Showing papers by "Jason E. Gestwicki published in 2000"

Evolutionary conservation of methyl-accepting chemotaxis protein location in Bacteria and Archaea.

Abstract: The methyl-accepting chemotaxis proteins (MCPs) are concentrated at the cell poles in an evolutionarily diverse panel of bacteria and an archeon. In elongated cells, the MCPs are located both at the poles and at regions along the length of the cells. Together, these results suggest that MCP location is evolutionarily conserved.

Microwave-induced esterification using heterogeneous acid catalyst in a low dielectric constant medium

Abstract: During the past decade, our knowledge of microwaveassisted organic synthesis has increased significantly.1-4 Initial studies of several reactions revealed an enhancement of reaction rates in the presence of microwave irradiation, as compared to identical reactions heated by classical methods. Early hypotheses suggested that the rate enhancements were due to athermal effects induced by microwave radiation.5a-e However, subsequent investigations, in which reaction mixtures were stirred or mixed to ensure thermal homogeneity, revealed that reaction rates of microwave-irradiated and classically heated reactions were comparable.6a-c These studies led to the hypothesis that rate enhancement observed as a result of microwave irradition is caused by superheating of the reaction solution.7,8 Despite limitations, domestic microwave ovens are widely used for laboratory organic synthesis.1 These devices are multimode applicators, which operate at a fixed maximum power level for varying periods of time (duty cycle). The resulting electric field is thus nonuniform or heterogeneous, resulting from multiple reflections inside the microwave cavity. Efforts to model the electromagnetic field inside such reactors reveals the fact that temperature measurement of an irradiated sample cannot be easily determined.9-10 These underlying factors are a large cause of variation in reported rate enhancements. The limitations of the domestic microwave oven can be overcome by the use of a monomode applicator, capable of focusing electromagnetic waves with the use of a waveguide to achieve a homogeneous electromaginetic field.3 However, despite the advantages, such equipment remains relatively unavailable in contemporary organic synthesis laboratories. Concomitant with the technological advances in microwave reactors, several strategies for microwave-assisted organic synthesis are currently in use.1-4,11 A significant body of work concerning the applications of microwave radiation to homogeneous reactions exists.2-4 However, heterogeneous reactions, in particular those that involve the use of solid-state catalysts, are particularly advantageous in terms of ease of use, separation, and catalytic recycling.12 Most reports of kinetic studies involving the use heterogeneous catalysts in microwaveirradiated systems, suggest that selective superheating of the catalyst may account for observations of small rate enhancements.13-16 To effectively study the kinetics of a heterogeneous system, absorption of microwave radiation should be limited only to the reacting species. This can be accomplished through the use of a low dielectric constant medium (i.e., hydrocarbons) as a reaction solvent.17 Additionally, the reaction should occur entirely inside the microwave cavity to maximize irradiation of all reacting species. This constraint poses severe practical limitations, since adequate temperature and pressure control in such a system is difficult to achieve.17 To overcome this problem, we have developed a continuous-flow reactor for heterogeneous systems, which localizes a bed of Amberlyst-15 cation-exchange resin contained in a radiolucent polyethylene tube inside the microwave cavity (Figure 1). We have chosen the acid-catalyzed Fischer-type esterification of isopentyl alcohol and acetic acid as our model system. Esterifications are of practical interest because esters are used in the production of a wide range of products such as cosmetics, lubricants, pharmaceuticals, and plasticizers. This reaction represents a well-understood Fischer esterification that in homogeneous systems occurs by the AAC2 mechanism18 (Scheme 1). Since the reaction is driven by the protonation of the carbonyl functionality, reacting species should be localized to the bed of acid catalyst, subject to microwave radiation. This model is also advantageous to study because all reagents (1) Loupy, A.; Petit, A.; Hamelin, J.; Texier-Boullet, F.; Jacquault, P.; Mathe, D. Synthesis 1998, 1213-1234. (2) Stuerga, D. A. C.; Gaillard P. Journal of Microwave Power and Electromagnetic Energy, 1996, 31, 87-100. (3) Strauss, C. R.; Trainor, R. W., Aust. J. Chem. 1995, 48, 16651692. (4) Westaway, K. C.; Gedye, R. N., J. Microwave Power Electromag. Energy. 1995, 30, 219-230. (5) (a) Pollington, D. S.; Bond, G.; Moyes, R. B.; Whan, D. A.; Candlin, J. P.; Jennings, J. R. J. Org. Chem. 1991, 56, 1313. (b) Thiebaut, J. M.; Roussy, G.; Maire, G.; Garin, F. International Conference on High-Frequency Microwave Processing and Heating, Arnhem, The Netherlands, 1989. (c) Berlan, J.; Giboreau, P.; Lefeuvre, S.; Marchand, C. Tetrahedron Lett. 1991, 32, 2363. (d) Sun, W.-C.; Guy, P. M.; Jahngen, J. H.; Rossomando, E. F.; Jahngen, E. G. E. J. Org. Chem. 1988, 4414. (e) Bose, A. K.; Manhas, M. S.; Ghosh, M.; Raju, V. S.; Tabei, K.; Urbanczyk-Lipkowska, Z. Heterocycles 1990, 30, 741. (6) (a) Raner, K. D.; Strauss, C. R.; Vyskoc, F.; Mokbel, L. J. Org. Chem. 1993, 950. (b) Laurent, R.; Laporterie, A.; Dubac, J.; Berlan, J.; Lefeuvre, S.; Audhuy, M. J. Org. Chem. 1992, 57, 7099. (c) Jahngen, E. G. E.; Lentz, R. R.; Pesheck, P. S.; Sackett, P. H. J. Org. Chem. 1990, 55, 3406. (7) (a) Kingston, H. M.; Jassie, L. B. Introduction to Microwave Sample Preparation Theory and Practice; American Chemical Society: Washington, D.C., 1988. (b) Baghurst, D. R.; Mingos, D. M. P. J. Chem. Soc., Chem. Comm. 674, 1992. (8) Mingos, D. M. P. Res. Chem. Intermed. 1994, 20, 85. (9) (a) Stuerga, D.; Gaillard P. Tetrahedron 1996, 52, 5505-5510. (b) Berlan, J. Radiat. Phys. Chem. 1995, 45, 581-589. (10) (a) Ayappa, K. G. Reviews in Chemical Engineering 1997, 13, 1-69. (b) Saillard, R.; Poux, M.; Berlan, J.; Audhuypeaudecerf, M. Tetrahedron 1995, 51, 4033-4042. (11) Kingston, H. M.; Haswell, S. J. Microwave-Enhanced Chemistry; Fundamentals, Sample Preparation and Applications; American Chemical Society: Washington, D.C., 1997; pp 16-17. (12) Xu, Z. P.; Chuang, K. T. Can. J. Chem. Eng. 1996, 74, 493500. (13) Gedye, R. N.; Wei, J. B. Can. J. Chem. 1998, 76, 525-532. (14) Shibata, C.; Kashima, T.; Ohuchi, K. Jpn. J. Appl. Phys. 1996, 35, 316. (15) Chemat, F.; Esveld, D. C.; Poux, M.; Di-Martino, J. L. J. Microwave Power Electromag. Energy 1998, 33, 88-94. (16) Holzwarth, A.; Lou, J.; Hatton, T. A.; Laibinis, P. E. Ind. Eng, Chem. Res. 1998, 37, 2701-2706. (17) Mingos, D. M. P. Chem. Ind. 1994, 1 Aug, 596-599. (18) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure; Wiley: New York, 1992; pp 378-383. 1210 J. Org. Chem. 2000, 65, 1210-1214

