Nanosizing is a frequently applied strategy in recent years to improve storage properties of Li-ion electrodes and facilitate novel storage mechanisms. Due to particle size reduction, surface effects increasingly dominate, which can drastically change the storage properties. Using density functional theory calculations we investigate the impact of the surface environment on the Li-ion insertion properties in defective spinel Li(4+x)Ti(5)O(12), a highly promising negative electrode material. The calculations reveal that the storage properties strongly depend on the surface orientation. The lowest energy (1 1 0) surface is predicted to be energetically favorable for Li-ion insertion into the vacant 16c sites. The (1 1 1) surface allows capacities that significantly exceed the bulk capacity Li(7)Ti(5)O(12) at voltages greater than 0 V by occupation of 8a sites in addition to the fully occupied 16c sites. One of the key findings is that the surface environment extends nanometers into the storage material, leading to a distribution of voltages responsible for the curved voltage profile commonly observed in nanosized insertion electrode materials. Both the calculated surface-specific voltage profiles and the calculated particle size dependent voltage profiles are in good agreement with the experimental voltage profiles reported in literature. These results give a unique insight into the impact of nanostructuring and further possibilities of tailoring the Li-ion voltage profiles and capacities in lithium insertion materials.
ACS Nano 09/2012; 6(10):8702-12. · 10.77 Impact Factor
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ABSTRACT: The self-aggregated state of bacteriochlorophyll (BChl) c molecules in chlorosomes belonging to a bchQ bchR mutant of the green sulfur bacteria Chlorobaculum tepidum, which mostly produces a single 17(2)-farnesyl-(R)-[8-ethyl,12-methyl]BChl c homologue, was characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy and high-resolution electron microscopy. A nearly complete (1)H and (13)C chemical shift assignment was obtained from well-resolved homonuclear (13)C-(13)C and heteronuclear (1)H-(13)C NMR data sets collected from (13)C-enriched chlorosome preparations. Pronounced doubling (1:1) of specific (13)C and (1)H resonances revealed the presence of two distinct and nonequivalent BChl c components, attributed to all syn- and all anti-coordinated parallel stacks, depending on the rotation of the macrocycle with respect to the 3(1)-methyl group. Steric hindrance from the 20-methyl functionality induces structural differences between the syn and anti forms. A weak but significant and reproducible reflection at 1/0.69 nm(-1) in the direction perpendicular to the curvature of cylindrical segments observed with electron microscopy also suggests parallel stacking of BChl c molecules, though the observed lamellar spacing of 2.4 nm suggests weaker packing than for wild-type chlorosomes. We propose that relaxation of the pseudosymmetry observed for the wild type and a related BChl d mutant leads to extended domains of alternating syn and anti stacks in the bchQ bchR chlorosomes. Domains can be joined to form cylinders by helical syn-anti transition trajectories. The phase separation in domains on the cylindrical surface represents a basic mechanism for establishing suprastructural heterogeneity in an otherwise uniform supramolecular scaffolding framework that is well-ordered at the molecular level.
Biochemistry 05/2012; 51(22):4488-98. · 3.42 Impact Factor
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ABSTRACT: The power density of lithium-ion batteries requires the fast transfer of ions between the electrode and electrolyte. The achievable power density is directly related to the spontaneous equilibrium exchange of charged lithium ions across the electrolyte/electrode interface. Direct and unique characterization of this charge-transfer process is very difficult if not impossible, and consequently little is known about the solid/liquid ion transfer in lithium-ion-battery materials. Herein we report the direct observation by solid-state NMR spectroscopy of continuous lithium-ion exchange between the promising nanosized anatase TiO(2) electrode material and the electrolyte. Our results reveal that the energy barrier to charge transfer across the electrode/electrolyte interface is equal to or greater than the barrier to lithium-ion diffusion through the solid anatase matrix. The composition of the electrolyte and in turn the solid/electrolyte interface (SEI) has a significant effect on the electrolyte/electrode lithium-ion exchange; this suggests potential improvements in the power of batteries by optimizing the electrolyte composition.
