Utilizing the Dynamic Stark Shift as a Probe for Dielectric Relaxation in Photosynthetic Reaction Centers During Charge Separation

In photosynthetic reaction centers, the electric field generated by light-induced charge separation produces electrochromic shifts in the transitions of reaction center pigments. The extent of this Stark shift indirectly reflects the effective field strength at a particular cofactor in the complex. The dynamics of the effective field strength near the two monomeric bacteriochlorophylls (B_A and B_B) in purple photosynthetic bacterial reaction centers has been explored near physiological temperature by monitoring the time-dependent Stark shift during charge separation (dynamic Stark shift). This dynamic Stark shift was determined through analysis of femtosecond time-resolved absorbance change spectra recorded in wild type reaction centers and in four mutants at position M210. In both wild type and the mutants, the kinetics of the dynamic Stark shift differ from those of electron transfer, though not in the same way. In wild type, the initial electron transfer and the increase in the effective field strength near the active-side monomer bacteriochlorophyll (B_A) occur in synchrony, but the two signals diverge on the time scale of electron transfer to the quinone. In contrast, when tyrosine is replaced by aspartic acid at M210, the kinetics of the B_A Stark shift and the initial electron transfer differ, but transfer to the quinone coincides with the decay of the Stark shift. This is interpreted in terms of differences in the dynamics of the local dielectric environment between the mutants and the wild type. In wild type, comparison of the Stark shifts associated with B_A and B_B on the two quasi-symmetric halves of the reaction center structure confirm that the effective dielectric constants near these cofactors are quite different when the reaction center is in the state P⁺Q_A^–, as previously determined by Steffen et al. at 1.5 K (Steffen, M. A.; et al. Science 1994, 264, 810−816). However, it is not possible to determine from static, low-temperature measurments if the difference in the effective dielectric constant between the two sides of the reaction center is manifest on the time scale of initial electron transfer. By comparing directly the Stark shift dynamics of the ground-state spectra of the two monomer bacteriochlorophylls, it is evident that there is, in fact, a large dielectric difference between protein environments of the two quasi-symmetric electron-transfer branches on the time scale of initial electron transfer and that the effective dielectric constant in the region continues to evolve on a time scale of hundreds of picoseconds

Exciton Structure and Dynamics in Solution Aggregates of a Low-Bandgap Copolymer

In this work, we elucidate exciton structure, dynamics, and charge generation in the solution phase aggregates of a low-bandgap donor–acceptor polymer, poly­(4,8-bis-alkyloxybenzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl-<i>alt</i>-(alkylthieno­[3,4-<i>b</i>]­thiophene-2carboxylate)-2,6-diyl (PBDTTT). The polymer aggregates in the solution phase serve as precursors for thin film morphologies. We have identified intrachain and interchain exciton transitions and resolved their relaxation pathways by comparing excitons in solution aggregates to those in isolated polymer chains. Hot intrachain excitons have led to the generation of stabilized interchain charge-separated states in solution aggregates, which could serve as the intermediate state to the hot exciton charge separation in bulk heterojunctions (BHJs). These results have important implications for controlling morphology dependent exciton dynamics in solution processed BHJs

Nanostructures Significantly Enhance Thermal Transport across Solid Interfaces

The efficiency of thermal transport across solid interfaces presents large challenges for modern technologies such as thermal management of electronics. In this paper, we report the first demonstration of significant enhancement of thermal transport across solid interfaces by introducing interfacial nanostructures. Analogous to fins that have been used for macroscopic heat transfer enhancement in heat exchangers, the nanopillar arrays patterned at the interface help interfacial thermal transport by the enlarged effective contact area. Such a benefit depends on the geometry of nanopillar arrays (e.g., pillar height and spacing), and a thermal boundary conductance enhancement by as much as ∼88% has been measured using the time-domain thermoreflectance technique. Theoretical analysis combined with low-temperature experiments further indicates that phonons with low frequency are less influenced by the interfacial nanostructures due to their large transmissivity, but the benefit of the nanostructure is fully developed at room temperature where higher frequency phonons dominate interfacial thermal transport. The findings from this work can potentially be generalized to benefit real applications such as the thermal management of electronics

Correlating Nanoscopic Energy Transfer and Far-Field Emission to Unravel Lasing Dynamics in Plasmonic Nanocavity Arrays

Excited-state interactions between nanoscale cavities and photoactive molecules are critical in plasmonic nanolasing, although the underlying details are less-resolved. This paper reports direct visualization of the energy-transfer dynamics between two-dimensional arrays of plasmonic gold bowtie nanocavities and dye molecules. Transient absorption microscopy measurements of single bowties within the array surrounded by gain molecules showed fast excited-state quenching (2.6 ± 1 ps) characteristic of individual nanocavities. Upon optical pumping at powers above threshold, lasing action emerged depending on the spacing of the array. By correlating ultrafast microscopy and far-field light emission characteristics, we found that bowtie nanoparticles acted as isolated cavities when the diffractive modes of the array did not couple to the plasmonic gap mode. These results demonstrate how ultrafast microscopy can provide insight into energy relaxation pathways and, specifically, how nanocavities in arrays can show single-unit nanolaser properties

