Studies on plant gums of the acacia group

The results of a re-investigation of A.Senegal gum have led. to the rejection of the idea that its molecular structure is necessarily based on a "main chain" or "backbone" of (31,3- linked g-galactose residues. The concept that the A.Senegal gum molecule contains a "main chain" or "backbone," to which are attached short side chains, implies that there is one chain in the macromolecule which is unique in being very much longer than the others. Unequivocal evidence for the occurrence of such chains in A.Senegal gum molecules does not exist. In the case of A.arabica gum, all the evidence from structural investigations, and from the solution behaviour of the gum favours a dichotomously branched galactan framework for the basal units of the macromolecules. These findings suggest that it is no longer meaningful to interpret the structural features of these gums in terms of average repeating units. The results, however, may have a wider implication and significance. On re-examination, many plant gums (Smith & Montgomery, 1959) of' the substituted arabinogalactan type, and perhaps some of the arabinogalactans themselves (Timell, 1965), may be shown to exhibit dichotomous branching within their galactan frameworks.Although structural work has been carried out on gums from other Acacia species, including A.mollissima (Stephen, 1951; CHAPTER V - 125 - Young, 1963), A.pycnantha (Hirst & Perlin, 1954; Aspinall, Hirst & Nicolson, 1959), A.karroo (Charlson, Nunn & Stephen, 1955a), A.cyanophylla (Gharlson, Nunn & Stephen, 1955b), A.catechu (Hulyalkar, Ingle & Bhide, 1956, 1959), A.sundra (Mukherjee & Shrivastava, 1957, 1958, 1959; Shrivastava, 1962), A.seyal (Herbich, 1963), A.nilotica (Karamalla, 1965; Anderson & Karamalla, 1966b), A.nubica (Oree, 1966) and A.laeta (Smith, 1966), the investigations reported in this thesis represent the first exploratory steps towards interpreting molecular structures of Acacia gums at the molecular level

Free energy barrier for molecular motions in bistable [2]rotaxane molecular electronic devices

Donor−acceptor binding of the π-electron-poor cyclophane cyclobis(paraquat-p-phenylene) (CBPQT^(4+)) with the π-electron-rich tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) stations provides the basis for electrochemically switchable, bistable [2]rotaxanes, which have been incorporated and operated within solid-state devices to form ultradense memory circuits (ChemPhysChem 2002, 3, 519−525; Nature 2007, 445, 414−417) and nanoelectromechanical systems. The rate of CBPQT^(4+) shuttling at each oxidation state of the [2]rotaxane dictates critical write-and-retention time parameters within the devices, which can be tuned through chemical synthesis. To validate how well computational chemistry methods can estimate these rates for use in designing new devices, we used molecular dynamics simulations to calculate the free energy barrier for the shuttling of the CBPQT^4+ ring between the TTF and the DNP. The approach used here was to calculate the potential of mean force along the switching pathway, from which we calculated free energy barriers. These calculations find a turn-on time after the rotaxane is doubly oxidized of ~10^9−7) s (suggesting that the much longer experimental turn-on time is determined by the time scale of oxidization). The return barrier from the DNP to the TTF leads to a predicted lifetime of 2.1 s, which is compatible with experiments

Kinetic and Thermodynamic Approaches for the Efficient Formation of Mechanical Bonds

Among the growing collection of molecular systems under consideration for nanoscale device applications, mechanically interlocked compounds derived from electrochemically switchable bistable [2]rotaxanes and [2]catenanes show great promise. These systems demonstrate dynamic, relative movements between their components, such as shuttling and circumrotation, enabling them to serve as stimuli-responsive switches operated via reversible, electrochemical oxidation−reduction rather than through the addition of chemical reagents. Investigations into these systems have been intense for a number of years, yet limitations associated with their synthesis have hindered incorporation of their mechanical bonds into more complex architectures and functional materials. We have recently addressed this challenge by developing new template-directed synthetic protocols, operating under both kinetic and thermodynamic control, for the preparation of bistable rotaxanes and catenanes. These methodologies are compatible with the molecular recognition between the π-electron-accepting cyclobis(paraquat-p-phenylene) (CBPQT4+) host and complementary π-electron-donating guests. The procedures that operate under kinetic control rely on mild chemical transformations to attach bulky stoppering groups or perform macrocyclizations without disrupting the host−guest binding of the rotaxane or catenane precursors. Alternatively, the protocols that operate under thermodynamic control utilize a reversible ring-opening reaction of the CBPQT4+ ring, providing a pathway for two cyclic starting materials to thread one another to form more thermodynamically stable catenaned products. These complementary pathways generate bistable rotaxanes and catenanes in high yields, simplify mechanical bond formation in these systems, and eliminate the requirement that the mechanical bonds be introduced into the molecular structure in the final step of the synthesis. These new methods have already been put into practice to prepare previously unavailable rotaxane architectures and novel complex materials. Furthermore, the potential for utilizing mechanically interlocked architectures as device components capable of information storage, the delivery of therapeutic agents, or other desirable functions has increased significantly as a result of the development of these improved synthetic protocols

