Single Guest Research Articles

Host‐Assisted Guest Self‐Assembly: Enhancement of the Dimerization of Pyronines Y and B by γ‐Cyclodextrin

Buckle up! The dimerization of small fluorescent guests is strongly enhanced in presence of a cyclodextrin host. The host cavity acts like a belt to assist the self-assembly of guests (see picture). Small variations in the guest structure have significant influence on the stability and geometry of the aggregates.The role of small variations in the structural properties of host and guest molecules on the stoichiometry and strength of supramolecular associations is analyzed. Earlier we found that a change in substituents from pyronine B to pyronine Y has a dramatic effect on both the stability and the dynamics of the association of these guests with beta-cyclodextrin as host. Now we study the association between these two pyronines and a cyclodextrin with a bigger cavity (gamma-cyclodextrin) using UV/Vis absorption and fluorescence spectroscopy. The absorption spectra of the pyronines show complex variations with cyclodextrin concentration indicating that pyronine dimerization is strongly enhanced inside the cavity of the cyclodextrin. A full model is proposed and the equilibrium constants of the involved processes and the absorption and emission spectra of the different species are estimated. The equilibrium constants of the formation of complexed dimers are much higher than those for free dimerization or for the inclusion of a single guest. The gamma-cyclodextrin host acts like a belt to assist the guest self-assembly. The differences in the stability of pyronine B and pyronine Y dimers are explained on the basis of their structure and geometry.

Cyclodextrin-Based Hyperbranched Polymers: Molecule Design, Synthesis, and Characterization

The cyclodextrin (CD) molecule, a host for a variety of smaller molecular guests, and hyperbranched polymers both possess molecule cavities in their molecular architectures. If the hyperbranched poly(β-cyclodextrin)s are established using modified β-CD molecules as monomers, their molecular inclusion capabilities for smaller molecular guests may be enhanced due to the combination of two different molecule cavities from both their hyperbranched topography architecture and CD molecule chain segments. Herein, three types of hyperbranched poly(β-cyclodextrin)s, i.e., HBP-ABx from ABx-type β-CD monomers, HBP-(By + ABx) from By core molecules and ABx-type β-CD monomers, and water-soluble HBP-AB2 from AB2-type β-CD monomers, were synthesized via hydrosilylation reaction under the thermal or ultraviolet activated polymerization. The hyperbranched structures of resultant poly(β-cyclodextrin)s were characterized using 1H NMR, 13C NMR, 29Si NMR, 1H−29Si heteronuclear multiple bond correlation, and size exclusion chromatography/multiangle laser light scattering (SEC/MALLS). The molecule inclusion and recognition behaviors of the novel hyperbranched poly(β-cyclodextrin)s as well as their macromolecular structures, degree of branching, and thermal properties were investigated in detail. UV−vis spectroscopy results show that HBP-AB2 has molecular inclusion capabilities and also can form inclusion complexation with single or double guests, including phenolphthalein (PP) and methyl orange (MO). Compared with AB2-type β-CD monomer, the molecular inclusion capability of the hyperbranched polymer was enhanced. Furthermore, HBP-AB2 presents molecular recognition behavior when MO solution is added into their PP solution.

