Charge reduction

Charge reduction (pH, temperature) and other precipitation are common methods of FVIII fractionation, d. ... 

A. Charge Reduction (Charge Separation) on Desolvation of Multiply Charged Ion-Solvent Molecule Clusters... 

For some ions the desolvation can proceed down to the naked ion M2+ while for others a reduction of the charge occurs at a given n = r (r for reduction). The charge reduction may take different forms, the simplest of which is loss of Sl+ ... 

This type of charge reduction by charge transfer to the solvent molecule occurs in general when SI are polar solvent molecules of aprotic character such as dimethyl-sulfoxide, dimethyl formamide, and acetonitrile. Protic solvents such as water lead to charge reduction which involves an intracluster proton transfer reaction ... 

Examples illustrating the reactions 21-23 are given in Figures 10-12. Shown in Figure 10 is the CID mass spectrum for the desolvation of Ni2+(H2O)10. The sequence of product ions Ni2+(H20) where n = 9 to n = 4 illustrates the sequential solvent loss represented by equation 21. The CID spectra in Figure 11 demonstrate that for the n = r = 4, charge reduction via internal proton transfer (see equation 23)... 

Table 2 gives a summary of results70,71 which provide the solvent number r at which simple solvent loss and charge reduction become competitive. Charge reduction becomes dominant for solvent numbers lower than r. Ion clusters for... 

Figure 12. CID of precursor ion Cu(DMSO)3+, which shows that this ion undergoes charge reduction by electron transfer (see equation 22 which leads to Cu(DMSO)2 and DMSO+. From Blades, A. T. Jayaweera, P. Ikonomou, M. G. Kebarle, P. J. Chem. Phys. 1990, 92, 5900, with permission.

Table 2. Ligand Number at which Charge Reduction from M2+ to M+ Occurs...

The enthalpy change AH24 for the charge reduction reaction 24 can be obtained from a thermodynamic cycle,71... 

An estimate for the activation energy can be obtained by the consideration that for r = 2, the charge reduction reaction must proceed at a rate similar to that for the single solvent molecule dissociation, which means that the activation free energies for the two reaction are of similar magnitude. Neglecting the difference between AG and AH we obtain,... 

The activation energy for the charge reduction reaction is due to two factors the bond stretching and distortions of the originally near linear complex, so as to achieve the internal proton transfer and the increase of energy due to the Coulombic repulsion between the two charged products, a repulsion that leads to a release of kinetic energy on their separation. 

For higher Ca2+ (H20) clusters, the enthalpy change AHu will increase due to increase of the AH°n 0 energy which is not counterbalanced by the much smaller increase of AHn 2 (Ca+OH) see equation 24. This effect will lead to positive values for AH24 and the charge reduction reaction will shut down. 

Examining Table 2, one comes to the conclusion that only Ba2+ (H20)n where n > 1 can be produced by the association reactions of M2+ with H20. For all the other ions only the monohydrate will be obtainable. For ions with high IE(M) values, even the monohydrate, M2+H20 may not be obtained because of charge transfer reactions to H20 (see equation 22). Other protic solvents will lead to charge reduction by proton transfer at different values of r. Only NH3 has been examined.71 It leads to much more facile charge reduction than H20. Many of the doubly charged ions that were observed as hydrates could not be observed as the equivalent clusters of NH3. 

The charge reduction reactions occurred prior to any collisional activation in the second quadrupole Q2, which indicates that the activation energy for this reaction is very low. These results show that in the presence of a multiple charge even N-H hydrogens can become sufficiently acidic and engage in protonation of water molecules. 

The most important multiply charged polyatomic positive ions are compounds with two or more basic groups which when protonated lead to doubly or poly-charged ions. Typical examples are diamines such as the double protonated a, to alkyldiamines, H3N(CH2)pNH2+, and the most important class, the polyprotonated peptides and proteins, which have multiple basic residues. Charge reduction for these systems occurs through proton transfer from one of the protonated basic sites to a solvent molecule. Such a reaction is shown below for the monohydrate of a doubly protonated diamine ... 

In reaction 35, activation energy has to be provided to the precursor ion by collisions or other means and charge reduction will occur when the activation energy is lower than that for the desolvation reaction. In reaction 36, the solvation of the ion by B, i.e. reaction a, provides the activation energy and proton transfer and charge reduction will occur if the activation energy for reaction b is less than that for the reverse of reaction a. 

When experiments with electrospray producing H3N(CH2)pNH2+ ions were performed in this laboratory,56 it was found that doubly protonated ions could be obtained from methanol-water solutions only when p > 4. For p < 4 only the singly protonated ions were observed, even though the doubly protonated ions are known to be present in the solution. We attribute the failure to observe the doubly protonated ions with p < 4 to the occurrence of charge reduction by deprotonation. Probably methanol, whose gas-phase basicity is greater than that of water, is involved in the deprotonation. The diprotonated diammines, p > 4, could all be dehydrated down to the naked ion either in CID experiments or at higher temperature. 

Table 3. Hydration Number r at which Charge Reduction of Doubly Charged Hydrated Negative Ions Occurs3...

Notes "Charge reduction for all these ion hydrides occurs by intracluster proton transfer (see equation 37). Data from refs. 80-82. 

Doubly charged ion could be dehydrated down to the naked ion without charge reduction. 

The hydrated sulfate S04(H20)2 could be produced by electrospray in the gas phase,80 but neither the triply charged orthophosphate P04 nor the doubly charged HOPOj" were observed as the naked ion or the hydrate.81 CID of the hydrated sulfate led to simple desolvation down to n = r = 4. The decomposition of the r = 4 hydrate led to charge reduction by intracluster proton transfer ... 

Kampf N, Scheinost AC, Schultze DG (2000) Oxides minerals. In Sumner ME (ed) Handbook of soil science, CRC Press, Boca Raton (FL), F125-F168 Jain A, Loeppert RH (2000) Effect of competing anions on the adsorption of arsenate and arsenite by ferrihydrite. J Environ Qual 29 1422-1430 Jain A, Raven KP, Loeppert RH (1999) Arsenite and arsenate adsorption on ferrihydrite surface charge reduction and net OH release stoichiometry. Environ Sci Technol 33 1179-1184... 

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