Amphipoda Crustacea IV. Families Aristiidae, Cyphocarididae, Endevouridae, Lysianassidae, Scopelocheiridae, Uristidae

(150pp.

Spiers Memorial Lecture: Molecular mechanics and molecular electronics

We describe our research into building integrated molecular electronics circuitry for a diverse set of functions, and with a focus on the fundamental scientific issues that surround this project. In particular, we discuss experiments aimed at understanding the function of bistable [2]rotaxane molecular electronic switches by correlating the switching kinetics and ground state thermodynamic properties of those switches in various environments, ranging from the solution phase to a Langmuir monolayer of the switching molecules sandwiched between two electrodes. We discuss various devices, low bit-density memory circuits, and ultra-high density memory circuits that utilize the electrochemical switching characteristics of these molecules in conjunction with novel patterning methods. We also discuss interconnect schemes that are capable of bridging the micrometre to submicrometre length scales of conventional patterning approaches to the near-molecular length scales of the ultra-dense memory circuits. Finally, we discuss some of the challenges associated with fabricated ultra-dense molecular electronic integrated circuits

Efficient Templated Synthesis of Donor−Acceptor Rotaxanes Using Click Chemistry

The mild reaction conditions, remarkable functional group compatibility, and complete regioselectivity of the Cu-catalyzed Huisgen 1,3-dipolar cycloaddition (“click chemistry”) between organic azides and terminal alkynes have led to a threading-followed-by-stoppering approach to the synthesis of donor−acceptor rotaxanes incorporating cyclobis(paraquat-p-phenylene) (CBPQT^(4+)) as the π-accepting ring component. Rotaxane formation is initiated by reacting azide-functionalized pseudorotaxanes containing π-donating 1,5-dioxynaphthalene (DNP) recognition units with appropriate alkyne-functionalized stoppers. The high yields obtained in this efficient, kinetically controlled post-assembly covalent modification, as well as the excellent convergence of the synthetic protocol, are demonstrated by the preparation of [2]-, [3]-, and [4]rotaxanes containing multiple DNP/CBPQT^(4+) donor−acceptor recognition motifs

The Molecule-Electrode Interface in Single-Molecule Transistors

What's in the middle matters little! Differential conductance measurements made on single‐molecule rotaxanes and their precursor dumbbells in transistors with platinum electrodes reflect the molecule–electrode contacts rather than the middle section of the molecules (see diagram; the color reflects the conductance, with dark corresponding to zero current). Interface states dominate electron transport. Molecular signatures are masked and even constitutional asymmetry in the molecule is difficult to detect

Decoding the Distribution of Glycan Receptors for Human-Adapted Influenza A Viruses in Ferret Respiratory Tract

Ferrets are widely used as animal models for studying influenza A viral pathogenesis and transmissibility. Human-adapted influenza A viruses primarily target the upper respiratory tract in humans (infection of the lower respiratory tract is observed less frequently), while in ferrets, upon intranasal inoculation both upper and lower respiratory tract are targeted. Viral tropism is governed by distribution of complex sialylated glycan receptors in various cells/tissues of the host that are specifically recognized by influenza A virus hemagglutinin (HA), a glycoprotein on viral surface. It is generally known that upper respiratory tract of humans and ferrets predominantly express α2→6 sialylated glycan receptors. However much less is known about the fine structure of these glycan receptors and their distribution in different regions of the ferret respiratory tract. In this study, we characterize distribution of glycan receptors going beyond terminal sialic acid linkage in the cranial and caudal regions of the ferret trachea (upper respiratory tract) and lung hilar region (lower respiratory tract) by multiplexing use of various plant lectins and human-adapted HAs to stain these tissue sections. Our findings show that the sialylated glycan receptors recognized by human-adapted HAs are predominantly distributed in submucosal gland of lung hilar region as a part of O-linked glycans. Our study has implications in understanding influenza A viral pathogenesis in ferrets and also in employing ferrets as animal models for developing therapeutic strategies against influenza.Singapore-MIT Alliance for Research and Technolog