Chemistry 11/2011; 17(52):14811-6. · 5.93 Impact Factor
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ABSTRACT: Magic-angle spinning (MAS) (13)C-(13)C correlation NMR spectroscopy was used to resolve the electronic ground state characteristics of the bacteriochlorophyll a (BChl a) cofactors in light-harvesting 1 (LH1) complexes of Rhodopseudomonas acidophila (strain 10050). The BChl a (13)C isotropic chemical shifts of the LH1 complexes are compared to the (13)C chemical shifts for BChl a dissolved in acetone-d(6) and to (13)C NMR data that has been obtained for the B800 and B850 BChl molecules in Rps. acidophila peripheral light-harvesting complexes (LH2). Since both complexes contain BChl a cofactors, we can address the chemical shift variability for specific carbon responses between the two types of antennae. The global shift pattern of the LH1 BChl's resembles the shift patterns of the LH2 alpha- and beta-B850 BChl's, while some carbon responses, in particular the C3 and C3(1), show significant deviations. A comparison with density functional theory (DFT) shift calculations provides insight into the BChl concomitant structural and electronic interactions in the ground state. The differences in the LH1 BChl observed chemical shifts relative to the (13)C responses of BChl a in solution cannot be explained by local side chain interactions, such as hydrogen bonding or nonplanarity of the C3 acetyl, but appear to be dominated by protein-induced macrocycle distortion. Such shaping of the macrocycle will contribute significantly to the red shift of the BChl Q(y) absorbance band in purple bacterial light-harvesting complexes.
The Journal of Physical Chemistry B 05/2010; 114(18):6207-15. · 3.70 Impact Factor
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ABSTRACT: Protein nuclear magnetic resonance (NMR) secondary chemical shifts are widely used to predict the secondary structure, and in solid-state NMR, they are often the only unambiguous structural parameters available. However, the employed prediction methods are empirical in nature, relying on the assumption that secondary shifts are only affected by shielding effects of neighboring atoms. We analyzed the secondary shifts of a photosynthetic membrane protein with a high density of chromophores and very tight packing, the light-harvesting 2 (LH2) complex of Rhodopseudomonas acidophila. A relation was found between secondary shift anomalies and protein-protein or pigment-protein tertiary and quaternary contacts. For several residues, including the bacteriochlorophyll-coordinating histidines (alphaH31 and betaH30) and the phenylalanine alphaF41 that has strongly twisted C(b)-C(a)-C and C(a)-C-N conformations in the LH2 crystal structure, the perturbing effects on the backbone chemical shifts were tested by density functional theory (DFT) calculations. We propose that higher-order interactions in the tightly packed complex can induce localized perturbations of the backbone conformation and electronic structure, related to functional pigment-protein or protein-protein interactions.
Biochemistry 12/2009; 49(3):478-86. · 3.42 Impact Factor
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ABSTRACT: In the last two decades, Magic Angle Spinning (MAS) NMR has created its own niche in studies involving photosynthetic membrane protein complexes, owing to its ability to provide structural and functional information at atomic resolution of membrane proteins when in the membrane, in the natural environment. The light-harvesting two (LH2) transmembrane complex from Rhodopseudomonas acidophila is used to illustrate the procedure of the technique applicable in photosynthesis research. One- and two-dimensional solid-state NMR experiments involving (13)C- and (15)N-labeled LH2 complexes allow to make a sequence-specific assignment of NMR signals, which forms the basis for resolving structural details and the assessment of charge transfer, electronic delocalization effects, and functional strain in the ground state.
Photosynthesis Research 09/2009; 102(2-3):415-25. · 3.24 Impact Factor
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ABSTRACT: We introduce a concept to solve the structure of a microcrystalline material in the solid-state at natural abundance without access to distance constraints, using magic angle spinning (MAS) NMR spectroscopy in conjunction with X-ray powder diffraction and DFT calculations. The method is applied to a novel class of materials that form (semi)conductive 1D wires for supramolecular electronics and artificial light-harvesting. The zinc chlorins 3-devinyl-3(1)-hydroxymethyl-13(2)-demethoxycarbonylpheophorbide a (3',5'-bis-dodecyloxy)benzyl ester zinc complex 1 and 3-devinyl-3(1)-methoxymethyl-13(2)-demethoxycarbonylpheophorbide a (3',5'-bis-dodecyloxy)benzyl ester zinc complex 2, self-assemble into extended excitonically coupled chromophore stacks. (1)H-(13)C heteronuclear dipolar correlation MAS NMR experiments provided the (1)H resonance assignment of the chlorin rings that allowed accurate probing of ring currents related to the stacking of macrocycles. DFT ring-current shift calculations revealed that both chlorins self-assemble in antiparallel pi-stacks in planar layers in the solid-state. Concomitantly, X-ray powder diffraction measurements for chlorin 2 at 80 degrees C revealed a 3D lattice for the mesoscale packing that matches molecular mechanics optimized aggregate models. For chlorin 2 the stacks alternate with a periodicity of 0.68 nm and a 3D unit cell with an approximate volume of 6.28 nm(3) containing 4 molecules, which is consistent with space group P2(1)22(1).
