Shell precursors

Free radicals and radical ions usually have excited states of low energy, and this can be understood from a simple orbital picture (Figure 4.89). In a closed-shell organic molecule the energy spacing between the HOMO and LUMO is quite large, but in the open-shell species the lowest excitations involve transitions of an electron between closely spaced orbitals. Many free radicals and radical ions of organic molecules absorb in the VIS or NIR, while the closed-shell precursors absorb only in the UV. 

Organic radical cations generated by electron transfer photosensitization are species with greatly enhanced pK a s compared with their closed shell precursors (142). If a C-H bond is located adjacent to the site of predominant charge density, rapid deprotonation will ordinarily ensue, producing a radical whose fate determines the identity of the isolated products. ... 

The thickness of the imprinted polymer shell can be also tuned in the range 10—40 nm by changing the relative amounts of functionalised silica nanoparticles and polymer shell precursors. The resulting core-shell particles exhibit enhanced capacity of rebinding the TNT template over 2,4-dinitrotoluene in comparison to particles prepared by precipitation polymerisation. Nevertheless, this strategy, although leading to impressive results, cannot be easily applied to other templates and monomers. 

The shell precursors in the vitelline cells - proteins, phenols and phenol oxidase (EC 1.14.18.1, monophenol o-diphenol oxygen oxidoreductase) - can all be stained specifically with cytochemical reagents although the reactions are not as intense as in trematodes (810). The most useful of these reagents are probably (a) Fast Red Salt B, which stains phenolic materials orange/purple, and (b) catechol, which can be used for detecting the phenol oxidase. Details of these techniques are given by Smyth (789). 

Recent studies have identified shell precursors from S. mansoni and Fasciola hepatica with molecular masses of 14-48 kDa (5,6) further suggesting a role for sclerotinization in eggshell formation. A 31 kDa precursor from F. hepatica is particularly unique in that it is rich in dihydroxyphenylalanine (DOPA), a potential precursor of quinone crosslinking (11). A 35 kDa eggshell precursor, identified by pulse labeling S. mansoni with [ CJtyrosine, is converted into an approximately 100 kDa protein in shells of newly laid eggs most likely due to a quinone-mediated erosslinking mechanism (12). 

Details of the synthesis of core-shell nanocrystals are available elsewhere [6, 7]. Briefly, TOP-capped InAs cores (5-20mg) were dissolved in 3-6 g of TOP (or a mixture with TOPO) in a three-necked flask. Under an Ar flow on a Schlenk line, the nanocrystal solution was heated to 260 °C, and the shell precursor solution introduced into the hot solution by drop-wise addition. The growth of core-shells was monitored using UV-visible spectroscopy of aliquots taken from the reaction flask. Following growth of the desired shell thickness the reaction mixture was cooled to room temperature, after which the core-shell nanocrystals passivated by TOP were obtained by precipitation, using a mixture of methanol and toluene. 

Similar evolution of LPH with activation was found in another series of air-reacted carbons prepared from almond shells (ref. 4). The results found with these two series of air-reacted carbons -olive stones and almond shells precursors - differ greatly however from those obtained when these two precursors are activated in COj to comparable burn-off levels (ref. 5,6). 

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