Concentration molecular uptake

In principle, the determination of molecular uptake may be based on any experimentally accessible quantity which is a function of the amount adsorbed. Being directly sensitive to a certain molecular species, in this respect the application of spectroscopic methods is particularly suitable. IR spectroscopy has been successfully applied to studying molecular uptake by beds of zeolite catalysts [26-28] as well as—in combination with IR microscopy [29, 30]—on individual crystallites. Similarly, NMR spectroscopy has also been used to monitor the time dependence of the sorbate concentration within porous media [31]. Moreover, recent progress in NMR imaging allows the observation of concentration profiles within porous media with spatial resolution below the mm region [32-34],... 

Figure 36 compares the actual measurements of transient concentration profiles during molecular uptake by a silicalite-1 crystal as displayed in Fig. 34 with uptake simulations. The diagrams show the integrals of concentration in the x direction as a function of z (this is in longitudinal extension of the... 

Figure 46b and c provide an overview of the observed evolution of the molecular concentration (integrated in the x direction) during molecular uptake, namely the evolution over the total y-z plane (c) as well as, for clarity, selected profiles in the z direction (b) and / direction (d) [88,90]. These latter two sets of profiles help us to understand that the time dependence of the observed integral concentrations may be easily explained by assuming a two-stage process, namely a first fast one during which the rooflike... 

In fact, this behavior can be shown to hold quite generally. With the simplifying assumption of constant diffusivities and surface permeabilities, the normahzed concentration profiles during molecular uptake by a plate of thickness 2l are given by the relation... 

In certain adsorbents, notably partially coked 2eohtes and some carbon molecular sieves, the resistance to mass transfer may be concentrated at the surface of the particle, lea ding to an uptake expression of the form... 

The transfection mechanism of plasmid-chitosan complexes as well as the relationship between transfection activity and cell uptake was analyzed by using fluorescein isothiocyanate-labeled plasmid and Texas-Red-labeled chitosan. Several factors affect transfection activity and cell uptake, for example the molecular mass of chitosan, stoichiometry of complex, seriun concentration and the pH of the transfection medium. The level of transfection with plasmid-chitosan complexes was found to be highest when the molecular mass of chitosan was 40 or 84 kDa, the ratio of chitosan nitrogen to DNA phosphate was 5, and serum at pH 7.0 was 10%. Plasmid-chitosan complexes most likely condense to form large aggregates (5-8 p,m), which absorb to the cell surface. After this, plasmid-chitosan complexes are endocytosed, and accumulate in the nucleus [97]. 

It should be remembered that some of the established antioxidants have other metabolic roles apart from free-radical scavenging. The finding of reduced antioxidant defences in diabetes, for example, may not be prima fascie evidence of increased oxidative stress, since alternative explanations may operate. For example, this may reflect a response to reduced free-radical activity as su ested by the results of a previous study (Collier et al., 1988). In the case of ascorbate, an alternative explanation has been proposed by Davis etal. (1983), who demonstrated competitive inhibition of ascorbate uptake by glucose into human lymphocytes. This view is supported by the similar molecular structure of glucose and ascorbic acid (see Fig. 12.4) and by a report of an inverse relationship between glycaemic control and ascorbate concentrations in experimental diabetes in rats. Other investigators, however, have not demonstrated this relationship (Som etal., 1981 Sinclair etal., 1991). 

Fig. 21. Molecular structures of new aromatic [M(ATSM)] analogs (a) M — Zn(II) and (b) M = Cu(II), (c) cytotoxicity tests in MCF-7 cells for the Zn(II) complex (group 2) and Cu(II) complex (group 3) and comparison with control and with cis-platin over a range of concentrations, (d) cell uptake profile monitored over 90 min, (e) confocal fluorescence imaging of Zn(II) complex in MCF-7 cells, at 100 pM cone, in DMEM, 1% DMSO (112,113).

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