Showing papers by "George M. Whitesides published in 1990"

Wet chemical approaches to the characterization of organic surfaces: self-assembled monolayers, wetting, and the physical-organic chemistry of the solid-liquid interface

Abstract: Etude des phenomenes de mouillage et d'adsorption d'une surface solide. Application a l'adsorption d'alcanethiols et de sulfures de dialkyl sur de l'or

X-Ray Specular Reflection Studies of Silicon Coated by Organic Monolayers (Alkylsiloxanes)

Abstract: X-ray specular reflectivity has been used to characterize the structure of silicon--silicon-oxide surfaces coated with chemisorbed hydrocarbon monolayer films (alkylsiloxanes). Using synchrotron radiation the reflectivity was followed over 9 orders of magnitude, from grazing incidence to an incident angle of \ensuremath{\varphi}\ensuremath{\approxeq}6.5\ifmmode^\circ\else\textdegree\fi{}, or q=(4\ensuremath{\pi}/\ensuremath{\lambda})sin(\ensuremath{\varphi})=0.8 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, allowing a spatial resolution of features approximately \ensuremath{\pi}/0.8\ensuremath{\approxeq}4.0 A\r{} along the surface normal. Analysis was performed by fitting the data to reflectivities calculated from models of the surface electron density and by calculating Patterson functions directly from the data. Although the measured reflectivities could be equally well described by different sets of model parameters, the electron densities calculated from these different parameters were remarkably alike. Inspection of the electron densities allowed identification of a layer of ${\mathrm{SiO}}_{2}$ (\ensuremath{\approxeq}17-A\r{} thick), a layer of head-group region where the alkylsiloxane adsorbs to the ${\mathrm{SiO}}_{2}$, and the hydrocarbon layer. Fitting the data also required that the various interfaces have different widths. The fact that the same local hydrocarbon density of 0.85 g/${\mathrm{cm}}^{3}$ was observed for both fully formed and partially formed monolayers with alkane chains of varying length excluded a model in which the partially formed monolayer was made up of separated islands of well-formed monolayers. Measurements before and after chemical reaction of a monolayer in which the alkyl chain was terminated by an olefinic group demonstrated the ability to use x-ray reflectivity to characterize chemical changes. The effects of radiation damage on these types of measurements are described.

The design of organic structures in the solid state: hydrogen-bonded molecular "tapes"

Abstract: We have begun a program whose objective is to develop methods to pred ic t the pack ing of organic molecu les in crys ta l l ine and noncrystal l ine sol ids, start ing from the atomic structures of these molecu les.2 The ab i l i ty to re la te molecu lar and crysta l l ine structure for organic solids will clarify the interactions that underlie molecular ecognit ion and self-assembly. simpli fy the preparation of opt ica l ly or e lect ron ica l ly act ive o iganic to i id r , ind 'he lp to rationalize the macroscopic properties of organic materials in terms of microscopic, molecular structures. One problem that has frustrated efforts to relate molecular and crystal l ine structures is the very large number of orientat ions the molecu les can, in pr inc ip le , adopt : ca lcu la t ing re la t ive energ ies of al l possible packing structures is currently impractical. To l imit the scope of the problem, we wish to constrain the possible orientat ions of the molecules in the sol id statc. Wc arc dcvcloping systems in which strong, direct ional hydrogen bonds provide the requi red const ra in t . Using the network proposed for melamine/cyanuric acid as a model , l 'a we are examin ing the s t ructurcs o f l : l cocrysta ls o f der ivat ives o f melamine (M) and barb i tur ic ac id (B) , funct iona l ized in pat terns that break up the sheet s t ructure but permi t the fbrmat ion o l 'h idrogen bonds that y ic ld \" tapes\" (eq l ) . Tapes are l ike ly to pack wi th the i r axes para l le l . Th is enforced para l le l ism wi l l , we be l ieve, s ign i f icant ly s impl i fy the computat iona l analys is o f these so l id-s ta te s t ructures.

Self-assembly based on the cyanuric acid-melamine lattice

Abstract: React ion o f cyanur ic ac id (CA) wi th melamine (M) forms a stab le , inso lub le l : l complex CA.M.3 We, and others , be l ieve that CA.M has a local structure represented by the lattice l.a We have started a program in the design and synthesis of three-dimensional supramolecu lar assembl ies based on the hydrogenbonding pat tern o f CA.M. Th is program is an extens ion of the strategy of molecular self-assembly that has been highly successful in forming quasi-two-dimensional monolayers on sol id supports.5

Predicting the stability of cyclic disulfides by molecular modeling: effective concentrations in thiol-disulfide interchange and the design of strongly reducing dithiols

Abstract: Relation entre la structure et les constantes d'equilibre des reactions d'echange thiol-disulfure

Comparisons of rate constants for thiolate-disulfide interchange in water and in polar aprotic solvents using dynamic proton NMR line shape analysis

