Theoretical Aspects of Molecular Recognition

Molecular recognition is a key process in non-covalent interactions, which determines, among others, host-guest complexation, drug action and protein-protein interaction. A simple and attractive formulation is the lock-and-key analogy defining the host as a lock accommodating the guest as a key. We stress three major aspects of molecular recognition, determining both complementarity between host and guest and similarity within a group of guest molecules. These aspects are: steric, i.e. maximization of close contacts, electrostatic, i.e. maximization of electrostatic attraction between host and guest, as well as hydrophobic, i.e. avoiding hydrophobic hydration, which can be reached by the maximization of apolar contacts between interacting molecules. Some examples are presented from our laboratory: the complexes of acylaminoacyl peptidase with small peptides, the effect of heparin binding on inhibitory potency of C1- inhibitor as well as small-molecule ligand binding to prolyl oligopeptidase and calmodulin

Reversible intercalation of methyl viologen as a dicationic charge carrier in aqueous batteries.

The interactions between charge carriers and electrode structures represent one of the most important considerations in the search for new energy storage devices. Currently, ionic bonding dominates the battery chemistry. Here we report the reversible insertion of a large molecular dication, methyl viologen, into the crystal structure of an aromatic solid electrode, 3,4,9,10-perylenetetracarboxylic dianhydride. This is the largest insertion charge carrier when non-solvated ever reported for batteries; surprisingly, the kinetic properties of the (de)insertion of methyl viologen are excellent with 60% of capacity retained when the current rate is increased from 100 mA g-1 to 2000 mA g-1. Characterization reveals that the insertion of methyl viologen causes phase transformation of the organic host, and embodies guest-host chemical bonding. First-principles density functional theory calculations suggest strong guest-host interaction beyond the pure ionic bonding, where a large extent of covalency may exist. This study extends the boundary of battery chemistry to large molecular ions as charge carriers and also highlights the electrochemical assembly of a supramolecular system

Spectroscopic Determination of Binding Constants of Modified β-cyclodextrins with Brooker’s Merocyanine

Host-guest complexation occurs when a guest molecule is held inside a host molecule through weak molecular forces. β-cyclodextrin, a host molecule, has a variety of uses and is often applied in pharmaceuticals. Binding constants of host-guest complexes of Brooker’s merocyanine and various modified β-cyclodextrins (2-hydroxypropyl-β-cyclodextrin, sulfated β-cyclodextrin, and methyl-β-cyclodextrin) were studied using fluorescence and UV-Vis spectrometry to determine the strength of interaction between the host molecule and the guest molecule. By modifying the β-cyclodextrin, the effect of ionic forces, hydrogen bonding, and steric hindrance were compared. It was determined via fluorescence spectrometry that sulfated β-cyclodextrin had a binding constant of 38.3 M-1, which was significantly lower than the determined binding constants 430 M-1 for β-cyclodextrin, 359.1 M-1 for 2-hydroxypropyl-β-cyclodextrin, and 194.6 M-1 for methyl-β-cyclodextrin. These results were confirmed via UV-Vis spectrometry, where the binding constants were 335.2 M-1 for 2-hydroxypropyl-β-cyclodextrin and 178.4 M-1 for methyl-β-cyclodextrin. These results were in agreement with fluorescence data. The much lower binding constant of sulfated β-cyclodextrin could be caused by a combination of ionic forces and steric hindrance. A comparison of these results to the theoretical models will lead to verification of the effect of these forces on binding. About the author: Carly Hanson is a junior Chemistry major and German minor

Guest Worker Programs: A Theoretical Analysis of Welfare of the Host and Source Countries

This paper examines the interaction between migration policies of the host and source countries in the context of a model of guest-worker migration. For the host, the objective is to provide low-cost labor for its employers while avoiding illegal immigration. It optimizes over these objectives by setting the time limit of a guest-worker permit. The source country seeks remittance flows and return migration by offering fiscal benefits to returnees. Within this framework, we solve for the Nash equilibrium values of the migration policy instruments and compare them, to the extent possible, with the ones that emerge in a cooperative setting.Temporary Migration, Remittances, Migration Policy

Host-Guest interaction between herbicide oxadiargyl and hydroxypropyl-β-cyclodextrin