Proceedings of the National Academy of Sciences 08/2009; 106(28):11472-7. · 9.68 Impact Factor
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ABSTRACT: Chlorosomes are the largest and most efficient light-harvesting antennae found in nature, and they are constructed from hundreds of thousands of self-assembled bacteriochlorophyll (BChl) c, d, or e pigments. Because they form very large and compositionally heterogeneous organelles, they had been the only photosynthetic antenna system for which no detailed structural information was available. In our approach, the structure of a member of the chlorosome class was determined and compared with the wild type (WT) to resolve how the biological light-harvesting function of the chlorosome is established. By constructing a triple mutant, the heterogeneous BChl c pigment composition of chlorosomes of the green sulfur bacteria Chlorobaculum tepidum was simplified to nearly homogeneous BChl d. Computational integration of two different bioimaging techniques, solid-state NMR and cryoEM, revealed an undescribed syn-anti stacking mode and showed how ligated BChl c and d self-assemble into coaxial cylinders to form tubular-shaped elements. A close packing of BChls via pi-pi stacking and helical H-bonding networks present in both the mutant and in the WT forms the basis for ultrafast, long-distance transmission of excitation energy. The structural framework is robust and can accommodate extensive chemical heterogeneity in the BChl side chains for adaptive optimization of the light-harvesting functionality in low-light environments. In addition, syn-anti BChl stacks form sheets that allow for strong exciton overlap in two dimensions enabling triplet exciton formation for efficient photoprotection.
Proceedings of the National Academy of Sciences 06/2009; 106(21):8525-30. · 9.68 Impact Factor
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ABSTRACT: Magic angle spinning (MAS) solidstate NMR spectroscopy was used to investigate the stacking of bacteriochlorophylls (BChl)
in the bchQRU mutant of the green sulfur bacterium C. tepidum. This mutant produces [8-Et, 12-Me] BChl d instead of the BChl c in the wild type. Using uniformly 13C enriched bchQRU chlorosomes, a 13C and 1H resonance assignment of the BChl d ring was made using two-dimensional 13C−13C homonuclear and 1H−13C heteronuclear MAS NMR dipolar correlation experiments. The aggregation shifts are largest for the 21−H3, 121−H3, 31−H, and 5-H, which are shifted upfield by −3.3, −2.6, −3.7, and −2.0 ppm, respectively. A comparison of the bchQRU chlorosomes with aggregation shifts for the wild type and chlorin models forming dense aggregates reveals parallel stacking
of the [8-Et, 12-Me]BChl d, which is more dense and much more homogeneously ordered than for the BChl c in the wild type. However, the structure is less dense than for the Cd-chlorin models which lack the 31-Me.
12/2007: pages 248-251;
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ABSTRACT: Magic angle spinning (MAS) solidstate NMR spectroscopy was used to broadly envisage the differences between the [8-Et, 12-
Me] BChl c producing bchQR mutant and the [8-Et, 12-Me] BChl d producing bchQRU mutant of the green sulfur bacterium C. tepidum. Twodimensional 13C−13C homonuclear MAS NMR dipolar correlation experiments on uniformly 13C enriched bchQR and bchQRU chlorosomes, revealed a major doubling of the 13C resonances at the 5−C, 131−C, and 14−C position for the bchQRU mutant and a minor doubling at the 5−C, 14−C, and 20−C positions for the bchQRU mutant, indicating the presence of two components in the chlorosomes from both mutants though not in the same ratios.
12/2007: pages 257-260;
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Journal of the American Chemical Society 03/2007; 129(6):1504-5. · 9.91 Impact Factor
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Swapna Ganapathy
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ABSTRACT: This thesis focuses on bridging the gap between natural and artificial systems by the structural and structure-function characterization of two kinds of natural photosynthetic antenna systems, a pigment-protein complex i.e. the LH2 complex, and the protein-free chlorosome supramolecular light harvesters. Chlorosomes contain the largest numbers of chromophores for any antenna system known in nature and are very efficient ultra-fast light harvesters. They provide an optimal starting point for a novel class of artificial antenna arrays for ultra-rapid feeding of energy into photocatalytic devices.
