Chakicherla Gayathri

Monoplatinum Doping of Gold Nanoclusters and Catalytic Application

We report single-atom doping of gold nanoclusters (NCs), and its drastic effects on the optical, electronic, and catalytic properties, using the 25-atom system as a model. In our synthetic approach, a mixture of Pt(1)Au(24)(SC(2)H(4)Ph)(18) and Au(25)(SC(2)H(4)Ph)(18) was produced via a size-focusing process, and then Pt(1)Au(24)(SC(2)H(4)Ph)(18) NCs were obtained by selective decomposition of Au(25)(SC(2)H(4)Ph)(18) in the mixture with concentrated H(2)O(2) followed by purification via size-exclusion chromatography. Experimental and theoretical analyses confirmed that Pt(1)Au(24)(SC(2)H(4)Ph)(18) possesses a Pt-centered icosahedral core capped by six Au(2)(SC(2)H(4)Ph)(3) staples. The Pt(1)Au(24)(SC(2)H(4)Ph)(18) cluster exhibits greatly enhanced stability and catalytic activity relative to Au(25)(SC(2)H(4)Ph)(18) but a smaller energy gap (E(g) ≈ 0.8 eV vs 1.3 eV for the homogold cluster).

Probing the Structure and Charge State of Glutathione-Capped Au25(SG)18 Clusters by NMR and Mass Spectrometry

Despite the recent crystallographic determination of the crystal structure of Au(25)(SCH(2)CH(2)Ph)(18) clusters, the question--whether all thiolate-capped, 25-atom gold clusters adopt the same structure, regardless of the types of thiols (e.g., long-chain alkylthiols, aromatic thiols, or other functionalized ones)--still remains unanswered. To crystallize long-chain or bulky ligand (e.g., glutathione)-capped Au(25)(SR)(18) clusters has proven to be difficult due to the major amorphousness caused by such ligands; therefore, one needs to seek other strategies to probe the structural information of such gold clusters. Herein, we report a strategy to probe the Au(25) core structure and surface thiolate ligand distribution by means of NMR in combination with mass spectrometry. We use glutathione-capped Au(25)(SG)(18) clusters as an example to demonstrate the utility of this strategy. One-dimensional (1D) and two-dimensional (2D) correlation NMR spectroscopic investigation of Au(25)(SG)(18) reveals fine spectral features that explicitly indicate two types of surface binding modes of thiolates, which is consistent with the ligand distribution in the Au(25)(SCH(2)CH(2)Ph)(18) cluster. Laser desorption ionization (LDI) mass spectrometry analysis shows that Au(25)(SG)(18) exhibits an identical ionization and core fragmentation pattern with phenylethylthiolate-capped Au(25) clusters. The charge state of the native Au(25)(SG)(18) clusters was determined to be -1 by comparing their optical spectrum with those of [Au(25)(SCH(2)CH(2)Ph)(18)](q) of different charge states (q = -1, 0). Taken together, our results led to the conclusion that glutathione-capped Au(25)(SG)(18) clusters indeed adopt the same structure as that of Au(25)(SCH(2)CH(2)Ph)(18). This conclusion is also valid for other types of thiolate-capped Au(25) clusters, including hexyl- and dodecylthiolates. Interestingly, the chiral optical responses (e.g., circular dichroism (CD) signals in the visible wavelength region) from the Au(25)(SG)(18) clusters seem to be imparted by the chiral glutathione ligands because no similar CD signals were observed in Au(25)(SCH(2)CH(2)Ph)(18).

Crystal Structure of Barrel-Shaped Chiral Au130(p-MBT)50 Nanocluster.

We report the structure determination of a large gold nanocluster formulated as Au130(p-MBT)50, where p-MBT is 4-methylbenzenethiolate. The nanocluster is constructed in a four-shell manner, with 55 gold atoms assembled into a two-shell Ino decahedron. The surface is protected exclusively by -S-Au-S- staple motifs, which self-organize into five ripple-like stripes on the surface of the barrel-shaped Au105 kernel. The Au130(p-MBT)50 can be viewed as an elongated version of the Au102(SR)44. Comparison of the Au130(p-MBT)50 structure with the recently discovered icosahedral Au133(p-TBBT)52 nanocluster (where p-TBBT = 4-tert-butylbenzenethiolate) reveals an interesting phenomenon that a subtle ligand effect in the para-position of benzenethiolate can significantly affect the gold atom packing structure, i.e. from the 5-fold twinned Au55 decahedron to 20-fold twinned Au55 icosahedron.

