Formation, stability and fragmentation of biomolecular clusters in a supersonic jet investigated with nano‐ and femtosecond laser pulses

Abstract

AbstractGas phase reactions have often been discussed to play a role in the MALDI ionisation mechanism. Therefore the cluster formation of several typical MALDI matrix substances like ferulic acid (= 4‐hydroxy‐3‐methoxy‐cinnamic acid), vanillic acid (= 4‐hydroxy‐3‐methoxy‐benzoic acid), sinapic acid (= 3,5‐dimethoxy‐4‐hydroxy‐cinnamic acid) and 2,5‐dihydroxy‐benzoic acid was investigated after laser desorption into a supersonic beam of argon by means of time‐of‐flight mass spectrometry. Not only the formation of homogeneous matrix clusters but also that of heterogeneous clusters with some small amino acids was studied. The different neutral clusters formed in the supersonic expansion were ionised by a multiphoton process employing either nano‐ or femtosecond laser pulses. Ionisation conditions were chosen to avoid direct ionisation of the small, non aromatic amino acids. Consequently the ionisation process of the neutral clusters is determined by the MALDI matrix compounds provided with an aromatic chromophore.Strong differences in the detection of cluster ions due to the laser pulse length employed for multiphoton ionisation were observed. Only femtosecond activation led to mass spectra with intense signals of the cluster ions. In addition in the case of femtosecond ionisation protonated amino acids were detected in the mass spectra. As direct ionisation of the free amino acids is not possible under the chosen ionisation conditions because they lack an adequate chromophore these protonated amino acids are assumed to be formed via an intracluster proton transfer in the heterogeneous dimer and subsequent decay of the ionised cluster (dissociative proton transfer). Such well‐known processes for heterogeneous clusters consisting of a substituted aromatic molecule and small polar solvent molecules may be involved in the MALDI ionisation process.