Letter: Reactions in molecular clusters: proton transfer to small amino acids

In this paper, intermolecular proton transfer within heterogeneous molecular clusters of amino acids is reported for the first time. These proton transfers were observed in mass spectrometric experiments with ferulic acid (4-hydroxy-3methoxy-cinnamic acid) and small amino acid molecules. It should be noted that ferulic acid is a well-known matrix substance for matrix-assisted laser desorption ionisation (MALDI). 6,7 Therefore, the observation of cluster formation between biomolecular samples and matrix substances and of reactions therein would allow the investigation of internal effects in the MALDI ionisation procedure. The small amino acids, glycine and leucine, were ionised via proton transfer from ferulic acid in heterogeneous dimers formed during laser desorption into a supersonic beam and following release of the protonated amino acid molecules. The heterogeneous dimers were observed in the obtained time-of-flight (TOF) mass spectra, as well as the homogeneous clusters of ferulic