Ionization, mechanisms, initiating

Karas, M. Bahr, U. Fournier, L Gltick-mann, M. Pfenninger, A. The Initial Ion Velocity As a Marker for Different Desorption-Ionization Mechanisms in MALDI. Int. J. Mass Spectrom. 2003, 226, 239-248. 

The occurrence of the ionization mechanism (eqns. (6) and (18)) in preference to the alternative addition mechanism (eqn. (24)) has been demonstrated by a study of the initiating activities of the series of bases pyridine, a-picoline, 2,6-lutidine [21]. Mechanism (24) is analogous to the normal tertiary base-catalysed acylation mechanism encountered with anhydrides such as acetic anhydride [41]. Steric factors determine the... 

The initial step of the mechanism is the coordination of the first equivalent of the Lewis acid to the carbonyl group of the acylating agent. Next, the second equivalent of Lewis acid ionizes the initial complex to form a second donor-acceptor complex which can dissociate to an acylium ion in ionizing solvents. The typical SsAr reaction gives rise to an aromatic ketone-Lewis acid complex that has to be hydrolyzed to the desired aromatic ketone. 

In an APCI interface the column effluent enters a heated nebulizer where the pneumatically assisted desolvation process is almost completed. While still in the spray chamber, ionization of analytes is initiated by corona discharge. The ionization mechanisms in APCI are almost identical to those in conventional medium pressure chemical ionization (7). Positive ion formation can be achieved by proton transfer, adduct formation or charge exchange reactions, while in the negative mode ions are formed due to proton abstraction, anion attachment and electron capture reactions. The APCI interface is compatible with flow rates exceeding 1 ml/min and will... 

According to the ionization mechanism, if the same carbocation can be generated from more than one precursor, its subsequent reactions should be independent of its origin. But, as in the case of stereochemistry, this expectation must be tempered by the fact that ionization initially produces an ion pair. If the subsequent reaction takes place from this ion pair, rather than from the completely dissociated and symmetrically solvated ion, the leaving group can influence the outcome of the reaction. 

A further consequence of the ionization mechanism is that if the same carbonium ion can be generated from more than one precursor, the subsequent reactions it undergoes should be independent of its origin. As in the just-mentioned case of stereochemistry, our faith in this prediction is tempered by the fact that the ionization produces an ion pair initially, rather than dissociated ions, and the leaving group is sufficiently close to the carbonium ion to influence further reaction if it occurs prior to diffusion apart of the two species. 

But it turned out that this concentration is insufficient to initiate a thunderous discharge -the lightning. A series of auxiliary ionization mechanisms was proposed [6,16-20], but they were hypothetical and therefore could not lead to the satisfactory solution of the problem. 

Affinity mass spectrometry 9 Atmospheric pressure mass spectrometry 4 Desorption/ionization mechanism 2 Enzymatic activities measurement 6 Mass Spectrometry Imaging (MSI) 5 Mass-spectrometry detection 3 Nanostructure-initiator mass spectrometry (NIMS) 5 Porous silicon based mass spectrometry 9... 

Karas, Bahr, and Stahl-Zeng emphasized the multiple functions of the matrix in 1996, especially proton transfer reactions of the matrix with analyte. A focused review of MALDl ionization mechanisms appeared in 1998, " which also expUcitly introduced the concept of distinct primary and secondary ion formation processes. The former are rapid, early events creating the first ions. Initial ions may later undergo secondary reactions in the expanding plume, in a manner similar to the SIMS selvedge concept. ... 

Following the biosynthetic mechanism, initial ionization of GDP to the cation followed by the subsequent addition of the IDP unit forms the FDP cation in a reaction that is catalyzed by FPPS prenyltransferase (Scheme 7.1) [2]. The mechanism is based on the findings that the enzyme, which normally catalyzes the addition of GDP to IDP, is also able to catalyze the hydrolysis of GDP [3]. Deuterium experiments of this hydrolysis process either with D O or with (1S)-[1- H] GDP indicated that C—O bond was broken and the chirality of the C-1 carbon of GDP was inverted in this process. In addition, when trifluoromethyl group was present at the C-3 position or fluoro atom at the C-2 position of the allylic substrate, destabilization of the cation has been witnessed as observed on the retard of enzyme reaction [4]. In the elimination step (Scheme 7.1) hydrogen is removed from C-2 of IDP part with simultaneous formation of a double bond. The formation of a trans or cis double bond during the FPPS reaction depends on the spatial orientation of IDP relative to the elongating FDP. In the tranx-prenyltransferases, the GDP... 

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