Catalysis charge compensation

E. coli endonuclease III is the prototype of DNA repair enzymes that contain a [4Fe-4S] cluster [20], The cluster has four Cys ligands and does not appear to be involved in the catalysis. It may have a structural, in particular, a charge-compensating, role [21,22],... 

The incorporation of Al atoms into the framework of zeolites occurs in a tetrahedral oxygen coordination and leads to negative framework charges. These framework charges are compensated by protons in acidic hydroxyl groups or by extra-framework cations such as Li , Na, Cs", Mg ", etc. Accordingly, these surface sites are responsible for the chemical behavior of zeolites in separation processes and in catalysis (199,200). 

Hydrotalcite was also used as support. A hydrotalcite is regarded as a very basic support. It consists of platelets in a brucite structure, with Mg2+ ions octahedrally coordinated by O atoms. Part of the Mg2+ ions are isomorphically exchanged for Al3+ ions, creating a positive charge on the framework. Between the platelets, this positive charge is compensated with CO3 or OH- ions. The OH- ions located at the edge of the platelets are active in basic catalysis. 

Enzymes, in addition to providing hydrogen bonding to anions formed, use electrostatic catalysis to polarize the reactant and compensate for a charge formed on the substrate as it reacts. Although it may be difficult to protonate a carbonyl (pA a = 7) at... 

HPTS is a pH-sensitive fluorophore (pk, 7.3) [6]. The opposite pH sensitivity of the two excitation maxima permits the ratiometric (i.e. unambiguous) detection of pH changes in double-channel fluorescence measurements. The activity of synthetic ion channels is determined in the HPTS assay by following the collapse of an applied pH gradient. In response to an external base pulse, a synthetic ion channel can accelerate intravesicular pH increase by facilitating either proton efflux or OH influx (Fig. 11.5c). These transmembrane charge translocations require compensation by either cation influx for proton efflux or anion efflux for OH influx, i.e. cation or anion antiport (Fig. 11.5a). Unidirectional ion parr movement is osmotically disfavored (i.e. OH /M or X /H symport). HPTS efflux is possible with pores only (compare Fig. 11.5b/c). Modified HPTS assays to detect endovesiculation (Fig. 11.1c) [16], artificial photosynthesis [17] and catalysis by pores [18] exist. 

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