Chirality in Gold Nanoclusters Probed by NMR Spectroscopy

We report the analysis of chirality in atomically precise gold nanoclusters by nuclear magnetic resonance (NMR) spectroscopic probing of the surface ligands. The Au(38)(SR)(24) and Au(25)(SR)(18) (where, R = CH(2)CH(2)Ph) are used as representative models for chiral and nonchiral nanoclusters, respectively. Interestingly, different (1)H signals for the two germinal protons in each CH(2) of the ligands on the chiral Au(38)(SR)(24) nanocluster were observed, so-called diastereotopicity. For α-CH(2) (closest to the chiral metal core), a chemical shift difference of up to ~0.8 ppm was observed. As for the nonchiral Au(25)(SCH(2)CH(2)Ph)(18)(-)TOA(+) nanocluster, no diastereotopicity was detected (i.e., no chemical shift difference for the two protons in the CH(2)), confirming the Au(25) core being nonchiral. These two typical examples demonstrate that NMR spectroscopy can be a useful tool for investigating chirality in Au nanoclusters. Since the diastereotopicity induced on the methylene protons by chiral nanoclusters is independent of the enantiomeric composition of the chiral particles, NMR can probe the chirality of the nanoclusters even in the case of a racemic mixture, while circular dichroism spectroscopy is not useful for racemic mixtures.

End-group effects on the properties of PEG-co-PGA hydrogels

A series of resorbable poly(ethylene glycol)-co-poly(glycolic acid) (PEG-co-PGA, 4KG5) macromonomers have been synthesized with the chemistries from three different photopolymerizable end-groups (acrylates, methacrylates and urethane methacrylates). The aim of the study is to examine the effects of the chemistry of the cross-linker group on the properties of photocross-linked hydrogels. 4KG5 hydrogels were prepared by photopolymerization with high vinyl group conversion as confirmed by (1)H nuclear magnetic resonance spectrometry using a 1D diffusion-ordered spectrometry pulse sequence. Our study reveals that the nature of end-groups in a moderately amphiphilic polymer can adjust the distribution and size of the micellar configuration in water, leading to changes in the macroscopic structure of hydrogels. By varying the chemistry of the cross-linker group (diacrylates (DA), dimethacrylates (DM) and urethane dimethacrylates (UDM)), we determined that the hydrophobicity of a single core polymer consisting of poly(glycolic acid) could be fine-tuned, leading to significant variations in the mechanical, swelling and degradation properties of the gels. In addition, the effects of cross-linker chemistry on cytotoxicity and proliferation were examined. Cytotoxicity assays showed that the three types of hydrogels (4KG5 DA, DM and UDM) were biocompatible and the introduction of RGD ligand enhanced cell adhesion. However, differences in gel properties and stability differentially affected the spreading and proliferation of myoblast C2C12 cells.

Tri-icosahedral Gold Nanocluster [Au37(PPh3)10(SC2H4Ph)10X2](+): Linear Assembly of Icosahedral Building Blocks.

The [Au37(PPh3)10(SR)10X2](+) nanocluster (where SR = thiolate and X = Cl/Br) was theoretically predicted in 2007, but since then, there has been no experimental success in the synthesis and structure determination. Herein, we report a kinetically controlled, selective synthesis of [Au37(PPh3)10(SC2H4Ph)10X2](+) (counterion: Cl(-) or Br(-)) with its crystal structure characterized by X-ray crystallography. This nanocluster shows a rod-like structure assembled from three icosahedral Au13 units in a linear fashion, consistent with the earlier prediction. The optical absorption and the electrochemical and catalytic properties are investigated. The successful synthesis of this new nanocluster allows us to gain insight into the size, structure, and property evolution of gold nanoclusters that are based upon the assembly of icosahedral units (i.e., cluster of clusters). Some interesting trends are identified in the evolution from the monoicosahedral [Au13(PPh3)10X2](3+) to the bi-icosahedral [Au25(PPh3)10(SC2H4Ph)5X2](2+) and to the tri-icosahedral [Au37(PPh3)10(SC2H4Ph)10X2](+) nanocluster, which also points to the possibility of achieving even longer rod nanoclusters based upon assembly of icosahedral building blocks.

