Interfacial receptors

Potentiometric sensing based on type 3 derivatives of a-, P-, and y-cyclodextrins has been reported.We have been interested in the type 1 cyclodextrin derivatives developed by Tagaki, e.g., host 41. This type of cyclodextrin derivative is characteristic in that the primary hydroxyl groups at the C-6 positions are exhaustively substituted with long alkyl chains. At the surface of an oiganic membrane, this structural feature allows the secondary hydroxyl side (wider open end of the cavity) to face the aqueous solution to accommodate a guest molecule. The interfacial receptor functions of these cyclodextrin hosts have been confirmed mainly by rt-A isotherm studies. 

In the absence of specific interactions of the receptor - ligand type the change in the Helmholtz free energy (AFadj due to the process of adsorption is AFads = yps - ypi - Ysi, where Yps, YPi and ys, are the protein-solid, protein-liquid and solid-liquid interfacial tensions, respectively [5], It is apparent from this equation that the free energy of adsorption of a protein onto a surface should depend not only of the surface tension of the adhering protein molecules and the substrate material but also on the surface tension of the suspending liquid. Two different situations are possible. 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

Typical theoretical concentration profiles, observed at a probe electrode, for the consumption of a receptor phase species in a first-order interfacial reaction are shown in Fig. 16. The simulation involved solving Eq. (30) with appropriate boundary conditions. 

The reaction was followed by the local measurement of chloride ions, at a potentio-metric Ag/AgCl microelectrode probe, positioned in the aqueous receptor phase, as DCE droplets containing TPMCl were grown (feeder phase). The reaction was shown unambiguously to occur interfacially, and was first-order in TPMCl with a hydrolysis rate constant of 6.5 x 10 cms. A typical concentration-distance profile determined in these experiments is shown in Fig. 18. 

The reaction rate between these interfacial electrons from the n-type semiconductor and receptor in solution (say, on an Fe3+ ion) will be given by an expression for the electrochemical rate of reaction by... 

Relatively few types of G proteins transduce the signals from a vast number of receptors. Each member of the heterotrimeric G protein family must, therefore, be capable of interacting with many different receptors. Additionally, many receptors are capable of activating multiple different G proteins. Although many interfacial contacts between these proteins have been described, the connections that define the coupling between receptors and specific G proteins remain unclear. 

Davis, J. J., Interfacial sensing surface assembled molecular receptors. Chem. Commun. 2005, 3509-3513. 

To observe a photocurrent, it is necessary to complete the light-activated reaction at the interface.3 Thus, e.g., there must be electron ejection from a p -type photocathode to suitable receptor levels in an Fe3+ ion in the interfacial region (see Fig. 10.6). 

We wished to develop a macroscopic model of the interactions between molecular ligands and receptors. Molecular recognition is a broad subject that describes selective assembly in chemistry and biology, with examples from DNA-protein complex formation to asymmetric catalysis. The principle behind molecular recognition dictates that the molecules that mate have complementary shapes and interfacial characteristics. Our extension of this principle to the mesoscale involved the self-assembly of objects that matched both... 

Each interface contains 11 or 12 hydrogen bonds or salt bridge interactions. Although the GH/GHR complex buries more surface area into its interface, it stiU contains only 12 specific interactions. Three of the IFN-7/ IFN-7RI interactions occur between mainchain atoms in the AB loop of IFN-7 and the L2 loop of IFN-7RI. In addition to the 12 receptor-cytokine interactions, seven water molecules bridge the donor and acceptor groups of IFN-7 and IFN-7RI via hydrogen bonds (Randal and Kossiakoff, 2001). Since the B-factors for the water and protein atoms in the complex are essentially the same, the water molecules are considered to be an integral part of the interface. The presence of ordered water molecules in the IFN-7/IFN-7RI interface is not likely unique, since the IL-lO/IL-lORl interface is even more polar. However, the poorer diffraction quality of IL-10 complex crystals ( 2.9A versus 2.0A) obtained at this time prevents a detailed description of the interfacial waters. The water molecules in the... 

In addition to receptor-type proteins, bilayer lipid membranes (BLMs) have been investigated for the detection of species of biochemical interest [221, 231,232]. The lipid film can be used alone, or chemical receptor agents can be incorporated into the membrane to enhance selectivity for inorganic ions or organic compounds/ions. Responses for BLM-coated devices are related to the mass loading of the analyte in/on the lipid film and to changes in interfacial conditions, e.g., elastic and viscous coupling effects [53,221-223]. 

To distinguish ligand v. protein atoms, we have classified the protein and ligand C, N, O and S as different atom types. Hydrogen atoms were not considered since, usually, they are not defined explicitly in X-ray structures. Thus, we have defined four atom types for receptor proteins and six atom types for the ligands. In total, there were 554 possible types of interfacial atomic quadruplet compositions, and each of them gave rise to an independent variable... 

Substantial efforts have been devoted to the development of molecular sensors for dopamine. Raymo et al.70 reported a two-step procedure to coat silica particles with fluorescent 2,7-diazapyrenium dications sensing toward dopamine. The analysis of the fluorescence decay with multiple-equilibria binding model revealed that the electron deficient dications and the electron-rich analytes form 1 1 and 1 2 complexes at the particle/water interface. The interfacial dissociation constants of the 1 1 complexes were 5.6mM and 3.6mM for dopamine and catechol, respectively. Dopamine was dominated by the interaction of its electron-rich dioxyarene fragment with the electron-deficient fluorophore in neutral aqueous environments. Ahn et al.71 reported tripodal oxazoline-based artificial receptors, capable of providing a preorganized hydrophobic environment by rational design, which mimics a hydrophobic pocket predicted for a human D2 receptor. A moderate binding affinity, a dissociation constant of 8.2 mM was obtained by NMR titrations of tripodal oxazoline-based artificial receptor with dopamine in a phosphate buffer solution (pH 7.0). Structurally related ammonium ions, norepinephrine, 2-phenylethylamine,... 

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