Low-energy collision conditions

The symmetry forbidden character of the 1,3-proton transfer provides the explanation for the absence of methanol loss from the protonated methyl cxo-bicyclo[2.2.2]oct-5-ene-2-carboxylate under low-energy collision conditions, although this elimination is favored from its protonated endo isomer. " Scheme 17.9 shows the origin of the observed stereospecificity. In this example, the double bond is used as proton "acceptor," allowing a reversible 1,5 proton transfer from the endo protonated ester, followed by methanol loss. From the exo ester such release does not occur because this 1,5 proton transfer is not possible and the 1,3-proton transfer pathway is forbidden. Interestingly, in the case of the fra/w-ethyl methyl esters of the bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic acid system, the alcohol loss is also regioselective for the endo ester, although a very minor alcohol release for the exo ester can be detected. 

In the early LC-MS-MS studies, CID is performed with low-energy collisions by means of triple-quadrapole instruments. Under these conditions, series of N-terminal b-ions and C-terminal y-ions result from cleavages at the peptide bond and charge retention on either side, although double-charge tryptic peptide ions tend to favour fragmentation towards more abimdant y-ions. From these ladders of sequence ions in the MS-MS spectrum, the amino-acid sequence of the peptide may be derived. This is the bottom-up protein identification approach (Ch. 18.3.1). 

The possibility to obtain distinct fragmentation patterns under controlled collision energy and gas conditions along with the ability of automatic switching between MS and MS/MS mode by an electrospray QTOF instrument under low-energy collision-induced dissociation (CID) represents a powerful option for high-throughput... 

The desorption flux is so low under these conditions that no gas phase collisions occurred between molecular desorption and LIF probing. Phase space treatments " of final-state distributions for dissociation processes where exit channel barriers do not complicate the ensuing dynamics often result in nominally thermal distributions. In the phase space treatment a loose transition state is assumed (e.g. one resembling the products) and the conserved quantities are total energy and angular momentum the probability of forming a particular flnal state of ( , J) is obtained by analyzing the number of ways to statistically distribute the available (E, J). 

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