Solid-Supported ITIES

For applications that require a lipophilic bonded phase, Cg and C g phases may be used interchangeably. ITie best ratio of sample to solid support-bonded phase is 1 4. However, this ratio is dependent on the application. Lower ratios... 

In the 1980s, Julia and Colonna discovered that the Weitz-Scheffer epoxidation of enones such as chalcone (4, Scheme 2) by alkaline hydrogen peroxide is catalyzed in a highly enantioselective fashion by poly-amino acids such as poly-alanine or poly-leucine (Julia et al. 1980, 1982). The poly-amino acids used for the Julia-Colonna epoxidation are statistical mixtures, the maximum length distribution being around 20-25 mers (Roberts et al. 1997). The most fundamental question to be addressed refers to the minimal structural element (i.e. the minimal peptide length) required for catalytic activity and enantioselectiv-ity. To tackle this question, we have synthesized the whole series of L-leucine oligomers from 1- to 20-mer on a solid support (Berkessel... 

The packing, or solid support in a packed column, holds the liquid stationary phase in place so ihai as large a surface area as possible is exposed to the mobile phase. ITie ideal support consists of small, uniform, spherical particles with g(K)d mechanical strength and a specilic... 

Fig. 9 SECM operation inodes with pipet-supported ITIES tips. See the text for the description of each operation mode. Mutually immiscible electrolyte solutions are separated by solid hnes. A conductive substrate is shown in gray. The white parts are the pipet wall.

FIGURE18.4 The permeability of a pnc-Si membrane, k, plotted against the diffusion coefficients of probe ions in the aqueous phase, The solid line represents the best fit of Equation 18.2 for monovalent ions. The permeability to Arixtra and protamine was measured with 0.10, 0.03, and 0.01 M phosphate buffer at pH 7.0 while the buffer concentration was 0.10 M for monovalent probe ions. (Adapted with permission from Ishimatsu, R., Kim, J., Jing, R et al., Ion-selective permeability of a ultrathin nanopore silicon membrane as probed by scanning electrochemical microscopy using micropipet-supported ITIES tips. Anal. Chem., 82, 7127-7134, 2010. Copyright 2010 American Chemical Society.)... 

Starting from amides on solid supports one could cleave to get fimctional-ities hke carboxylic adds 145, esters 144, primary, secondary, tertiary amides (147,148) and other targets 146 obtainable via nucleophilic substitution. Car-boxyhc acids can be obtained by treatment of the resin with strong bases like aq. NaOH or alternatively by addition of enzymes imder mild, neutral and aqueous conditions (Scheme 23) [177]. 

Attempts to develop supported ionic fluorides and for nucleophilic fluorina-tions have been largely unsuccessful but have led to the discovery of remarkably useful solid bases with some recent applications showing high regioselectiv-ity 21,133 134 Metal fluorides such as KF are known to be weak bases and their dispersion over a support leads to a remarkable enhancement in their basicity. The surface chemistry between the adsorbed salt and the support material is quite complex and oxide and hydroxide sites are likely to be formed and to contribute to the basic properties. 

Similar to solid/liquid electrochemistry, important advantages can be obtained by replacing a macroscopic interface between two immiscible electrolyte solutions (ITIES) with a liquid/liquid nanointerface. In 1986, Taylor and Girault introduced micrometer-sized liquid/liquid interface supported at the tip of a pulled glass pipette. Nanoscale ITIES and their arrays have later been formed by using nanopipettes, nanopores, and porous membranes. Electrochemistry of nanopores and porous membranes is surveyed in Chapter 11 our focus here is on the ITIES supported at the tip of a nanometer-sized pipette. 

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