Motility and chemotaxis of filamentous cells of Escherichia coli.

Abstract: Filamentous cells of Escherichia coli can be produced by treatment with the antibiotic cephalexin, which blocks cell division but allows cell growth. To explore the effect of cell size on chemotactic activity, we studied the motility and chemotaxis of filamentous cells. The filaments, up to 50 times the length of normal E. coli organisms, were motile and had flagella along their entire lengths. Despite their increased size, the motility and chemotaxis of filaments were very similar to those properties of normal-sized cells. Unstimulated filaments of chemotactically normal bacteria ran and stopped repeatedly (while normal-sized bacteria run and tumble repeatedly). Filaments responded to attractants by prolonged running (like normal-sized bacteria) and to repellents by prolonged stopping (unlike normal-sized bacteria, which tumble), until adaptation restored unstimulated behavior (as occurs with normal-sized cells). Chemotaxis mutants that always ran when they were normal sized always ran when they were filament sized, and those mutants that always tumbled when they were normal sized always stopped when they were filament sized. Chemoreceptors in filaments were localized to regions both at the poles and at intervals along the filament. We suggest that the location of the chemoreceptors enables the chemotactic responses observed in filaments. The implications of this work with regard to the cytoplasmic diffusion of chemotaxis components in normal-sized and filamentous E. coli are discussed.

Chapter 29. Principles for multivalent ligand design

Abstract: Publisher Summary This chapter focuses on principles for multivalent ligand design. Multivalent interactions control a wide variety of cellular processes including cell surface recognition events. Examples of specific cell–cell binding events can be found in diverse processes, such as inflammation, tumor metastasis, and fertilization. An understanding of the mechanistic principles that underlie multivalent binding events facilitates the generation of new classes of therapeutic agents and biomaterials. Synthetic multivalent ligands can be used to illuminate and exploit biological processes that benefit from multipoint contacts. The use of antibodies can provide information about the involvement of multivalent interactions in a particular process. Understanding the structural requirements for multivalent ligand activity requires a wider variety of multivalent ligands whose structure, including size and valency, can be controlled and tailored. The chapter focuses on the principles for designing synthetic multivalent ligands and the interplay between ligand structure and biological activity. An overview of size and three dimensional shape is presented and flexibility of the Scaffold is discussed. Incorporation of ancillary functional groups is elaborated and design of multivalent scaffolds is also analyzed.