Female Mathematicians as Role Models for All Students

Female Mathematicians as Role Models for All Students Lynda R. Wiest (bio) Girls' and women's dispositions, performance, and participation in mathematics have received significant attention in recent decades. Nevertheless, females still perform below males on the mathematics portion of standardized tests, such as the Scholastic Assessment Test (SAT) (Institute of Education Sciences), and they attain fewer mathematics degrees than males at all higher-education levels (Institute of Education Sciences). Further, they account for only one-fourth of workers classified into "Computer and Mathematical Occupations" (Bureau of Labor Statistics, "Occupational"), the third-highest paying of the U.S. Bureau of Labor Statistics' twenty-two job categories (Highlights). One way to address these lingering imbalances is to provide female role models in mathematics for both girls and boys. In keeping with the Equity Principle promoted by the National Council of Teachers of Mathematics, it is important for teachers to infuse information about historical and contemporary female mathematicians into mathematics instruction. Leah McCoy presents activities for having young women work with successful historical female figures in mathematics. She notes, "Because female students are not aware of female mathematicians and scientists, they may internalize a belief that mathematics is not appropriate for women" (125). In this paper, I share broad ideas and extensive resources for supporting secondary teachers—as well as college mathematics instructors to a large degree—in this effort. These foundational approaches and resources are intended to serve as a launching point for more specific, teacher-selected and -developed activities. In addition to the strategies shared below, a valuable class discussion can focus on reasons why female mathematicians were few in number compared with males, as well as why they have been given scant attention in many historical accounts of the development of mathematics. (See Kelley in the resource list, in addition to the stories of individual women within other entries on the list; also consult Bossé and Hurd, and Kunoff.) Both females and males should understand familial, societal, and institutional barriers so that they do not perceive women's absence as a lack of ability. [End Page 162] Strategies for Including Information About Female Mathematicians Numerous approaches may be used to include information about female mathematicians in the mathematics classroom. Resources provided at the end of this paper can help teachers implement many of these suggestions. Invite guest speakers to address the class. Women and men in mathematics-related careers can share their thoughts and experiences regarding reasons they entered the field, obstacles they overcame in relation to their career path, the type of work they do on a daily basis, practical information (e.g., about pay and benefits), and positive and negative aspects of their jobs. Presentations can involve various individuals over time or a single guest panel and should allow time for pertinent questions from the class. Show films about female mathematicians. Films that profile females in mathematics related careers can be shown in class. Existing commercial films are both rare and somewhat dated (see resource list). An alternative is to have students make their own film of local female mathematicians, or an even mix of males and females, by filming them at work and/or during interviews. This would also make a nice class project for a technology class or club to produce for mathematics classroom use. Have students study one or more mathematicians and share information learned. Names of historical and/or contemporary female and male mathematicians can be randomly distributed to students to research. Another option is to have students interview local individuals who work in mathematics-related careers. Selected methods of disseminating summarized information include written or oral reports, class newsletters, bound booklets, children's books, web sites, or interpretive performances in character. Audiences might include other classes of various ages, parents, or community groups. Infuse information about women mathematicians and their work into relevant course content. Contributions of female mathematicians can be woven naturally into pertinent course topics. In algebra, for example, students may learn or investigate why Emmy Noether is considered to be the founder of modern abstract algebra. Students might also engage in activities related to these mathematicians' work (for ideas, see the two Perl entries in the resource list). Create...

Optical switching studies of an azobenzene rigidly linked to a hexa-peri-hexabenzocoronene derivative in solution and at a solid–liquid interface

An azobenzene moiety rigidly linked to a hexa-peri-hexabenzocoronene (HBC) derivative has been switched optically between its trans- and cis-conformations in solution. Crystalline monolayers of the flat lying trans-conformer have been obtained at the interface between the basal plane of graphite and an organic solution. However, from the illuminated solution no cis-conformer was observed adsorbed to the interface, indicating that the resulting cis- conformer is not thermodynamically stable at this interface, possibly due to the competition with the coexisting trans-conformers. Therefore, two-dimensional honeycombs self-assembled from derivatives of three-fold symmetric polycyclic aromatic hydrocarbons were investigated, which may be employed as templates, providing the necessary space for conformational switching of an azobenzene moiety, and tune the current–voltage characteristics through the aromatic cores. As a first step towards this goal, the capability of the template to host single molecular guests was studied by filling the voids with individual coronene molecules.

Computing the Heat of Adsorption using Molecular Simulations: The Effect of Strong Coulombic Interactions.

Molecular simulations are an important tool for the study of adsorption of hydrocarbons in nanoporous materials such as zeolites. The heat of adsorption is an important thermodynamic quantity that can be measured both in experiments and molecular simulations, and therefore it is often used to investigate the quality of a force field for a certain guest-host (g - h) system. In molecular simulations, the heat of adsorption in zeolites is often computed using either of the following methods: (1) using the Clausius-Clapeyron equation, which requires the partial derivative of the pressure with respect to temperature at constant loading, (2) using the energy difference between the host with and without a single guest molecule present, and (3) from energy/particle fluctuations in the grand-canonical ensemble. To calculate the heat of adsorption from experiments (besides direct calorimetry), only the first method is usually applicable. Although the computation of the heat of adsorption is straightforward for all-silica zeolites, severe difficulties arise when applying the conventional methods to systems with nonframework cations present. The reason for this is that these nonframework cations have very strong Coulombic interactions with the zeolite. We will present an alternative method based on biased interactions of guest molecules that suffers less from these difficulties. This method requires only a single simulation of the host structure. In addition, we will review some of the other important issues concerning the handling of these strong Coulombic interactions in simulating the adsorption of guest molecules. It turns out that the recently proposed Wolf method ( J. Chem. Phys. 1999, 110 , 8254 ) performs poorly for zeolites as a large cutoff radius is needed for convergence.

