Selective deletion of the internal lysine residue from the peptide sequence by collisional activation

Pomeranz-Fritsch Synthesis of Isoquinoline: Gas-Phase Collisional Activation Opens Additional Reaction Pathways

We have investigated the gas-phase production of isoquinoline by performing collisional activation on benzalaminoacetal, the first intermediate in the classic solution-phase Pomeranz-Fritsch synthesis of isoquinoline. We have elucidated the reaction pathways in the gas phase using tandem mass spectrometry. Unlike the corresponding condensed-phase reaction, where catalytic proton exchange between intermediate(s) and solvent (Brønsted-Lowry base) is known to drive the reaction, the gas-phase reaction follows the "mobile proton model" to form the products via a number of intermediates, some the same as in their condensed-phase counterparts. Energy-resolved mass spectrometry, deuterium labeling experiments, and theoretical calculations (B3LYP/6-31G**) identified 27 different reaction routes in the gas phase, forming a complex interlinked reaction network. The experimental measurements and theoretical calculations confirm the proton hopping onto different basic sites of the precursors and intermediates to transform them ultimately into isoquinoline.

Syntheses of Isoquinoline and Substituted Quinolines in Charged Microdroplets

A Pomeranz-Fritsch synthesis of isoquinoline and Friedländer and Combes syntheses of substituted quinolines were conducted in charged microdroplets produced by an electrospray process at ambient temperature and atmospheric pressure. In the bulk phase, all of these reactions are known to take a long time ranging from several minutes to a few days and to require very high acid concentrations. In sharp contrast, the present report provides clear evidence that all of these reactions occur on the millisecond timescale in the charged microdroplets without the addition of any external acid. Decreasing the droplet size and increasing the charge of the droplet both strongly contribute to reaction rate acceleration, suggesting that the reaction occurs in a confined environment on the charged surface of the droplet.