Probing Quantum Confinement within Single Core–Multishell Nanowires

Martí,nez-Criado, Gema,Homs, Alejandro,Ale&#x301,n, Benito,Sans, Juan A.,Segura-Ruiz, Jaime,Molina-Sa&#x301,nchez, Alejandro,Susini, Jean,Yoo, Jinkyoung,Yi, Gyu-Chul American Chemical Society 2012 NANO LETTERS Vol.12 No.11

<P>Theoretically core–multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2012/nalefd.2012.12.issue-11/nl303178u/production/images/medium/nl-2012-03178u_0007.gif'></P>

Dramatic Structural Rearrangements in Porous Coordination Networks

Martí,-Rujas, Javier,Islam, Nazrul,Hashizume, Daisuke,Izumi, Fujio,Fujita, Makoto,Kawano, Masaki American Chemical Society 2011 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.133 No.15

<P>With the use of <I>ab initio</I> X-ray powder diffraction, a family of isostructural crystalline porous coordination networks, [(ZnX<SUB>2</SUB>)<SUB>3</SUB>(TPT)<SUB>2</SUB>]<SUB><I>n</I></SUB>· (solvent) (X = I, Br, Cl), has been studied at elevated temperatures of 573−723 K. Upon heating, all three networks exhibited crystalline-to-amorphous-to-crystalline (CAC) phase transformations to three new networks, [(ZnI<SUB>2</SUB>)<SUB>3</SUB>(TPT)<SUB>2</SUB>]<SUB><I>n</I></SUB>, [(ZnBr<SUB>2</SUB>)<SUB>3</SUB>(TPT)<SUB>2</SUB>]<SUB><I>n</I></SUB>·(H<SUB>2</SUB>O) and [(ZnBr<SUB>2</SUB>)(μ-Br)(ZnBr)(TPT)]<SUB><I>n</I></SUB>, and [(ZnCl<SUB>2</SUB>)(μ-Cl)(ZnCl)(TPT)]<SUB><I>n</I></SUB>, respectively. A set of control experiments was used to obtain detailed mechanistic aspects of the CAC transformations. We demonstrate how bonds are broken and formed in these significant molecular rearrangements and how the initial arrangement plays a crucial role in the formation of the new networks after the CAC transformations. The structural information in the amorphous phase is retained and passed from a metastable to a more stable crystal, thus, reinforcing the notion that coordination networks are flexible and chemically active.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2011/jacsat.2011.133.issue-15/ja109160a/production/images/medium/ja-2010-09160a_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja109160a'>ACS Electronic Supporting Info</A></P>

Kinetic Products in Coordination Networks: Ab Initio X-ray Powder Diffraction Analysis

Martí,-Rujas, Javier,Kawano, Masaki American Chemical Society 2013 Accounts of chemical research Vol.46 No.2

