FORMATION, STABILITY AND FRAGMENTATION OF BIOMOLECULAR CLUSTERS IN A SUPERSONIC JET INVESTIGATED WITH NANOSECOND AND FEMTOSECOND LASER-PULSES

Gas phase reactions have often been discussed to play a role in the MA LDI ionisation mechanism. Therefore the cluster formation of several t ypical MALDI matrix substances like ferulic acid (= 4-hydroxy-3-methox ycinnamic acid), vanillic acid (= 4-hydroxy-3-methoxy-benzoic acid), s inapic acid (= 3,5-dimethoxy-4-hydroxy-cinnamic acid) and 2,5-dihydrox y-benzoic acid was investigated after laser desorption into a superson ic beam of argon by means of time-of-flight mass spectrometry. Not onl y the formation of homogeneous matrix clusters but also that of hetero geneous clusters with some small amino acids was studied. The differen t neutral clusters formed in the supersonic expansion were ionised by a multiphoton process employing either nano-or femtosecond laser pulse s. 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 pr ovided with an aromatic chromophore. Strong differences in the detecti on of duster ions due to the laser pulse length employed for multiphot on ionisation were observed. Only femtosecond activation led to mass s pectra with intense signals of the cluster ions. In addition in the ca se of femtosecond ionisation protonated amino acids were detected in t he mass spectra. As direct ionisation of the free amino acids is not p ossible under the chosen ionisation conditions because they lack an ad equate chromophore these protonated amino acids are assumed to be form ed 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 b e involved in the MALDI ionisation process.