Charged residue mechanism

Charge-residue mechanism One of the two mechanisms used to account for the production of ions by electrospray ionization. 

Charged residue mechanism. This was proposed by Dole and suggests that after multiple coloumbic explosions droplets are formed, which contain only one ion [10]. This mechanism is thought to be important for the ionisation of macromolecules. 

Two models can explain the events that take place as the droplets dry. One was proposed by Dole and coworkers and elaborated by Rollgen and coworkers [7] and it is described as the charge residue mechanism (CRM). According to this theory, the ions detected in the MS are the charged species that remain after the complete evaporation of the solvent from the droplet. The ion evaporation model affirms that, as the droplet radius gets lower than approximately 10 nm, the emission of the solvated ions in the gas phase occurs directly from the droplet [8,9]. Neither of the two is fully accepted by the scientific community. It is likely that both mechanisms contribute to the generation of ions in the gas phase. They both take place at atmospheric pressure and room temperature, and this avoids thermal decomposition of the analytes and allows a more efficient desolvation of the droplets, compared to that under vacuum systems. In Figure 8.1, a schematic of the ionization process is described. 

Until now two different mechanisms have been proposed to give a rationale for the formation of ions from small charged droplets. The first recently was discussed by Cole (Cole, 2000) and Kebarle and Peschke (Kebarle and Peschke, 2000). It describes the process as a series of scissions that lead at the end to the production of small droplets having one or more charges, but only one analyte molecule. When the last few solvent molecules evaporate, the charges are localized on the analyte substructure, which give rise to the most stable gas-phase ion. This model is usually called the charged residue mechanism (CRM) (see lower part of Fig. 1.3). 

Figure 1.3. Mechanisms proposed for the formation of ions from small charged droplets ion evaporation mechanism (IEM) and charge residue mechanism (CRM).

Carbonyl compounds 130,131 Charged residue mechanism 19,20 Chemical Ionization 12 Chitinases 323... 

For the formation of the gaseous analyte, two mechanisms are discussed. The charged residue mechanism (CRM) proposed by Cole [58], Kebarle and Peschke [59], and the ion evaporation mechanism (lEM) postulated by Thomason and Iribarne [60]. In CRM, the droplets are reduced as long as only one analyte in the microdroplets is present, then one or more charges are added to the analyte. In lEM, the droplets are reduced to a so-called critical radius (r < 10 nm)... 

Independently Fernandez de la Mora [42] was able to show that the empirical relationship (Eq. (1.13)) holds and that this relationship can be derived on the basis of the charged residue mechanism. The plot shown in Figure 1.9 is based on data from the literature used by Fernandez de la Mora, but also includes the data of Standing et al. [41b]... 

The agreement of Eq. (1.15) with the observed charges Z can be considered as very strong evidence that globular proteins and protein complexes are produced by the charged residue mechanism. A recent compilation of data by Heck and coworkers [45] has shown that the square root dependence of the charge Zon M (see Eq. (1.15a)) also holds for protein complexes. 

In summary, the Charged Residue Mechanism has allowed quantitative predictions of the protein charge state in the gas phase and is well supported for large proteins of widely varying mass. It is likely that it will be impoiiant also for the analysis of larger supramolecular and polymeric systems. 

Fernandez de la Mora, J. (2000) Electrospray ionization of large multiply charged species proceeds via Dole s charged residue mechanism. Arud. Chim. Acta, 406, 93-104. 

It is generally accepted that low molecular weight ions are liberated into the gas phase through the ion evaporation mechanism [5], while larger ions form by charged residue mechanism [8]. The third model appears well demonstrated in the ionization of lipid species. 

The agreement of Eqs. 1.14 and (14a) with the observed charges Z can be considered as strong evidence that globular proteins and protein complexes are produced by the charged residue mechanism. 

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