Solvent bridge

Protein Source Diiron site function Diiron-binding sequence motif Diiron(III) solvent bridge... 

The possibility of intervention of a solvent bridge between the proton and the basic site, where the solvent molecule acts simultaneously as a proton donor and proton acceptor, should always be considered. Here as elsewhere, the operational recognition of what Hammett (1970) calls the stoichiometric involvement of solvent is not a simple task. 

In solvent-separated ion pairs, the solvation shells of the cation and the anion touch each other in solvent-bridged ion pairs, the ions share solvent molecules. In contact ion pairs, the cation and the anion are bound directly to each other and are surrounded by a common solvation shell. In penetrated ion pairs, an empty space between edge groups in one ion of a salt is occupied to a certain degree by a counterion. The two latter types of ion pair may have quite a different electronic distribution than the corresponding naked ions. The following examples show the influence of ion-pair formation. 

Rose, K. N., Barbour, L. J., Orr, G. W., Atwood, J. L., Self-assembly of carcerand-like dimers of calix[4]resorcinarene facilitated by hydrogen bonded solvent bridges. Chem. Commun. 1998, 407-408. 

Multi-component, solvent-bridged systems are usually formed between two identical units resulting in a bridged dimer [112-114]. The normal approach for these systems has been the same as that for simple multi-component capsules, namely the use of concave molecules such as calixarenes with hydrogen bonding... 

The mechanism of proton transfer for carbon acids often differs in one further way from that for oxygen and nitrogen acids. Proton transfer for these latter can occur through reaction complexes Ij and I2, and a transition state (XXIX) in which the acid and base are separated by a solvent bridge. An alternative transition state is shown in (XXX), viz. 

The contributions made by these pathways in proton transfer between amines and their conjugate acids have been determined [193] and the results are shown in Table 13. The rate coefficient kt refers to the direct proton transfer mechanism and k2 is for proton transfer through a solvent bridge. The available evidence for carbon acids suggests that proton transfer occurs directly between acid and base and an intervening solvent molecule is not involved [123,194]. This evidence is mostly based on the magnitude of the solvent isotope effect, and results for reactions involving nitroparaffins, acetals, and diazocompounds have been reviewed [123]. In a different approach to this question, the rate expression for the acid catalysed decomposition of ethyl vinyl ether in water/dimethyl-sulphoxide was measured [195] and shown to consist of two terms (111). 

Figure 7 General chemical scheme for peptidyl transfer by the ribosome. The scheme shows nucleophilic attack by the amine group of the amino acid (with side chain R2) esterified to the tRNA in the ribosomal A site (right) on the ester linkage of the aminoacyl tRNA (with amino acid chain Rl). The 2 OH of the peptidyl tRNA participates in the reaction and seems to transfer a proton to the 3 0 leaving group, either directly or potentially via a solvent bridge. Adapted from Reference 76.

Figure 3. Two models describing the microphases of swollen Nation membranes. Top Gierke s [48] suggestion of aqueous inverse spherical micelles connected by water-filled cylindrical channels. Bottom Yeager and Steck s [49] three-region model of a water/ionomer mixture without regular structure. Regions A, B and C are the hydrophobic polymer, the solvent bridges and the hydrophilic regions, respectively.

As noted before, solubility of covalent organic compounds in molten salts seems to require proton interaction between acidic groups of the solute and anions of the solvent. For un-ionized solutes this may well involve structures analogous to the solvent-bridged ion-pair complexes postulated for concentrated aqueous solutions ... 

Figure 2.13 Experimental configuration of DESI and nanoDESI. (a) Traditional DESI setup. Solvent is electrosprayed with the aid of a nebulizer gas jet. Solvent and analyte are removed from the surface, (b) NanoDESI setup. A solvent bridge formed between the primary and nanospray capillaries contacts the analyzed surface. Analyte-containing solvent is removed from the surface by self-aspirating nanospray [102]. Reproduced from Roach, P.j. etal. (2010) [102] with permission of the Royal Society of Chemistry...

Figure 19 Three limiting situations for the complexation of ion pairs by a ditopic receptor (a) contact, (b) solvent bridged, and (c) host separated.

Heteroditopic receptors are those in which the distance between the two binding sites allows the accomodation of solvent-bridged ion pairs, in which solvent molecules help link the anion to the cobound cation. 

In the case of receptors that can acconunodate solvent-bridged ion pairs or that are provided with well-separated binding sites for the two partners of the ion pair, the inherent binding ability toward one of the ion is not expected to be significantly altered by the presence of the other ion. But some exceptions can be found, thus confirming the well-established limits of any classification. 

The large decrease In exchemge rate occurring at an HMPA/Ma of 2.5 Is proposed to be due to the formation of a solvent-bridged species of stoichiometry [2Na.5HMPAj... 

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