Accommodation of a Single Protein Guest in Nanometer‐Scale Wells Embedded in a “DNA Nanotape”

Ein bandförmiges DNA-Aggregat mit regelmäßig angeordneten Vertiefungen wird durch Bündelung von neun DNA-Helices zu U-förmigen Strukturmotiven erhalten (siehe Bild). Nach Anbringen von zwei Biotinresten an zwei Kanten jeder Vertiefung werden Streptavidintetramere größenselektiv gebunden, sodass eine regelmäßige Anordnung entsteht. Die „verankerten“ Streptavidintetramere sind unter dem Rasterkraftmikroskop stabil. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2008/z800028_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Accommodation of a Single Protein Guest in Nanometer-Scale Wells Embedded in a “DNA Nanotape”

All's well: A tapelike DNA scaffold with regularly arranged nanometer-scale wells is prepared by bundling nine DNA helices into U-shaped motifs (see picture). By attaching two biotin residues at two edges of each well, just one streptavidin tetramer is size-selectively captured to form a regularly oriented streptavidin nanoarray. The “anchored” tetramers nested in the wells are stable under repetitive AFM scanning. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z800028_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Low-Temperature Spectral Dynamics of Single TDI Molecules in n-Alkane Matrixes

We report on studies of the influence of the matrix on the spectral dynamics of the zero-phonon-line (ZPL) emission by means of single molecule spectroscopy at low temperature. The host-guest system combinations consist of terrylenediimide (TDI) molecules embedded in four n-alkane matrixes of hexane, heptane, pentadecane, and hexadecane. Excitations into the broad vibronic absorption band and spectrally dispersed detection allows us to monitor fluorescence of single TDI molecules as a function of time. In the case of long-chain n-alkanes (pentadecane and hexadecane), the ZPL line is quite stable, showing spectral jumps of moderate frequency of less than 10 cm(-1) with an average time between the jumps of 10 s. In a clear contrast, the spectral dynamics of TDI molecules embedded within the short-length n-alkane matrixes (heptane and hexane) feature much more frequent spectral jumps that occur on a broader energy scale. The results suggest that matrixes composed of short-chain molecules are more susceptible to translations and/or rotations, which influence the fluorescence of single guest chromophores.

Giant molecular spoked wheels in giant voids: two-dimensional molecular self-assembly goes big

We present here the formation of giant pores in surface-confined molecular networks of a triangular-shaped dehydrobenzo-[12]annulene derivative: the diameter of the pores reaches over 7 nm and the giant pores are used as templates to accommodate a giant molecular spoked wheel, which allows us to observe rotational and adsorption-desorption dynamics of single guest molecules.

On the thermodynamic stability of hydrogen clathrate hydrates

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.

How much carbon dioxide can be stored in the structure H clathrate hydrates?: A molecular dynamics study

The stability of structure H (sH) carbon dioxide clathrate hydrates at three temperature-pressure conditions are determined by molecular dynamics simulations on a 3x3x3 sH unit cell replica. Simulations are performed at 100 K at ambient pressure, 273 K at 100 bars and also 300 K and 5.0 kbars. The small and medium cages of the sH unit cell are occupied by a single carbon dioxide guest and large cage guest occupancies of 1-5 are considered. Radial distribution functions are given for guests in the large cages and unit cell volumes and configurational energies are studied as a function of large cage CO(2) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at three temperature/pressure conditions stated above. At the low temperature, large cage occupancy of 5 is the most stable while at the higher temperature, the occupancy of 3 is the most favored. Calculations are also performed to show that the CO(2) sH clathrate is more stable than the methane clathrate analog. Implications on CO(2) sequestration by clathrate formation are discussed.

Clathrates with mixed guests

Investigation of the selectivities of two diol organic hosts for pairs of small organic guests gave interesting results which could be correlated with the crystal structures of the inclusion compounds containing mixtures of the two guests; these structures were compared with those of the single guest inclusion compounds.

Nano-dimensions for the pyrogallol[4]arene cavity

The cup-like cavity of pyrogallol[4]arenes has been deepened by the addition of four hydrogen bonded bipyridine molecules to the upper-rim of the calixarene, enabling the extended cup-like molecules to stack inside one another and consequently trap and completely enshroud a single guest molecule within the 250 angstroms3 cavitand formed between two of these stacked "nano-cups".

Dendrimers as Guests in Molecular Recognition Phenomena

AbstractFor Abstract see ChemInform Abstract in Full Text.

Helical folding of alkanes in a self-assembled, cylindrical capsule.

