Electron transfer secondary reactions

Several MALDl ionization-relevant papers have appeared in a special issue of the European Journal of Mass Spectrometry (volume 12,2006) Proton transfer reactions in the plume were the object of experimental work by the Kinsel group and calculations by Beran and co-workers. Hoteling et al. considered electron transfer secondary reactions, which are also of relevance for the fullerenes studied with solvent-firee methods by Drewello s group. Finally, cluster ionization and desolavtion processes were investigated by Tabet and CO-workers " Also appearing in 2006 was a mechanisms review by Knochenmuss in The Analyst ... 

In mechanistic matters, it has been demonstrated that co-alkenyl iodides undergo cyclization onto the vinyl function upon treatment with Me2CuLi, in competition with direct substitution. This, as well as the generation of trityl radical in the reaction of Me2CuLi with trityl chloride, constitutes evidence for single electron transfer in reactions of cuprates with iodides (and, to a lesser extent, bromides)16. The intermediacy of alkyl radicals in the substitution process (equation 12) is likely the source of the aforementioned racemization in reactions of secondary iodides4. 

YD Halsey and WW Parson (1974) Identification of ubiquinone as the secondary electron acceptor in the photosynthetic apparatus of Chromatium vinosum. Biochim Biophys Acta 347 404-416 A Vermeglio (1977) Secondary electron transfer in reaction centers of Rhodopseudomonas sphaeroldes. Out-of-phase periodicity of two for the formation of ubisemiquinone and fully reduced ubiquinone. Biochim Biophys Acta. 459 516-524... 

Since matrix is nearly always a key reactant in the plume, it is particularly important to understand matrix ions and their reactivity. Matrix can be involved in proton, electron, and cation transfer secondary reactions. As a result, matrix ions associated with all these reactions can often be observed in the mass spectra. Thermodynamic data for several matrices and their various clusters, fragments, and ionic forms have been accumulating. Experiment and theory are today generally in quite good agreement, see Table 5.1. 

Depending on the mechanism, primary ions may be matrix or analyte, or both. Primary ions are created in a dense environment, perhaps even clusters and particles, but must reach a low-density regime to become available for mass analysis. During the expansion, primary ions react with neutrals to make the secondary ions observed in the mass spectrum. These secondary reactions are the key to understanding many MALDI phenomena and are the final theme to be discussed here. There are three types of charge transfer secondary reactions known in MALDI proton, electron, and cation transfer. The characteristics of each will be discussed below, but aU lead to qualitatively similar effects in MALDI mass spectra. 

The PSII complex contains two distinct plastoquiaones that act ia series. The first is the mentioned above the second, Qg, is reversibly associated with a 30—34 kDa polypeptide ia the PSII cote. This secondary quiaone acceptor polypeptide is the most rapidly tumed-over proteia ia thylakoid membranes (41,46). It serves as a two-electron gate and connects the single-electron transfer events of the reaction center with the pool of free... 

The second part of the database contains reactions for the various secondary species, minerals, and gases. These reactions are balanced in terms of the basis and redox species, avoiding (to the extent practical) electron transfer. Species and minerals containing ferric iron, for example, are balanced in terms of the redox species Fe+++,... 

This scheme implies that dioxygen is activated only in a secondary step by an intermediate formed in the initial electron transfer reaction between the metal ion and a co-substrate, Si- The reduced form of the metal ion is re-oxidized to its original oxidation state by O2, but such a reaction with a secondary intermediate cannot be excluded. If the rate determining step is Eq. (9), the overall reaction is again zeroth order with respect to dioxygen. 

If the provoked or spontaneous acid-base reactions overcome the radical reactions of the primary radical, the secondary radical is easier to reduce, or to oxidize, than the substrate in most cases. Exceptions to this rule are scarce, but exist. They involve substrates that are particularly easy to reduce thanks to the presence of a strongly electron-withdrawing substituent (for reductions, electron-donating for oxidation), which is expelled upon electron transfer, thus producing a radical that lacks the same activation. Alkyl iodides and aryl diazonium cations are typical examples of such systems. 

In the case of stepwise processes, the cleavage of the primary radical intermediate (often an ion radical) may be viewed in a number of cases as an intramolecular dissociative electron transfer. An extension of the dissociative electron transfer theory gives access to the dynamics of the cleavage of a primary radical into a secondary radical and a charged or neutral leaving group. The theory applies to the reverse reaction (i.e., the coupling of a radical with a nucleophile), which is the key step of the vast family of... 

There have, from time to time, been suggestions that electron transfer may mediate in processes which are formally ionic substitution reactions. Whilst such mechanisms have been established for a small number of systems (Kornblum, 1975 Bunnett, 1978), in other instances the evidence is less substantial. For example, dialkylaminyl spin adducts may be observed when secondary amines are allowed to react with picryl chloride in the presence of MNP (Bil kis and Shein, 1974). This can be interpreted in terms of Scheme 12, but alternatives involving nucleophilic addition to the trap merit consideration. 

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

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