High-Energy Fragmentation in Nanophotonic Ion Production by Laser-Induced Silicon Microcolumn Arrays

Abstract

Laser-induced silicon microcolumn arrays (LISMA) exhibit nanophotonic ion production in laser desorption ionization experiments (Walker et al., Angew. Chem., Int. Ed. 2009, 48, 1669) for small-to-medium-size molecules. Although these surfaces are known to promote fragmentation of adsorbates at high laser fluences, the nature, extent, and origin of peptide ion decomposition remains unknown. Here we demonstrate that peptide ions, e.g., bradykinin, leucine enkephalin, angiotensin I, substance P, and various tripeptides, desorbed from these substrates show an increasing degree of fragmentation as the fluence is raised. Compared to other ion fragmentation methods, LISMA shows similarity to high-energy collision activated dissociation (CAD), ion decomposition produced by metastable atom beams, and surface induced dissociation (SID). While some of the produced fragments are associated with high-energy decomposition channels, for example, the abundant a5 fragment produced from singly protonated bradykinin ion, other ions in the same spectra (e.g., the ammonia loss from the protonated bradykinin ion) are predominantly produced by low energy processes. To explore the role of internal energy in the fragmentation of ions produced from LISMA, the survival yields of eight benzyl-substituted benzylpyridinium thermometer ions were also studied as a function of laser fluence and surface derivatization. The corresponding internal energies were determined using the Rice−Ramsperger−Kassel−Marcus formalism. On both native and silane-derivatized surfaces, the thermometer ions showed stable internal energy values over a wide range of laser fluences. This presented a strong contrast to the behavior of the peptides that yielded high-energy fragments at increased fluence. As the thermometer ions did not record an increase in internal energy, the enhanced fragmentation of the peptides was indicative of alternative high-energy mechanisms.