Abstract: The rate constants for representative thiolate-disulii de rcactions are larger in DMSO and DMF than in water b1 a iactor of approximately 2300 at 24 oC. The log of the rate constant is directly proportional to the mole fraction ol DrO in mixtures of DMSO and D2O, even at small mole fractions of D2O. This linear proportionality suggests that thiolate anion is not specifically solvated by water and that hydrogen bonding is relatively unimportant in stabilizing this species. The valucs of -\Sr for thiolate-disulfide interchange are approximately -10 cal/(deg mol), presumptively because of loss in the entropy of the reactants in going from ground to transition state, partially compensated by a gain in entropy from solvent release. Introduction of a hydroxyl group p to the C-S bond slows the reaction by a factor of 2-15; the introduction of methyl groups p to the C-S bond slows the rate by factors of 3-20. A number of substances have been screened as potential catalysts ior thiolate-disulfide interchange in water: none showed useful levels of catalytic activity, although phenylselenol did accelerate the interchange significantly.

Degenerate intermolecular thiolate-disulfide interchange involving cyclic five-membered disulfides is faster by .apprx.103 than that involving six- or seven-membered disulfides

Abstract: The rate constants for degenerate intermolecular thiolate-disulfide interchange involving 1,2-dithiolane (S(CH2)35) are higher than those involving 1,2-dithiane (m) by a factor of -650 in mixtures of DMSO-d6 and D2O. The extrapolated rate constant for 1,2-dithiolane in DMSO-d6 is fast (k - 108 M-l s-l). The rate constants for cyclic six- and seven-membered disulfides are similar to those for acyclic disulfides. Rate constants for self-exchange were measured by dynamic rH NMR line-shape analysis. The evolutionary selection of lipoamide as the cofactor in 2-oxo acid dehydrogcnases may reflect the fast rate of ring opening of the dithiolane ring by nucleophiles.

A new approach to cyclitols based on rabbit muscle aldolase (RAMA)

Abstract: The synthes is o f cyc l i to ls hav ing wel l -def ined srereochemist ry start ing with nonchiral precursors is a current chal lenge in organic synthes is . Most synthet ic s t ra teg ies e i ther s tar t f rom ch i ra l precursors (e.g., carbohydrates),1-5 or resolve the racemic adduct formed in a Die ls-A lder react ion.6 '7 Here we repor t the appl i ca t i on o i r abb i t musc le a l do lase (RAMA; EC 4 .1 .2 .13 ) r o t he preparat ion o f cyc l i to ls and C-g lycos ides (Scheme l ) . RAMA catalyzes the aldol condensation of dihydroxyacetone phosphate (DHAP) and a ldehydes and forms products wi th the

What Will Chemistry Do in the Next Twenty Years

Abstract: The path of chemistry in the future will be determined both by its participation in solving large-scale societal problems and by its generation of new ideas through basic research. This article sketches four of the areas of societal “pull” in which chemistry will play a role in solving applied problems—national security, health care, the environment, and energy—and four areas in which basic research will be especially fruitful—materials chemistry, biological chemistry, computational chemistry, and chemistry exploring the limits of size and speed in chemical phenomena.

Preparation of a mixture of nucleoside triphosphates suitable for use in synthesis of nucleotide phosphate sugars from ribonucleic acid using nuclease P1, a mixture of nucleoside monophosphokinases and acetate kinase.

Abstract: This paper (1) describes the enzymatic syntheses of a mixture of adenosine, guanosine, cytidine, and uridine triphosphates (ATP, GTP, CTP, and UTP) from ribonucleic acid (RNA). RNA was hydrolyzed by nuclease P1 to a mixture of 5′-nucleoside monophosphates. This mixture was converted to the nucleoside triphosphates using a mixture of nucleoside monophosphate kinases and acetate kinase, with acetyl phosphate as the ultimate phosphoryl donor. The nucleoside monophosphokinases were extracted from brewer’s yeast in a four-step procedure. The specific activity of the yeast enzyme preparation after gel permeation chromatography was sufficiently high that the yeast kinases could be immobilized in volumes that were practical for laboratory scale syntheses. Conversions from NMP to NTP in a mixture containing 0.34 mol of total nucleoside phosphates were: ATP, 90%; GTP, 90%; CTP, 60%; and UTP, 40%.

Enzyme-catalyzed organic synthesis of sucrose and trehalose with in situ regeneration of UDP-glucose.

Abstract: This paper describes cell-free enzymatic syntheses of sucrose and trehalose using partially-purified preparations of sucrose and trehalose synthetase. The coupling of the regeneration of uridine-5'-diphosphoglucose (UDP-Glc) with synthesis of the disaccharide offers a practical route to millimol quantities of these carbohydrates. The syntheses used pyruvate kinase, UDP-Glc pyrophosphorylase, and inorganic pyrophosphatase, and the regenerated UDP-Glc was cycled approximately 10 times.