In the face of a growing human population and increased urbanization, the demand for pesticides will simply rise. Farmers must escalate yields on increasingly fewer farm acres. However, the risks of pesticides, whether real or perceived, may force changes in the way these chemicals are used. Scientists are working toward pest control plans that are environmentally sound, effective, and profitable. In this context the development of new pesticide formulations which may improve application effectiveness, safety, handling, and storage can be pointed out as a solution. As a contribution to the area, the microencapsulation of the herbicide oxadiargyl (OXA) in (2-hydroxypropyl)-β-cyclodextrin (HP-β-CD) was performed. The study was conducted in different aqueous media (ultrapure water and in different pH buffer solutions). In all cases an increment of the oxadiargyl solubility as a function of the HP-β-CD concentration that has been related to the formation of an inclusion complex was verified. UV-Vis and NMR experiments allowed concluding that the stoichiometry of the OXA/HP-β-CD complex formed is 1 : 1. The gathered results can be regarded as an important step for its removal from industrial effluents and/or to increase the stabilizing action, encapsulation, and adsorption in water treatment plants

Temporary migration policies and welfare of the host and source countries : a game-theoretic approach

This paper examines the interaction between migration policies of the host and source countries in the context of a model of guest-worker migration. For the host, the objective is to provide low-cost labor for its employers while avoiding illegal immigration. It optimizes over these objectives by setting the time limit of a guest-worker permit. The source country seeks remittance flows and return migration by offering fiscal benefits to returnees. Within this framework, we solve for the Nash equilibrium values of the migration policy instruments and compare them with the ones that emerge in a cooperative setting

Polynuclear Cu4I4py4 complex loaded in mesoporous silica: Photophysics, theoretical investigation, and highly sensitive oxygen sensing application

The polynuclear Cu4I4py4 complex has been largely studied in solution and in the powder form due to its interesting luminescent properties, which are largely dependent on temperature and pressure. In this work, we present the synthesis of the complex and its wet impregnation in a mesoporous silica host obtained by sol-gel methodology. For optimized guest loadings, the well-dispersed guest molecules exhibit strong interaction with molecular oxygen, resulting in a significant quenching of the luminescence. The process is highly reversible with a Stern-Volmer constant of Ksv = 33.8, which is the largest value found in the literature for similar complexes in the solid state, suggesting that the new material is a promising candidate for high sensitivity oxygen sensing. Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations reveal a weak intermolecular interaction between two guest complexes in the excited state, suggesting the formation of an excited state complex (excimer). The assumption of triplet excimer formation is confirmed by temperature- and concentration-dependent experiments, which provides a new way to explain the giant Stokes shift observed for the guest complex in different media

Guest-host interaction of K0.34Co[Fe(CN)6]0.75·zH2O as investigated by a charge-density analysis

Charge-density level structure was determined in a prototypical nanoporous material with strong guest-host interaction, K0.34Co[Fe(CN)6]0.75·zH2O(z=3.6 and 2.7). We determined atomic coordinates and occupancies (g) of three kinds of oxygen sites of guest waters, i.e., zeolite water, ligand water, and off-axis water, as a function of z. The variation in g against z indicates easily removable nature of the zeolite and off-axis waters, and robust coordination bond with the ligand water. Significant change in the charge density of the host-NC-Fe-CN-Co-NC-Fe-CN-lattice revealed hybridization effect between the off-axis water and the adjacent Co atom

Hydrogen bond based noncovalent association in the semi-fluorous solvent perfluorobutyl-methyl ether: Host-host and host-guest association of the host 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluoro-decyl)-3-pyridin-2-yl-urea

A fluorous pyridyl-urea, 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-decyl)-3-pyridin-2-yl-urea, was prepared to act as a host and analyzed by 1H NMR inCD2Cl2 and perfluorobutyl-methyl ether (HFE7100). Crystals were analyzed by X-ray diffraction. The host molecules were found to form pillar-like structures in the crystal. There is an intramolecular bond between the pyridyl nitrogen and one urea hydrogen. 1H NMR spectra demonstrated that the urea hydrogens’ positions shift as the concentration of the host changes. The dependence of the shifts on concentration are consistent with the formation of a trimer of hosts with a logKeq for formation of trimer from monomer approximately 6. Association of the host with guests octanoic acid, ethyl acetate, N-ethylacetamide, N,N-dimethylacetamide, and acetone, was analyzed by titration of the host with individual guests in HFE7100 solvent. Downfield or upfield shifts of the urea hydrogens were used to indicate hydrogen bond formation with the guest. Acetone and ethyl acetate were unable to overcome the self-association of the host and form host-guest complexes. Octanoic acid binding caused shifts in the 1H NMR spectra of one hydrogen of the urea group. N-ethylacetamide and N,N-dimethylacetamide induced shifts in both urea hydrogens. The results indicate that the host monomer’s favored conformation contains an intramolecular hydrogen bond. This bond is not broken upon association with octanoic acid, but it is broken upon association with the two acetamides