Carboxylate binding mechanism

Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4 90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101... 

For covalently attached dyes, the mechanism is more complex, involving trap states as intermediates and entailing coupled proton transfer. Why do the mechanisms differ It appears likely that phosphonate- and carboxylate-binding perturb semiconductor surfaces sufficiently to create new trap states that can be rapidly populated following injection. The states are necessarily spatially proximal to the attached dyes, but apparently sufficiently separated from each other to preclude fast trap-to-trap hopping. 

Layer-silicates Recent studies have also demonstrated the potential microbial influence on clay mineral (layer silicates) formation at hydrothermal vents. Bacterial cells covered (or completely replaced) with a Fe-rich silicate mineral (putative nontronite), in some cases oriented in extracellular polymers (as revealed by TEM analysis), were found in deep-sea sediments of Iheya Basin, Okinawa Trough (Ueshima Tazaki, 2001), and in soft sediments, and on mineral surfaces in low-temperature (2-50°C) waters near vents at Southern Explorer Ridge in the northeast Pacific (Fortin etal., 1998 Fig. 8.6). The Fe-silicate is believed to form as a result of the binding and concentration of soluble Si and Fe species to reactive sites (e.g. carboxyl, phosphoryl) on EPS (Ueshima Tazaki, 2001). Formation of Fe-silicate may also involve complex binding mechanisms, whereas metal ions such as Fe possibly bridge reactive sites within cell walls to silicate anions to initiate silicate nucleation (Fortin etal., 1998). Alt (1988) also reported the presence of nontronite associated with Mn- and Fe-oxide-rich deposits from seamounts on the EPR. The presence of bacteria-like filaments within one nontronite sample was taken to indicate that bacterial activity may have been associated with nontronite formation. Although the formation of clay minerals at deep-sea hydrothermal vents has not received much attention, it seems probable that based on these studies, biomineralisation of clay minerals is ubiquitous in these environments. 

Another factor relating to the binding mechanism is the amino-carboxylate spacing, a factor that is strongly negative whenever the spacing is too short [(003)2] long [(003)4]. It is important... 

Other studies of metal-HS interactions include NMR experiments (Kingery et al. 2001 Senesi and Loffredo 2005 and references therein), which have been used to correlate metal-binding structures with metal-binding characteristics, to elucidate binding mechanisms and to determine complexation equilibrium constants. IR experiments are less helpful, but they generally confirm that the carboxylate groups are often the main ligand for complexed metals (Davies et al. 2001). 

The diversity of structures is matched by a wide range of mechanisms. While the ruthenium—amines may act by a DNA-binding mechanism the rhodium carboxylates inhibit DNA synthesis but do not bind directly to the doubly stranded polynucleotide. In the latter case a possible mechanism is inhibition of precursor enzymes. Copper thiosemicarbazones and gold diphosphines appear to act by release of the toxic ligand. For thiosemicarbazones the ultimate target is ribonucleotide reductase. Other copper complexes have been developed to mimic the action of superoxide dismutase. 

The molecular mechanism of the enantioselective protonation reaction by antibody 14D9 was revealed by a crystal structure analysis [19[. A catalytic carboxyl group AspH 101 was found at the bottom of the catalytic pocket and found to be necessary for catalysis by mutagenesis to Asn or Ala. The mechanism or protonation involves an overall syn addition of water to the enol ether in a chiral binding pocket ensuring complete enantioselectivity (Figure 3.4). 

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