Trapping small molecules

Polymer-based adsorbents are not widely used in the HPLC analysis of small molecules, mainly because of the presence of micropores in the structure of polymer resins [29]. These micropores may trap small molecules, and their relatively slow diffusion leads to signihcant band broadening and overall loss of separation efficiency. 

Molecular sieves, acting as a cosolvent, have recently been shown to greatly improve the stereoselectivity in some titanium - catalyzed reactions (22). Usually the zeolite selectively traps small molecules like water or alcohol to avoid the destruction of the catalyst or to form the chiral catalyst more efficiently. 

Using a stable dopant as the emissive dye has been shown to greatly enhance the lifetime of small molecule LEDs. Rubrene doped into the Alq, electron transport layer ] 184] or into the TPD hole transport layer 1185] can extend the lifetime by an order of magnitude. Similarly, dimclhylquinacridone in Alq has a beneficial effect ]45 ]. The likely mechanism responsible for this phenomenon is that the dopant acts as a trap for the excilon and/or the charge. Thus, molecules of the host maLrix are in their excited (cationic, anionic or cxcitonic) states for a smaller fraction of the time, and therefore have lower probability to undergo chemistry. 

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

If hot sulfur melts or hot sulfur vapors at low pressure are frozen at low temperatures highly colored samples are obtained which may be black, green or red depending on the temperature and pressure conditions and on the rate of quenching [69]. These colors originate from the small molecules and radicals, present at high temperatures, which become trapped in the solid sample. At room temperature these samples turn yellow, provided the sulfur has been very pure. 

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The first kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

SMALL MOLECULE PRODUCTS. The small molecule products from irradiation of polymers include hydrogen, alkanes and alkenes, CO and CO2, SO2, H2O and HC1 depending on the chemical composition of the polymer. They may be partly evolved and partly trapped in the polymer according to their volatility, the sample dimensions and the temperature. 

Enzymes can also be trapped within a nanotubular membrane by capping both faces of the membrane with a thin layer of porous polymer. In effect, arrays of enzyme-filled microcapsules are formed. Small molecules pass through the po-... 

Fig. A.3 Gel filtration Chromatography. The resins are porous and the small molecules get trapped inside the pores whereas the bigger protein molecules exclude out.

Hopfgartner, G. Varesio, E. Tschappat, V. Grivet, C. Emmanuel Bourgogne, E. Leuthold, L. A. Triple quadrupole linear ion trap mass spectrometer for the analysis of small molecules and macromolecules. J. Mass Spectrom. 2004, 39, 845-855. 

Buck, E., Wells, J.A. Disulfide trapping to localize small-molecule agonists and antagonists for a G protein-coupled receptor. Proc. Natl Acad. Sci. USA 2005, 102, 2719-2724. 

Researchers at Sunesis pharmaceuticals have developed a fragment-based drug discovery method termed tethering [25]. The approach, which is illustrated in Scheme 2.6, shares a number of features with DCC. Whereas protein-directed DCLs equilibrate small molecules via disulfide formation, say, in the presence of a protein that acts as a thermodynamic trap, tethering uses a cysteine residue on the protein surface to reversibly capture small-molecule thiol fragments from solution. Tethering is designed... 

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