<P>Porous coordination networks are materials that maintain their crystal structure as molecular “guests” enter and exit their pores. They are of great research interest with applications in areas such as catalysis, gas adsorption, proton conductivity, and drug release. As with zeolite preparation, the kinetic states in coordination network preparation play a crucial role in determining the final products. Controlling the kinetic state during self-assembly of coordination networks is a fundamental aspect of developing further functionalization of this class of materials. However, unlike for zeolites, there are few structural studies reporting the kinetic products made during self-assembly of coordination networks. Synthetic routes that produce the necessary selectivity are complex.</P><P>The structural knowledge obtained from X-ray crystallography has been crucial for developing rational strategies for design of organic–inorganic hybrid networks. However, despite the explosive progress in the solid-state study of coordination networks during the last 15 years, researchers still do not understand many chemical reaction processes because of the difficulties in growing single crystals suitable for X-ray diffraction: Fast precipitation can lead to kinetic (metastable) products, but in microcrystalline form, unsuitable for single crystal X-ray analysis. X-ray powder diffraction (XRPD) routinely is used to check phase purity, crystallinity, and to monitor the stability of frameworks upon guest removal/inclusion under various conditions, but rarely is used for structure elucidation. Recent advances in structure determination of microcrystalline solids from ab initio XRPD have allowed three-dimensional structure determination when single crystals are not available. Thus, ab initio XRPD structure determination is becoming a powerful method for structure determination of microcrystalline solids, including porous coordination networks. Because of the great interest across scientific disciplines in coordination networks, especially porous coordination networks, the ability to determine crystal structures when the crystals are not suitable for single crystal X-ray analysis is of paramount importance.</P><P>In this Account, we report the potential of kinetic control to synthesize new coordination networks and we describe ab initio XRPD structure determination to characterize these networks’ crystal structures. We describe our recent work on selective instant synthesis to yield kinetically controlled porous coordination networks. We demonstrate that instant synthesis can selectively produce metastable networks that are not possible to synthesize by conventional solution chemistry. Using kinetic products, we provide mechanistic insights into thermally induced (573–723 K) (i.e., annealing method) structural transformations in porous coordination networks as well as examples of guest exchange/inclusion reactions. Finally, we describe a memory effect that allows the transfer of structural information from kinetic precursor structures to thermally stable structures through amorphous intermediate phases.</P><P>We believe that ab initio XRPD structure determination will soon be used to investigate chemical processes that lead intrinsically to microcrystalline solids, which up to now have not been fully understood due to the unavailability of single crystals. For example, only recently have researchers used single-crystal X-ray diffraction to elucidate crystal-to-crystal chemical reactions taking place in the crystalline scaffold of coordination networks. The potential of ab initio X-ray powder diffraction analysis goes beyond single-crystal-to-single-crystal processes, potentially allowing members of this field to study intriguing in situ reactions, such as reactions within pores.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/achre4/2013/achre4.2013.46.iss

Zeolite Synthesis in FluorideMedia: Structure Directiontoward ITW by Small Methylimidazolium Cations

Rojas, Alex,Martí,nez-Morales, Evangelina,Zicovich-Wilson, Claudio M.,Camblor, Miguel A. American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.4

<P>Pure silica ITW zeolite can be synthesized using 1,2,3-trimethylimidazoliumand 1,3-dimethylimidazolium cations and fluoride anions as structure-directingagents (SDAs). Similarly to the previously reported 1,3,4-trimethylimidazolium,the dimethyl cation can also produce the zeolite TON, but this higherframework density phase finally transforms <I>in situ</I> into ITW. The structures of the as-made and calcined phases preparedwith the new cations show a unit cell doubling along <I>z</I>, and the refined structures are reported. Periodic Density FunctionalTheory calculations provide the energies of the six SDA-ITW and SDA-TONzeolites, and their relative stabilities fully agree with the experimentalobservations. Structure-direction in this system is discussed fromexperimental and theoretical results that give strong support to theidea that strained silica frameworks are made possible in fluoridemedia by decreasing the covalent character of the Si–O bond.This decreased covalency is enhanced with the 1,2,3-trimethyl isomer,which is shown to be the strongest SDA for ITW and, at the same time,is the more hydrophilic of the three SDAs tested. Our observationswith the three SDAs agree with the so-called Villaescusa’srule, i.e., the low framework density phase is favored at higher concentrations,but at the same time question the supersaturation hypothesis thathas been proposed to explain this rule, since here the low-densityphase is the most stable one.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-4/ja209832y/production/images/medium/ja-2011-09832y_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja209832y'>ACS Electronic Supporting Info</A></P>

Dynamic control of strand excision during human DNA mismatch repair

Jeon, Yongmoon,Kim, Daehyung,Martí,n-Li&#x301,o&#x301,pez, Juana V.,Lee, Ryanggeun,Oh, Jungsic,Hanne, Jeungphill,Fishel, Richard,Lee, Jong-Bong National Academy of Sciences 2016 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.113 No.12

