Molecular oxygen carrier

There remain many intriguing questions about biological molecular-oxygen carriers, questions that will be answered by complementary studies on the biological and model systems. To make and study such model systems is an example of the challenge and excitement of this aspect of bioinorganic chemistry. 

The electron-transport system is a series of coupled oxidation-reduction (also called redox) reactions which transfer electrons to molecular oxygen. Carrier 1 (Figure 13.2) in its oxidized form may accept electrons which reduce it. In the reduced state, it may donate the electrons to the oxidized form of carrier 2. In the process of the transfer, carrier 1 becomes reoxidized as carrier 2 becomes reduced. Similarly, reduced carrier 2 may donate electrons to carrier 3 and so on. In each reaction, the electron donor can only release the electrons if there is a suitable acceptor. The electron donor is termed the reductant since it reduces the acceptor and the electron acceptor is termed the oxidant since it oxidizes the donor. In the electron-transport system, each electron carrier oscillates between oxidized and reduced forms which constitute a redox couple. 

The first mechanism, Eq. (172), belongs to the general scheme for sensitized reactions which is favored by Schenck. Presumably, a sensitizer-oxygen diradical adduct is the molecular oxygen carrier which, because of its bulk, is extremely sensitive to steric factors when it hands oxygen to the sub-... 

Recently a novel experimental approach using Schottky diodes with ultra-thin metal films (see Fig. 11) makes direct measurement of reaction-induced hot electrons and holes possible. See for example Refs. 64 and 65. The chemical reaction creates hot charge carriers which travel ballistically from the metal film towards the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and N02 molecules and of certain hydrocarbons on Ag. Similar results have been found with metal-insulator-metal (MIM) devices, which also show chemi-currents for many exothermic surface reactions.64-68... 

The reversibility of the carrier was tested by cyclic voltammetry. The scan of the solvent and supporting electrolyte is shown in Fig. 13, with and without dissolved oxygen. The oxygen reduction occurs at about — 0.43 V. (vs. SCE). The scan with the complex added, but the solution free of dissolved oxygen is shown as Fig. 14. The carrier is seen to be reduced at about 0.04 V, well within the window of the solvent and electrolyte, and well before reduction of molecular oxygen. 

The photoexcited electrons are trapped at the surface by TiIV sites, subsequent to this process resulting Tim sites, which may further transfer charge carriers to molecular oxygen resulting in the formation of a superoxide radical ... 

Fig. 1. Solubility of perfluorochemicals (PFCs) in water decreases rapidly with increasing molecular weight. The solubility of F-octyl bromide used in injectable oxygen carriers is 5x10- mol/l.

Moilanen et al. (1978) observed that it was stable to irradiation when it was in a nitrogen carrier gas, indicating that molecular oxygen is involved in the photochemical decomposition they suggest that 02 may add to a biradical formed from the chloropicrin upon light absorption but that 02 is regenerated in the decomposition of the adduct. 

A polymer ligand might be expected to protect the dioxygen-metal complex against autoxidation in much the same way as the globin protein does. In this chapter, we describe how polymer-metal complexes react with molecular oxygen and introduce attempts to construct synthetic oxygen carriers. 

Basdo etall00 found that the attachment of Fe(II)(TPP) to a rigid modified silica gel support produced an efficient oxygen carrier. The silica gel used contained a 3-imidazolylpropyl group bonded to the surface atoms of silicon. This then reacted with Fe(lI)(TPP)(B)2, and the axial base was removed by heating the silica gel. The five-coordinate Fe(II)(TPP) complex 41 was prepared. The open coordination site could reversibly bind molecular oxygen. It was concluded that... 

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