High electronic density domains

Small domains of high electronic density have been imaged in halato-telechelic ionomers by Scanning Transmission Electron Microscopy (STEM) using the technique of atomic number or Z-contrast. The possibility that these are ionic domains is evaluated and the morphology compared with that derived from recent SAXS experiments. 

The question is, are the high electronic density features the ionic domains Let us emphasize a few important points. 

Figure 3A shows electron microscopy of the intact E. coli F-ATPase. Combining the low-resolution electron density maps derived by electron microscopy with the high-resolution structural information available for various subunits and their domains allows us to construct a high-resolution structural model of the intact E. coli FjF0-ATP synthase as shown in Fig. 3B. 

The formal potential, E0/, contains useful information about the ease of oxidation of the redox centers within the supramolecular assembly. For example, a shift in E0/ towards more positive potentials upon surface confinement indicates that oxidation is thermodynamically more difficult, thus suggesting a lower electron density on the redox center. Typically, for redox centers located close to the film/solution interface, e.g. on the external surface of a monolayer, the E0 is within 100 mV of that found for the same molecule in solution. This observation is consistent with the local solvation and dielectric constant being similar to that found for the reactant freely diffusing in solution. The formal potential can shift markedly as the redox center is incorporated within a thicker layer. For example, E0/ shifts in a positive potential direction when buried within the hydrocarbon domain of a alkane thiol self-assembled monolayer (SAM). The direction of the shift is consistent with destabilization of the more highly charged oxidation state. 

Most properties of density domains follow from the properties of MIDCOs. We have seen before that for low values of the electron density threshold a, the MIDCO G(a,K) is usually a single, closed surface, consequently, the density domain DD(a,K) is also a single, connected body. For high values of density threshold a, the MIDCO G(a,K) is often a collection of several closed surfaces, where each closed surface surrounds some of the nuclei of the molecule. Consequently, for such a density threshold, the formal density domain DD(a,K) is in fact a collection of several, disconnected bodies DDj(a,K). 

One example that has been studied in some detail is the ethanol molecule [2]. Let us choose H as nucleus A. The density domain of the OH proton appears relatively late in the process of gradually decreasing the electron density threshold. This observation can be justified by the high electronegativity of oxygen, resulting in a depletion of the electron density at the nearby proton, that has a chance for the formation of a density domain of its own only at a somewhat lower density threshold. 

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