Fate of Arsenate Adsorbed on Nano-TiO₂ in the Presence of Sulfate Reducing Bacteria

Arsenic removal using nanomaterials has attracted increasing attention worldwide, whereas the potential release of As from spent nanomaterials to groundwater in reducing environments is presently underappreciated. This research investigated the fate of As­(V) adsorbed on nano-TiO₂ in the presence of sulfate reducing bacteria (SRB) <i>Desulfovibrio vulgaris</i> strains DP4 and ATCC 7757. The incubation results demonstrated that As­(V) was desorbed from nano TiO₂, and subsequently reduced to As­(III) in aqueous solution. The release of adsorbed As­(V) was two to three times higher in biotic samples than that in abiotic controls. Reduction of As­(V) to As­(III) in biotic samples was coupled with the conversion of sulfate to sulfide, while no As­(III) was observed in abiotic controls. STXM results provided the direct evidence of appreciable As­(III) and As­(V) on TiO₂. XANES analysis indicated that As­(V) was the predominant species for three As loads of 150, 300, and 5700 mg/g, whereas 15–28% As precipitated as orpiment for a high As load of 5700 mg/g. In spite of orpiment formation, As mobilized in higher amounts in the SRB presence than in abiotic controls, highlighting the key role of SRB in the fate of As in the presence of nanomaterials

Oxidation is required for resveratrol stimulation of ATM.

<p>(a) ATM kinase assays were performed as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone-0097969-g003" target="_blank">Fig. 3</a> except with 0.5 and 2.5 mM TCEP as indicated. (b) ATM kinase assays were performed as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone-0097969-g003" target="_blank">Fig. 3</a> except with 0.36 nM ATM mutant (C2991L) and wild-type proteins as indicated. (c) ATM or resveratrol was pre-incubated with H₂O₂ (400 µM) as indicated for 15 min. Samples were diluted 40-fold with kinase reaction buffer containing 200 nM GST-p53 and incubated 1.5 hr. Final concentration of ATM and resveratrol is 0.36 nM and 0.1 mM, respectively, in all reactions. (d) HEK293T cells were preincubated with either 2 or 5 mM NAC as indicated for 16 hrs, followed by treatment with resveratrol and bleomycin as indicated. (e) (Quantitation of phosphorylated substrate levels from 3 independent experiments including those shown in (d); error bars indicate standard deviation.).</p

Structural Phase- and Degradation-Dependent Thermal Conductivity of CH₃NH₃PbI₃ Perovskite Thin Films

Organic–inorganic lead halide perovskites have shown great promise in photovoltaics and optoelectronics. In these applications, device performance and reliability can be strongly influenced by thermal transport in the materials. Through laser pump–probe experiments, different microstructures of CH₃NH₃PbI₃ perovskite thin films are found to give rise to different phonon scattering mechanism. The thermal conductivity in CH₃NH₃PbI₃ neat film decreases with temperature. Even though this agrees with the behavior of its bulk crystalline counterparts, an apparent thermal conductivity change near the structural phase transition temperature of this perovskite (orthorhombic vs tetragonal) has only been observed in the spin-coated films. Analyses suggest that this may be attributed to either an energy landscape change related to organic cation disorder or the thickness change of ferroelectric domain walls formed in the neat perovskite films that affects the phonon scattering at the domain boundaries. In contrast, no thermal conductivity discontinuity has been observed in the CH₃NH₃PbI₃/Al₂O₃ mesostructured films, where the thermal conductivity first shows an increasing trend at low temperature (<80 K) and then stays nearly constant. Such a trend is typical in amorphous materials and nanostructured composites where phonon scatterings are due to morphological disorder and internal interfaces play key roles in the thermal transport. When exposed to the ambient environment, humidity induced degradation is found to have a significant impact on the overall thermal conductivity of the spin-coated CH₃NH₃PbI₃ neat film

Resveratrol activates ATM in human primary fibroblasts (GM08399) in combination with H₂O₂ or bleomycin.