The reversible encapsulation of a series of normal alkane guests in a cylindrical host was studied by NMR methods. For small hydrocarbons such as n-pentane or n-hexane, two guests enter the host, and they move freely within. With n-heptane no encapsulation takes place. For longer alkanes such as n-decane, a single guest enters and the aromatic walls of the host are seen to twist to avoid empty spaces and increase favorable interactions with the hydrocarbon. The best guest (n-undecane) adopts a conformation with minimal gauche interactions. The longest alkane accommodated, n-tetradecane, adopts a helical conformation to fit in the cavity, a shape that maximizes CH/pi interactions with the aromatic walls of the receptor. These reciprocal conformational changes are discussed in terms of optimal host/guest interactions.

Simple predictive relations, fugacities, and enthalpies of dissociation for single guest clathrate hydrates in porous media

We present two sets of explicit relations for the equilibrium fugacities of single-guest gas hydrates. These relations can be applied whether the hydrate is dissociated under bulk conditions, or in porous media. The first set of presented relations explicitly shows the dependence of the equilibrium fugacity and the enthalpy of dissociation on classical statistical thermodynamic parameters. The second set of relations for the fugacity and enthalpy represent a very simple empirical form which can be used to calculate these quantities, without having to resort to the use of the full statistical thermodynamic model. It is hoped that these relations will prove useful in engineering or computational endeavors where the speed and/or ease of their use may be advantageous.

Short sequences of non-proline residues can adopt the polyproline II helical conformation.

The left-handed polyproline II (P(II)) helix is a structure that has been given a great deal of attention lately because of its role in a wide variety of physiologically important processes and potential significance in protein unfolded states. Recent work by several authors has shown that residues besides proline can adopt this structure. A scale of relative P(II)-helix-forming propensities has been generated but only for single guest residues in a proline-based host system. Here, we present multiple guest residues in a proline-based host system. Using circular dichroism spectroscopy, we have shown that not only single residues, but also short sequences of non-proline residues can adopt the P(II) conformation.

Reversible Enantiofacial Differentiation of a Single Heterocyclic Substrate by Supramolecular Receptors

Telling top and bottom apart: Supramolecular receptors can discriminate between the “top” and “bottom” sides of a single guest molecule, such as caffeine (see picture). This reversible enantiofacial differentiation results in the preferential formation of the α or β complexes.

The inorganic chemistry of guest-mediated zeolite crystallization: a comparison of the use of boron and aluminum as lattice-substituting components in the presence of a single guest molecule during zeolite synthesis

A comparison of two zeolite systems was made with trivalent element substitution for the use of N-benzyl-DABCO as the structure-directing agent (SDA). Boron and aluminum were used at the same initial concentrations. One reason for the study was awareness that a higher number of new zeolite structures are being found when researching borosilicate reaction mixtures. The borosilicate system studied produces SSZ-42 (IFR) while the aluminosilicate system yields beta zeolite (BEA). We focused on comparisons of the kinetics and found that the borosilicate system crystallized more rapidly, kept most of the boron in solution before incorporation into the crystal, and added mass even at the point where all solids were crystalline. In contrast, the aluminosilicate gel remained almost entirely in the solid phase through a long induction period and the mass of solids was relatively constant. The kinetics were followed by using a variety of techniques including powder XRD, pH changes, elemental analyses, partitioning in centrifugation and effects of dilution. The reaction systems were followed via NMR techniques. The transformation to crystalline could be seen by NMR for both the aluminosilicate and borosilicate reactions. Proton NMR changes were followed for both sets of solids. In addition, a profile for removal of boron from solution and towards the growing SSZ-42 was quantified by NMR. An unusual tetrahedral doublet for 11B was found in the SSZ-42 product. No such doublet was observed for tetrahedral aluminum in the case of beta zeolite formation.

Resonant oscillation modes of sympathetically cooled ions in a radio-frequency trap

Sympathetic cooling of ${\mathrm{Ca}}^{+},$ ${\mathrm{Zn}}^{+},$ ${\mathrm{Sr}}^{+},$ ${\mathrm{Ba}}^{+},$ and ${\mathrm{Yb}}^{+}$ as guest ions with laser-cooled ${}^{24}{\mathrm{Mg}}^{+}$ as host ions in a rf ion trap is carried out, and resonant frequencies of their motion in the trap potential are measured. Various oscillation modes of the sympathetically cooled ions are observed. The resonant frequency of the oscillation mode is different from the frequency of either the collective oscillation frequency of the trapped ions or the oscillation frequency of each ion without host ions. This difference is well explained by a theoretical model in which coupled equations of motion of the host ion cloud with a single guest ion are considered.