Size of alkyl group R: principal factor determining wettability of surface-functionalized polyethylenes (PE-CONHR and PE-CO2R) by water

Abstract: We have examined the wetting by water of polyethylene films having surfaces containing amide (PE-CONHR) or ester (PE-CO 2 R) functionalities and have determined the size of a nonpolar group R required to mask these small polar groups from contact with the water. This masking doubtless reflects weakening of electrostatic and hydrogen-bonding interactions between the contacting water and the polar functional group

Enzymes as Catalysts in Carbohydrate Synthesis

Abstract: The field of carbohydrate chemistry is perhaps one of the most thoroughly investigated of all disciplines of chemistry. Thousands of investigators, from the time of Emil Fischer, have probed the structure, function and reactivity of mono-, oligo-, and polysaccharides. Nevertheless, carbohydrate chemistry and biology are currently undergoing a major renaissance. In addition to their well known roles as energy storage vehicles and structural components of cells, carbohydrates are now recognized as vital components in biological recognition phenomena.1–3 The involvement of carbohydrates has been implicated in many biological processes, including cell-cell recognition in growth and differentiation, clearance of aging cells from circulation, and cell sorting and targeting. Cell-surface carbohydrates are specific attachment sites for certain pathogenic viruses, bacteria, and parasites, and for several soluble bacterial toxins.4–7 With the realization of these new roles has come a renewed demand for improved synthetic routes to carbohydrates.

Wohin geht die Chemie in den nächsten zwanzig Jahren

Abstract: Der zukunftige Weg der Chemie wird sowohl von Forderungen der Gesellschaft als auch von der Entwicklung neuer Ideen durch die Grundlagenforschung gepragt werden Dieser Aufsatz skizziert vier Gesellschaftsbereiche, in denen die Chemie zur Losung von Problemen gefordert sein wird - nationale Sicherheit, Gesundheitsfursorge, Umweltschutz und Energieversorgung -, sowie vier Bereiche, in denen die Grundlagenforschung wichtige Impulse geben wird - Materialforschung, Biochemie, Computerchemie und die Erforschung der Grenzen von Grose und Geschwindigkeit chemischer Phanomene

Predicting the Stability of Cyclic Disulfides by Molecular Modeling: “Effective Concentrations” in Thiol-Disulfide Interchange and the Design of Strongly Reducing Dithiols

Abstract: Relation entre la structure et les constantes d'equilibre des reactions d'echange thiol-disulfure

Hydrophobically modified proteins

Abstract: This invention relates to the production of oil-in-water emulsifiers and emulsion stabilizers. More particularly, this invention relates to methods for the production of novel hydrophobically modified proteins which may be used as emulsifiers or to stabilize emulsions. This invention also relates to products incorporating these hydrophobically modified protein emulsifiers and emulsion stabilizers. These compounds may be used in many applications such as paints, dyes and cosmetics.

Effective Molarity in Diffusion-Controlled Reactions

Abstract: Effective molarities for diffusion-controlled reactions between spherical reactants with reactive patches are calculated analytically and by Brownian dynamics simulations. Unimolecular reaction systems with internal translational motion in one, two, and three dimensions are investigated and compared with bimolecular reactions in three dimensions. Rotational diffusion is included in all cases in which a reactant particle is anisotropically reactive

Quantifying acid-base properties of organic functional groups at a polyethylene-water interface by photoacoustic calorimetry

Abstract: This paper describes the use of photoacoustic calorimetry (PAC)r to characterize acid-base equilibria involving organic chromophores covalently attached at the interphase between surface-functionalized polyethylene (PE) film2'3 and water. We use proton transfer reactions to define the polarity' and solvating capability of the interphase between organic surfaces and water.a Characterizing these physical-organic properties of interphases is difficult: light scattering at interfaces (especially rctugh interfaces) complicates many UV-vis absorption and fluorescence methods; infrared spectroscopy cannot be applied routinely to systems involving water; thermometric and conductometric methods are insensitive for solids having low surface areas. The measurement of the contact angle of buffered water on surfaces as a function of pH-\"contact angle titration\"-yields valuable information about acid-base equilibria,3 but depends on unverified assumptions.5

Molecular cloning of aldolases for synthetic applications.

Abstract: Aldolases catalyze the formation of carbon-carbon bonds in monosaccharides, as exemplified by fructose1,6-biphosphate (FBP), L-fuculose-I-phosphate, and L-rhamnulose1 -phosphate aldolases, which catalyze the formation of six-carbon sugars with complementary stereochemistries (FIGURE 1). FBP aldolase from rabbit muscle has been applied in the synthesis of several chiral products.' This enzyme accepts a variety of aldehydes in place of its natural substrate glyceraldehyde-3-phosphate while the stereospecificity of the aldol condensation is conserved, thus increasing the significance of this enzyme as a catalyst in organic synthesis.

Preparation of Phosphoenolpyruvate from D-(-)-3-Phosphoglyceric Acid for Use in Regeneration of ATP.

Abstract: Description du protocole experimental permettant la synthese de phosphoenolpyruvate a partir d'acide phosphoglycerique