<p>(a, b) The effects of resveratrol on human primary fibroblasts were tested as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone-0097969-g001" target="_blank">Figure 1</a> but with varying levels of H₂O₂ or bleomycin as shown. (c) To deplete ATM, the fibroblasts were transduced with lentivirus expressing shRNA directed against ATM shRNA plasmid. After selection with puromycin, cells were tested for ATM expression and ATM target phosphorylation in combination with KU-55933, resveratrol, H₂O₂, and bleomycin as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone-0097969-g001" target="_blank">Fig. 1</a>. (d) Human primary fibroblasts were treated with resveratrol, bleomycin, or both as in (a) and probed for γ-H2AX foci by immunofluorescence. Cell images (82, 83, 78, and 82 cells, respectively, were analyzed for foci using Image J software, and the average number of foci per cell were quantitated. Error bars show standard error and * indicates comparisons in which p<0.05. (e) Human primary fibroblasts were treated as in (d) and the percentage of cells containing >5 foci was quantitated. Cell images from 3 independent experiments with a total of 266, 264, 248, and 269 cells, respectively, were quantitated. (f) Human primary fibroblasts were treated with resveratrol (100 µM), hydrogen peroxide, or both as in (b) and were quantitated for γ-H2AX foci by immunofluorescence. 107, 110, 104, and 106 cells, respectively, were analyzed for foci using Image J software, and the average number of foci per cell was calculated. (g) Quantitation of total pan-nuclear γ-H2AX signal per nucleus in cells treated with resveratrol, H₂O₂, and bleomycin as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone-0097969-g001" target="_blank">Fig. 1</a>. The average nuclear signal in untreated cells was normalized to 1. (h) Representative immunofluorescence images with fibroblasts treated as in (a). (i) Representative comet assay images with fibroblasts treated as in (a). (j) Quantification of comet tail length from fibroblasts treated as in (a); 30 cells were measured for each condition.</p

Purified ATM is stimulated by resveratrol in vitro.

<p>(a) MRN/DNA-dependent ATM activity was tested with 0.36 nM ATM, 2.2 nM MRN, 50 nM GST-p53, and 10 ng (∼140 nM) linear DNA in a 40 µl reaction as described previously <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone.0097969-Lee3" target="_blank">[25]</a>. (b) H₂O₂-dependent ATM activity was performed with 817 µM H₂O₂ in vitro as described previously <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097969#pone.0097969-Guo1" target="_blank">[13]</a> in the presence of 0, 69.5, 139, 278, or 556 µM resveratrol. (c) ATM kinase assays were performed with 0.36 nM ATM, 817 µM H₂O₂, and varying concentrations of GST-p53 substrate (40, 60, 80, 100, 120, 140, 160, and 320 nM) as indicated, in the presence or absence of 278 µM resveratrol. Phosphorylated p53 was quantitated using western blotting in comparison to standards, and the rate of phosphorylation (nmoles/min/pmole ATM) is plotted as a function of p53 substrate concentration (d) Skatchard plot is shown based on data in (c). (e) V_max (nmoles/min/pmole ATM) and K_m (nM) values calculated from data shown in (d) and (e). (f) ATM kinase assay as in (a) with 817 µM H₂O₂, 278 µM resveratrol, and varying levels of ATP as indicated. (g) ATM kinase assays were performed as in (a) except with 100, 278, and 830 µM resveratrol, genistein, or piceatannol in the presence of H₂O₂. (h) diagrams of resveratrol, genistein, and piceatannol structures.</p

The Protein Environment of the Bacteriopheophytin Anion Modulates Charge Separation and Charge Recombination in Bacterial Reaction Centers

The kinetics and pathway of electron transfer has been explored in a series of reaction center mutants from <i>Rhodobacter sphaeroides</i>, in which the leucine residue at M214 near the bacteriopheophytin cofactor in the A-branch has been replaced with methionine, cysteine, alanine, and glycine. These amino acids have substantially different volumes, both from each other and, except for methionine, from the native leucine. Though the mutation site of M214 is close to the bacteriopheophytin cofactor, which is involved in the electron transfer, none of the mutations alter the cofactor composition of the reaction center and the primary charge separation reaction is essentially undisturbed. However, the kinetics of electron transfer from H_A^– → Q_A becomes both slower and substantially heterogeneous in three of the four mutants. The decreased H_A^– → Q_A electron transfer rate allows charge recombination between P⁺ and H_A^– to compete with the forward reaction, resulting in a drop in the overall yield of charge separation. Both the yield change and the variation in kinetics correlate well with the volume of the mutant amino acid side chains. Analysis of the kinetics suggests that the introduction of a smaller side chain at M214 results in greater protein structural heterogeneity and dynamics on multiple time scales, resulting in perturbation of the electronic environment and its evolution in the vicinity of the early charge-separated radical pair, P⁺H_A^–, and the subsequent acceptor Q_A, affecting both the extent and time scale of dielectric relaxation. It appears that the reaction center has been optimized not only in terms of its static structure–function relationships, but also finely tuned to favor particular reaction pathways on particular time scales by adjusting protein dynamics