Heterogeneous microenvironments

Heterogeneous microenvironments (e.g., particles, surface films, and micelles) constitute another absorber in natural seawater. Such microenvironments in the ocean may facilitate unique reactions, since the reactants become concentrated and light energy may be efficiently absorbed as the result of their having incorporated transition metals, portions of photosynthetic apparatus, and condensed organic chromophores. The quantification of the light energy absorbed in these microenvironments presents a formidable task, especially when they are variable in composition, in size, and relatively few in number. 

TMOS, TEOS, ORMOSIL, and TEOS/polystyrene hybrid gels compared to corresponding crystalline solids (124, 125, 129, 130). The presence of heterogeneous microenvironments that encapsulate the rare earth complexes within xerogel matrices explains this broadening phenomenon (128). 

Hotta K, Lange H, TantiUo DJ, Houk KN, Hilvert D, Wilson lA. Catalysis of decarboxylation by a preorganized heterogeneous microenvironment crystal structures of abzyme 21DS. J Mol Biol. 2000 302 1213-1225. 

Another problem with replicating viruses involves spread of virus within tumors. Even if a small population of tumor cells is infected, the radial spread of the virus to every tumor cell remains a rate-limiting step, in large part due to the heterogeneous microenvironment between tumor deposits and often due to a lack of appropriate receptors. The combination of replication vectors with retargeting strategies using additional cellular receptors may be a very important avenue for research. 

It is now believed from studies on the natural photosynthetic systems that microenvironments for the photoinduced ET reaction play an important role in the suppression of the back ET [1-3]. As such reaction environments, molecular assembly systems such as micelles [4], liposomes [5], microemulsions [6-8] and colloids [9] have been extensively investigated. In them, the presence of microscopically heterogeneous phases and interfacial electrostatic potential is the key to the ET rate control. 

Screening the molecular heterogeneity of receptor expression in endothelial cell surfaces is required for the development of vascular-targeted therapies. First, as opposed to targeting purified proteins as discussed above, membrane-bound receptors are more likely to preserve their functional conformation, which can be lost upon purification and immobilization outside the context of intact cells. Moreover, many cell surface receptors require the cell membrane microenvironment to function so that protein-protein interaction may occur. Finally, combinatorial approaches may allow the selection of cell membrane ligands in a functional assay and without any bias about the cellular surface receptor. Therefore, even as yet unidentified receptors may be targeted. 

A closer look at the data shows the lifetime distributions are comparatively broad, about 0.25 ns for both distributions. This is in fact much broader than what one would expect from photon statistics alone. Based on realistic / -values (1.2-1.5) lifetime images recorded with this many counts are expected to yield distributions with widths on the order of 0.1 ns. The broadening is therefore not because of photon statistics. Variations in the microenvironment of the GFP are the most likely source of the lifetime heterogeneities. Importantly, such sensitivity for local microenvironment may be the source of apparent FRET signals. In this particular FRET-FLIM experiment, we found that the presence of CTB itself without the acceptor dye already introduced a noticeable shift of the donor lifetime. Therefore, in this experiment the donor-only lifetime image was recorded after unlabeled CTB was added to the cells. The low FRET efficiency and broadened lifetime distribution call for careful control experiments and repeatability checks. 

Considerable progress has been made within the last decade in elucidating the effects of the microenvironment (such as electric charge, dielectric constant and lipophilic or hydrophilic nature) and of external and internal diffusion on the kinetics of immobilized enzymes (7). Taking these factors into consideration, quantitative expressions have been derived for the kinetic behavior of relatively simple enzyme systems. In all of these derivations the immobilized enzymes were treated as simple heterogeneous catalysts. 

The advantage of this method is its ability to check whether the donor fluorescence decay in the absence and presence of acceptor is a single exponential or not. If this decay is not a single exponential in the absence of acceptor, this is likely to be due to some heterogeneity of the microenvironment of the donor. It can then be empirically modeled as a sum of exponentials ... 

Szentirmay et al. studied the microchemical environments of Nafion 117 in the acid and Na+ forms using Py and Ru(bpy)32+ probes in fully hydrated ( 40%) samples in various cation forms.Ru(bpy)32+ emission spectra cannot be interpreted in terms of environmental polarity in as straightforward a fashion as in the case of Py, but blue shifts can reflect this aspect. One of the results of this study was that the microenvironment polarities were such that Lj//i values for Py are between those for fluorocarbon and aqueous environments, and this conclusion was strengthened by the results of Ru(bpy)3 + probe studies, as well as the similar conclusion of Lee and Meisel. Another conclusion that was reached was that the SOa" clusters are chemically heterogeneous, an idea that was in line with the view of Yeager and Steck, who spoke of mixed interfacial regions. ... 

Since these reactions are relatively rapid, i.e., phenolic acids are rapidly degraded aerobically, their presence in the soil under these conditions appears transitory. It has been difficult to detect unbound phenolic acids in the soil solution and the compounds do not appear to accumulate in appreciable amounts under aerobic conditions. However, the soil is a heterogeneous medium consisting of loci or microenvironments that are at times completely opposite in character, i.e., anerobic microsites in a well-aerated soil (57). The phytotoxicity problem should be viewed in the context of a specially variable environment. 

It should be emphasized here that there is a vast difference between the microenvironment of the catalyst surface as examined by the type of analytical techniques mentioned in Section 9.1 and the overall surface that influences commercial processes. Until the modern techniques became available, however, catalyst preparation was mostly a matter of trial and error we have now entered an era in which science has a chance to catch up with technology. It seems fairly safe to predict that a greatly increased understanding of heterogeneous catalysis will emerge as modern surface chemistry matures. 

The chemistry of the metalloenzymes must be considered as a special case of enzymic catalysis since most active sites of enzymes are stereospecific for only one molecule or class of molecules and many do not involve metal ions in catalysis. Since the metal ion is absolutely essential for catalysis in the examples chosen for this review, the mechanisms undoubtedly involve the metal ion and a particular protein microenvironment or reactive group(s) as joint participants in the catalytic event. It is our belief that studies of catalysis by metalloenzymes will have as many, if not more, features characteristic of protein catalysis in general, in a fashion similar to metal ion catalysis, and these studies will be directly applicable to heterogeneous and homogeneous catalytic chemical systems where the metal ion carries most of the catalytic function. 

Membrane technology could offer interesting possibilities in order to overcome these limitations and to improve the advantages of catalysis mediated by the decatungstate by the multiturnover recycling associated to heterogeneous supports, the selectivity tuning as a function of the substrate affinity towards the membrane, the effect of the polymeric microenvironment on catalyst stability and activity. 

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