Cavities, exclusion effect

Figure 1. Exclusion effect in cylindrical cavities (1) ((A) hard sphere of radius r (B) thin rod of length Li in two orientations in the plane of the cross section (C)...

The exclusion effect of hard-spheres is illustrated in Figure lA., which shows a spherical solute of radius r inside an infinitely deep cylindrical cavity of radius a. Here the exclusion process can be described by straightforward geometrical considerations, namely, solute exclusion from the walls of the cavity. Furthermore, it can be shown thatiQJ... 

The stationary phase in gel permeation (also called size exclusion) chromatography contains cavities of a defined size distribution, called pores. Analytes larger than the pores are excluded from the pores and pass through the column more rapidly than smaller analytes. There may be secondary effects due to hydrophobic adsorption, ionic interaction, or other interactions between the stationary phase and analyte. Gel permeation and non-ideal interactions in gel permeation are described more fully in Chapter 6. 

The calculation of free volumes and free surface areas requires an efficient way of extending the cavity algorithm. To calculate the free volume of a particle i, the particle and its associated exclusion sphere are effectively removed from a snapshot of the particle configuration. The volume and surface area of the cavity that is produced in the absence of particle i are equal to the free volume and free surface area of particle i, respectively. 

FABMS has been used as a semiquantitative indication of the selectivity of receptors for particular guest metal cations (Johnstone and Rose, 1983). The FABMS competition experiment on [7] with equimolar amounts of the nitrates of sodium, potassium, rubidium and caesium gave gas-phase complex ions of ([7] + K)+ ion (m/z 809) and a minor peak ([7] + Rb)+ ion (m/z 855) exclusively. The relative peak intensities therefore suggested a selectivity order of K+ Rb+ Na+, Cs+, indicative of the bis-crown effect, the ability of bis-crown ether ligands to complex a metal cation of size larger than the cavity of a single crown ether unit, forming a sandwich structure. 

All of these models predict that the hydrophobic effect provides a significant driving force for the exclusion of even highly polar, charged peptides from an aqueous environment to the nonpolar environment of the RPC sorbent. According to the solvophobic model, in order to place a peptide into a mobile phase, a cavity of the same molecular dimensions must first be created. The energy required to create this cavity is related to the cohesive energy density or the surface tension of the mobile phase. Conceptually, each solvent-accessible unit... 

Some authors based their approach to selective binding of the more lipophilic a-amino acids in water on hydrophobic effects using water-soluble, cavity-containing cyclophanes for the inclusion of only the apolar tail under renouncement of any attractive interaction of the hosts with the zwitterionic head . Kaifer and coworkers made use of the strong affinity of Stoddart s cyclobis(paraquat-p-phenylene) tetracation 33 for electron-rich aromatic substrates to achieve exclusive binding of some aromatic a-amino acids (Trp, Tyr) in acidic aqueous solution [48]. Aoyama et al. reported on selectivities of the calix[4]pyrogallolarene 34 with respect to chain length and t-basicity of aliphatic and aromatic amino acids, respectively [49]. Cyclodextrins are likewise water-soluble and provide a lipophilic interior. Tabushi modified )S-cyclodextrin with a 1-pyrrolidinyl and a carboxyphenyl substituent to counterbalance the... 

Although there are preliminary data supporting the antitumoral activity of quercetin, the most common flavonoid, in humans in the course of a Phase I clinical trial [189], direct evidence of the anticancer effect of flavonoids is derived almost exclusively from studies performed in animal models as well as studies performed on cultured cell lines, Fig. (2). Most animal studies on gastrointestinal cancer have focused on colon cancer using the azoxymethane (AOM) model in rats or mice [190-197]. There are also available reports on models of cancer of the stomach (induced by benzo[a]pyrene [198] or N-methyl-N-nitro-N-nitro so guanidine [199]), oesophagus (N-methyl-N-amylnitrosamine [200]), and the tongue/oral cavity (methyl-(acetoxymethyl)-nitrosamine [198], 7,12-dimethyl-... 

As revealed by selected catalytic test reactions, the effective pore width of zeolite MCM-22 is between those of medium pore and the very large pore materials [16-18]. Due to its very large cavities which, on the other hand, can be reached exclusively via 10-membered ring windows, this zeolite offers a remarkable potential for shape selective catalysis involving bulky transition states. 

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