<P>Mismatch repair (MMR) is activated by evolutionarily conserved MutS homologs (MSH) and MutL homologs (MLH/PMS). MSH recognizes mismatched nucleotides and form extremely stable sliding clamps that may be bound by MLH/PMS to ultimately authorize strand-specific excision starting at a distant 3'- or 5'-DNA scission. The mechanical processes associated with a complete MMR reaction remain enigmatic. The purified human (Homo sapien or Hs) 5'-MMR excision reaction requires the HsMSH2-HsMSH6 heterodimer, the 5' -> 3' exonuclease HsEXOI, and the single-stranded binding heterotrimer HsRPA. The HsMLH1-HsPMS2 heterodimer substantially influences 5'-MMR excision in cell extracts but is not required in the purified system. Using real-time single-molecule imaging, we show that HsRPA or Escherichia coli EcSSB restricts HsEXOI excision activity on nicked or gapped DNA. HsMSH2-HsMSH6 activates HsEXOI by overcoming HsRPA/EcSSB inhibition and exploits multiple dynamic sliding clamps to increase tract length. Conversely, HsMLH1-HsPMS2 regulates tract length by controlling the number of excision complexes, providing a link to 5' MMR.</P>

Stacking Structures of Few-Layer Graphene Revealed by Phase-Sensitive Infrared Nanoscopy

Kim, Deok-Soo,Kwon, Hyuksang,Nikitin, Alexey Yu.,Ahn, Seongjin,Martí,n-Moreno, Luis,Garci&#x301,a-Vidal, Francisco J.,Ryu, Sunmin,Min, Hongki,Kim, Zee Hwan American Chemical Society 2015 ACS NANO Vol.9 No.7

<P>The stacking orders in few-layer graphene (FLG) strongly influences the electronic properties of the material. To explore the stacking-specific properties of FLG in detail, one needs powerful microscopy techniques that visualize stacking domains with sufficient spatial resolution. We demonstrate that infrared (IR) scattering scanning near-field optical microscopy (sSNOM) directly maps out the stacking domains of FLG with a nanometric resolution, based on the stacking-specific IR conductivities of FLG. The intensity and phase contrasts of sSNOM are compared with the sSNOM contrast model, which is based on the dipolar tip–sample coupling and the theoretical conductivity spectra of FLG, allowing a clear assignment of each FLG domain as Bernal, rhombohedral, or intermediate stacks for tri-, tetra-, and pentalayer graphene. The method offers 10–100 times better spatial resolution than the far-field Raman and infrared spectroscopic methods, yet it allows far more experimental flexibility than the scanning tunneling microscopy and electron microscopy.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-7/acsnano.5b02813/production/images/medium/nn-2015-02813x_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b02813'>ACS Electronic Supporting Info</A></P>

MutS homolog sliding clamps shield the DNA from binding proteins

Hanne, Jeungphill,Britton, Brooke M.,Park, Jonghyun,Liu, Jiaquan,Martí,n-Li&#x301,o&#x301,pez, Juana,Jones, Nathan,Schoffner, Matthew,Klajner, Piotr,Bundschuh, Ralf,Lee, Jong-Bong,Fishel, Richar American Society for Biochemistry and Molecular Bi 2018 The Journal of biological chemistry Vol.293 No.37

<P>Sliding clamps on DNA consist of evolutionarily conserved enzymes that coordinate DNA replication, repair, and the cellular DNA damage response. MutS homolog (MSH) proteins initiate mismatch repair (MMR) by recognizing mispaired nucleotides and in the presence of ATP form stable sliding clamps that randomly diffuse along the DNA. The MSH sliding clamps subsequently load MutL homolog (MLH/PMS) proteins that form a second extremely stable sliding clamp, which together coordinate downstream MMR components with the excision-initiation site that may be hundreds to thousands of nucleotides distant from the mismatch. Specific or nonspecific binding of other proteins to the DNA between the mismatch and the distant excision-initiation site could conceivably obstruct the free diffusion of these MMR sliding clamps, inhibiting their ability to initiate repair. Here, we employed bulk biochemical analysis, single-molecule fluorescence imaging, and mathematical modeling to determine how sliding clamps might overcome such hindrances along the DNA. Using both bacterial and human MSH proteins, we found that increasing the number of MSH sliding clamps on a DNA decreased the association of the Escherichia coli transcriptional repressor LacI to its cognate promoter LacO. Our results suggest a simple mechanism whereby thermal diffusion of MSH sliding clamps along the DNA alters the association kinetics of other DNA-binding proteins over extended distances. These observations appear generally applicable to any stable sliding clamp that forms on DNA.</P>