Monovalent reaction centers

Nucleophiles can also substitute at the internal carbon position. Reaction of m-benziporphyrin with silver tetrafluoroborate in pyridine under reflux conditions yielded a pyridine-appended macrocyle, with a C-N bond at the core of the ring. The pyridine then forms a second bond at the ortho position with an adjacent meso carbon (70). Recently, we noticed a directly analogous reaction upon the metalation of dicarbahemiporphyrazine with Cu(II) in pyridine (33). In this macrocycle, one of the two core carbon positions is substituted with a pyridine. In both cases, the formation of the C-N bond is believed to result from reductive elimination at the metal. Initial metalation results in the formation of a M-C bond, and the metal is subsequently coordinated by a solvent pyridine. The activation of the C-H bond at the core results in a high valent metal center (Ag(III) and Cu(III) respectively) which can then reductively eliminate to form a monovalent metal center and a C-N bond. 

Table 1 compares the inhibitory properties of several cations. For a description of the monovalent cations, see (4). Sodium inhibition in reaction center core preparations is the same as in PSII preparations so the inhibition is not due to an effect on the LHC. The Kj seen for Na" " when assayed in saturating Cl at varied Ca(0H)2 concentrations is slightly lov/er than when measured against increasing concentrations of CaCl2 because the effect of... 

Peptidyltransferase assays have also provided insight into the mechanisms whereby spermine promotes, and NHJ ions inhibit, polypeptide synthesis on the ribosomes of S. solfataricus, T. tenax and D. mobilis (see section 3.3). First, the 30S uncoupled peptidyltransferase activity is absolutely dependent on spermine while being totally unaffected by monovalent cations. Secondly, monovalent cations strongly inhibit the 30S subunit coupled reaction [66]. Thus, polyamines appear to be obligatorily required to convert the catalytic center of the spermine-dependent ribosomes into an active conformation, whereas monovalent cations inhibit polypeptide synthesis by preventing 30S subunits from interacting with the cognate SOS particles (ref. [66], see below). 

There is also a large repertoire of reactions available for the reduction of carbon nanotubes (Figure 3.66). The most effective method is the reaction with alkali metals in liquid ammonia with subsequent methanolysis. At first, the alkali metal (Uthium in most cases) is oxidized to the respective monovalent cation, while at the same time a solvated electron is generated. The highly reactive species [e (NH3),] attacks the carbon atoms of the nanotubes and reduces them to carb-anions. The ensuing reaction with methanol leads to the hydrogenation of the carbon atom and to the production of one equivalent of methanolate. This process introduces an sp -center into the framework of the tube. This bears strongly on... 

Additionally, acetylene may be also cocyclized with nitriles to form substituted pyridines using Co - zeolites as with soluble cobaltous complexes. Only Co is active in this reaction which needs an insertion of the nitrile into the M pentacycle. Apparently this is the element that enables competition of the nitrile with acetylene. It also appeared that the cobalt centers had to swing between the trivalent and monovalent states (45). The use of substituted acetylenes showed that the 2-, 5-substituted pyridines are significantly favoured by comparison with homogeneous media. 

The membrane is treated as the same simple system as described above (Figure 1). We assume, besides the electroneutrality inside the membrane (constant field hypothesis), a continuity of the electrical potential ip from the bulk of the surrounding medium to the center of the cell. We are compelled, therefore, to assume that the potential at the outer Helmholtz plane, which is induced by the effective surface charge (dissociated functional groups or strongly adsorbed ions) is screened to a major extent by counterion-site pairing in the Nernst layer. We consider the three classes of transferable chemical species quoted in the introduction. Among the species not directly involved in chemical reactions, the calculation takes into account an anion (Cl ), two monovalent cations (K, Na+) and two divalent cations (Mg2+, Ca ). As for the species involved in chemical reactions, we assume that they do not interact inside the membrane (independent channels). However, their flux balance must take into account the chemical reactions in both lateral phases. We handle only here two types of equilibria the water dissociation and the first dissociation of the carbonic acid. The three flux balances of the five chemical species which are interconnected by the conditions for the equilibria are as follows ... 

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