Energetics Of Peptide Ion Fragmentation in MALDI/TOF

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We are using matrix-assisted laser desorption ionization (MALDI) of peptides as a model system to study peptide ion fragmentation. Ion energetics relationships between laser fluence and peptide ion fragmentation. This type of study is fundamental to optimizing MALDI TOF/TOF experiments for the purpose of peptide sequencing. In these studies we obtain peptide fragmentation spectra, typically 5000 laser shots, in both the unimolecular decomposition and collision induced dissociation (CID) modes. We have the ability to easily follow two time points for each peptide decomposition, i.e., the in-source fragmentation consisting of ions formed within 1 usec after the laser firing and the longer, mass dependent fragmentation occurring within the instrument's collision cell. We have used the fragmentation of a model peptide, leucine enkephalin, YGGFL, (LeuEnk) over the full range of laser fluence as the basis of the initial studies. While not a peptide of the type normally encountered in protein characterizations, LeuEnk is an excellent model to enable studies of short lived processes in the laser plume. LeuEnk fragmentation spectra have been acquired in both MS and MS-MS modes of operation as a function of laser fluence beginning at the onset of ionization and extending to the maximum fluence available in the instrument. These spectra reveal several distinct processes in LeuEnk fragmentation. First, the MS mode spectra show a region of extensive fragmentation occurring in what must be a very short time frame following the onset of ionization. We have been able to associate these rapid fragmentations, leading to immonium ions, with what is widely accepted to be the laser pulse-induced direct vaporization of molecules from the sample surface. There is a second set of process that take place within the first several hundred nanoseconds following the laser pulse. These processes, also manifest in MS mode, are most likely associated with desorption of LeuEnk ions from particles ablated from the surface. These desorbed ions undergo a large number of collisions with the high temperature gases present in the laser plume, and begin to fragment; these fragmentations proceed in a series of consecutive reactions in which the amide backbone bonds are ruptured. Our spectra show that the initial direct desorption processes reach a maximum extent, and then increase no further, and that the consecutive fragmentation reactions supplant them in intensity. Finally, the MS-MS mode spectra exhibit little fragmentation, most likely due to depletion of the high-energy portions of the energy distributions associated with the second stage, particle desorption processes, described above. We are simultaneously developing a kinetic model for these decompositions using the Rice-Ramsberger-Kassel-Marcus (RRKM) formalism for gas phase kinetics. In addition to modeling fragmentation, the calculations will define a lower limit of the peptide ion temperatures. With this information, we will be able for the first time to estimate the fraction of laser energy delivered to gas phase ions. Furthermore, we will have a means, other than pure empiricism, to select and optimize both MALDI matrix and laser frequency.