Contact Angle Goniometry as a Tool for Surface Tension Measurements of Solids, Using Zisman Plot Method A Physical Chemistry Experiment

Abstract: The paper describes a physical chemistry experiment for measurement of surface tension of solids. It includes description of the Zisman-plot method for obtaining the surface tensions of solids as well as a detailed experimental procedure for surface tension measurements using contact angle goniometry. Four surfaces were analyzed and the critical surface tension (gc) for each of them was obtained. Excellent reproducibility was achieved despite measurement of less accurate Young's contact angles. The experiment includes an extensive data analysis section combined with graphical interpretation of the results. It can be included in an undergraduate physical chemistry laboratory sequence on its own or combined with existing experiments pertaining to surface phenomena.

Visualization of Single Multivalent Receptor–Ligand Complexes by Transmission Electron Microscopy

Abstract: Multivalent ligands have the capacity to interact simultaneously with multiple receptors. The binding of a divalent ligand, for example, can bring two receptors together. Many natural and synthetic ligands are not divalent, however, but rather contain many possible receptor binding sites. The complexation of such multivalent ligands with multiple receptors may be important for their biological activities. Unfortunately, whether multiple receptors bind to a multi-valent ligand and how many associate in a complex often can only be inferred. The difficulties associated with investigating such molecular details are exacerbated by the lack of tools available to characterize these binding events. Previous efforts to study multivalent receptor ± ligand complexes have used light scattering, [6] fluorescence resonance energy transfer, [7±9] capillary electrophoresis, [10] or analytical ultracentrifugation. Transmission electron micros-copy (TEM) methods are complementary to these indirect measurements as they allow direct visualization of complexes. Another advantage of TEM experiments is that since single molecules or complexes can be viewed the amount of material needed is typically less than that required for other techniques. Prior applications of electron micros-copy to examine multiple receptors interacting with a ligand have focused on large complexes that can be imaged directly. Many important receptor ± ligand complexes, however, are too small or of insufficient density to be directly imaged in this way. Here we report a strategy that extends the range of receptor ± ligand complexes that can be imaged by TEM. We reasoned that a method that increased the contrast of receptors would allow visualization of receptors bound to a given ligand and facilitate the characterization of individual small receptor ± ligand complexes. Our solution employs colloidal gold particles as labels to monitor the receptor position in the presence of a ligand. Because of its density, colloidal gold allows high-contrast imaging by electron microscopy. This approach is routinely taken in immunohis-tochemical applications, in which cellular proteins are located by colloidal gold particles attached to antibodies. Proteins other than antibodies can be attached to colloidal gold, and streptavidin-conjugated gold particles of different sizes are readily available. Such particles are used commonly because CUO molecule and the Ar atoms of the model cages. We are examining these interactions carefully, especially in light of the recent report of the new stable argon molecule HArF. The potential energy surfaces (PES) along the bending coordinate are quite complicated for both the

A quantitative-trait locus controlling peripheral B-cell deficiency maps to mouse Chromosome 15

Abstract: Peripheral B-lymphocyte homeostasis is determined through incompletely defined positive and negative regulatory processes. The A/WySnJ mouse, but not the related A/J strain, has disturbed homeostasis leading to peripheral B-lymphocyte deficiency. B lymphopoeisis is normal in A/WySnJ mice, but the B cells apoptose rapidly in the periphery. This B cell-intrinsic defect segregated as a single locus, Bcmd, in (A/WySnJ×A/J)F2 mice. Here we mapped a quantitative-trait locus (QTL) that contributes to the A/WySnJ B-cell deficiency by examining the F2 progeny of a cross between strains A/WySnJ and CAST/Ei. In this cross, minimally 1.9 QTLs controlling peripheral B lymphocyte deficiency segregated. The (A/WySnJ×CAST/Ei)F2 mice were phenotyped for splenic B-cell percentage and the DNA from progeny with extreme phenotypes was used to map the QTL by the simple-sequence length polymorphism method. A genome scan showed linkage between peripheral B-cell deficiency and Chromosome (Chr) 15 markers. When closely spaced Chr 15 markers were analyzed, the 99% confidence interval for the QTL map position extended along the entire chromosome length. The peak lod scores >17 occurred between 30 and 45 cM. We conclude that a significant QTL segregating in (A/WySnJ×CAST/Ei)F2 mice resides in this middle region of Chr 15.