Anion specificity

The study of anion coordination has implications for a number of areas in chemistry and biochemistry. These include analytical applications concerned with anion sensing or separation as well as model studies for anion-specific biochemical systems. With respect to the latter, it is of interest that the majority of enzymic systems so far characterized bind substrates which are anionic. 

Curve C pyranine enclosed in the anion specific pore (18 x 27 A) of the PhoE... 

While some of these reactions are mediated by enzymes, e.g., glucuronidase, which hydrolyze glucuronide conjugates, many others are nonenzy-matic. These are driven by the electrophilicity of the substrates, which renders them susceptible to attack by the HO- anion (specific base catalyzed hydrolysis) or by other bases (general base catalyzed hydrolysis). A semblance of order in such a heterogeneous group of reactions is, thus, far from obvious. 

The process of monolayer deposition of metal ions in underpotential deposition is strongly affected by anion-specific adsorption, and the two processes at the electrode interface must be elucidated if one is to understand underpotential deposition phenomena in a unified way. 

As found in preceding sections, the underpotential deposition process is strongly influenced by anion adsorption, or controlled sometimes by the presence of the adsorbed anions. Anion-specific adsorption/desorption seems to take place as an electron transfer process and may be accompanied by the underpotential deposition formation/removal process. ... 

Sprunger, L. et al., Characterization of room temperature ionic liquids by the Abraham model cation-specific and anion-specific equation coefficients, /. Chem. Info. Model, 47,1123,2007. 

A theoretical (AMI molecular orbital) treatment of boron analogues of the katapinand series as well as macrotricyclic compounds related to 16 has also recently appeared. As with the ammonium-based host molecules the macrotricyclic hosts containing four boron atoms exhibited a greater degree of anion specificity as a consequence of the rigidity of their binding sites. In all cases, size selective complexation of halide anions was observed with accompanying decreases in B - B distances and partial sp -> sp rehybridization. ... 

Under some circumstances the inner membrane develops one or more types of large-permeability pore. An increase in Ca2+ may induce opening of an unselective pore which allows rapid uptake of Ca2+ 294/307/307a A general anion-specific channel may be involved in volume homeostasis of mitochondria.308... 

The pH jump technique with picosecond fluorescence measurements were used to study apomyoglobin and the anionic specific channel, porin of E. coli [151]. The results indicated that the water in the sites deviates markedly from the liquid state in the bulk, having a lower dielectric constant and smaller diffusivity of protons. 

Brighteners which rely on electrostatic or hydrophobic interactions may function in ionic liquids but their efficacy is likely to be surface and cation/anion specific. As with other solutes in ionic liquids, the general rule of like dissolving like is applicable i.e. ionic species will generally be soluble as will species capable of interacting with the anion. Aromatic species tend to exhibit poor solubility in ionic liquids consisting of aliphatic cations and vice versa. 

The dramatic role of the anion can perhaps best be appreciated from simple quantitative considerations. In the absence of a suitable anion, specific binding of iron to transferrin does not occur at all the effective binding constant is zero. At physiologic pH and bicarbonate concentrations, however, the effective binding constant is about 5 X 1023 M"1 24, 50). This means that in 1 L of blood plasma, in which the transferrin is only about 30% saturated with iron, there will be less than one free ferric ion or that a molecule of the ferric—transferrin complex will spontaneously dissociate only about once in 10,000 years. Since iron is readily removed from the transferrin molecule during its interaction with the reticulocyte without disrupting protein structure 51, 52), a... 

Induced adsorption — Enhancement of the - adsorption of a component induced by another adsorbed species. These phenomena are treated in terms of induced anion and cation adsorption. Interrelation of anionic -> specific adsorption and -> underpotential deposition of metal ions is a typical example for the induced adsorption of anions [i]. 

Eeng B, Dresser MJ, Shu Y, Johns SJ, Giacomini KM. Arginine 454 and lysine 370 are essential for the anion specificity of the organic anion transporter, rOAT3. Biochemistry 2001 40 5511-5520. 

Titration, 0.2 M salts Li > Na > K > Cs, Cl > NO3. Anion specificity was observed also in the basic range and cation specificity was observed also in the acidic range [537,1557]. 

In many electrolytes, one or more of the constituent ions are specifically adsorbed at the interface. Specific adsorption implies that the local ionic concentration is determined not just by electrostatic forces but also by specific chemical forces. For example, the larger halide ions are chemisorbed on mercury due to the covalent nature of the interaction between a mercury atom and the anion. Specific adsorption can also result from the hydrophobic nature of an ion. Thus, tetra-alkylammonium ions, which are soluble in water, are specifically adsorbed at the mercury water interface because of the hydrophobic nature of the alkyl groups. Specific adsorption of molecular solutes, such as the alcohols, occurs for the same reason. 

Very interesting studies have been made of the pattern for UPD on single crystal metal substrates [49]. A commensurate pattern is usually observed for the formation of a partial monolayer. The theory of UPD and formation of the initial monolayer is an area of active research. Of course, the phenomenon of UPD is restricted to formation of a monolayer. Once this has formed, the deposition process reverts to one of metal ion M"+ on metal M. By comparing the UPD process with anion-specific adsorption, the role of partial charge transfer in these processes is clarified. 

In electrocatalysis, the structure sensitivity of an adsorption process at the platinum-electrolyte interface is evidenced by the effect of surface orientation on the distribution of the configuration states in current vs. potential profiles [1-4]. The electrolyte composition at constant pH affects these adsorption states because of the anion-specific adsorption [2],... 

In the case of Pt(l 11), voltammograms for flame-cleaned electrodes were obtained under UHV conditions [53] providing evidence that such voltammograms correspond to well-ordered (lxl) structures. Since the same voltammetric features were obtained with various surface preparation procedures, there is a large stability of the (111) (lxl) structure [60], Conversely, UHV studies have shown the existence of various surface structures for Pt(100) [42] or Pt(110) [61]. Such structural changes may influence the electrochemical response of these orientations. Similar to Pt(lll), voltammograms obtained with Pt(100) electrodes have been found to depart on the electrolyte composition that is, a decrease of the anion-specific adsorption shifts the hydrogen adsorption states to more positive potentials. 

Thus, cation water clusters favour internal structures in contrast to the surface strucmres favoured by anionic water clusters. This critical difference in the structural preferences of hydrated cation and anion clusters provides important cues for the design of cation- and anion-specific ionophores and receptors. Indeed, we note that most cation receptors have spherical structures, while almost all anion receptors do not have compact spherical structures but have a vacant space around the anion binding site without full coordination (which might be exceptional for the F ion with strong electronegativity for which the excess electron is strongly bound to F due to its small ion radius). However, as the temperature increases, the hydration structure tends to be more spherical due to entropy effects. 

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