Stereochemistry Determination by Powder X‐Ray Diffraction Analysis and NMR Spectroscopy Residual Dipolar Couplings

A matter of technique: For a new steroidal lactol, jaborosalactol 24 (1), isolated from Jaborosa parviflora, NMR spectroscopy residual dipolar couplings and powder X-ray diffraction analysis independently gave the same stereochemistry at C23-C26. Conventional NMR spectroscopic techniques, such as NOE and {sup 3}J coupling-constant analysis failed to unambiguously determine this stereochemistry.

New strategy for RDC‐assisted diastereotopic proton assignment using a combination of J‐scaled BIRD HSQC and J‐scaled BIRD HMQC/HSQC

A new strategy to assign diastereotopic protons was developed on the basis of residual dipolar couplings (RDCs) collected in compressed poly(methyl methacrylate) (PMMA) gels. A combination of 2D J‐scaled BIRD HSQC and J‐scaled BIRD HMQC/HSQC NMR experiments was used to collect the RDC data. In the proposed strategy, the first experiment is used to measure 1DCH for methine groups, the sum of 1DCHa + 1DCHb for methylene groups and the average 1DCH3 value for methyl groups. In turn, the small molecule alignment tensor is calculated using these D values without the a priori assignment of CH2 diastereotopic protons. The D values of each individual CH bond (CHa and CHb) of each methylene group in the molecule are then predicted using the calculated alignment tensor and these values compared with the results from the HMQC/HSQC experiment, leading to their unambiguous assignment. This strategy is demonstrated with the alkaloid strychnine that contains five methylene groups with diastereotopic protons, and our results fully agree with the previously reported assignment using combinations of permutated assignments. Copyright

Removal of ecotoxicity of 17α-ethinylestradiol using TAML/peroxide water treatment

17α-ethinylestradiol (EE2), a synthetic oestrogen in oral contraceptives, is one of many pharmaceuticals found in inland waterways worldwide as a result of human consumption and excretion into wastewater treatment systems. At low parts per trillion (ppt), EE2 induces feminisation of male fish, diminishing reproductive success and causing fish population collapse. Intended water quality standards for EE2 set a much needed global precedent. Ozone and activated carbon provide effective wastewater treatments, but their energy intensities and capital/operating costs are formidable barriers to adoption. Here we describe the technical and environmental performance of a fast- developing contender for mitigation of EE2 contamination of wastewater based upon small- molecule, full-functional peroxidase enzyme replicas called “TAML activators”. From neutral to basic pH, TAML activators with H2O2 efficiently degrade EE2 in pure lab water, municipal effluents and EE2-spiked synthetic urine. TAML/H2O2 treatment curtails estrogenicity in vitro and substantially diminishes fish feminization in vivo. Our results provide a starting point for a future process in which tens of thousands of tonnes of wastewater could be treated per kilogram of catalyst. We suggest TAML/H2O2 is a worthy candidate for exploration as an environmentally compatible, versatile, method for removing EE2 and other pharmaceuticals from municipal wastewaters.

TAML/H2O2 Oxidative Degradation of Metaldehyde: Pursuing Better Water Treatment for the Most Persistent Pollutants

The extremely persistent molluscicide, metaldehyde, widely used on farms and gardens, is often detected in drinking water sources of various countries at concentrations of regulatory concern. Metaldehyde contamination restricts treatment options. Conventional technologies for remediating dilute organics in drinking water, activated carbon, and ozone, are insufficiently effective against metaldehyde. Some treatment plants have resorted to effective, but more costly UV/H2O2. Here we have examined if TAML/H2O2 can decompose metaldehyde under laboratory conditions to guide development of a better real world option. TAML/H2O2 slowly degrades metaldehyde to acetaldehyde and acetic acid. Nuclear magnetic resonance spectroscopy ((1)H NMR) was used to monitor the degradation-the technique requires a high metaldehyde concentration (60 ppm). Within the pH range of 6.5-9, the reaction rate is greatest at pH 7. Under optimum conditions, one aliquot of TAML 1a (400 nM) catalyzed 5% degradation over 10 h with a turnover number of 40. Five sequential TAML aliquots (2 μM overall) effected a 31% removal over 60 h. TAML/H2O2 degraded metaldehyde steadily over many hours, highlighting an important long-service property. The observation of metaldehyde decomposition under mild conditions provides a further indication that TAML catalysis holds promise for advancing water treatment. These results have turned our attention to more aggressive TAML activators in development, which we expect will advance the observed technical performance.