Temperature of peptide ions generated by matrix-assisted laser desorption ionization and their dissociation kinetic parameters

Abstract

Product ion yields in postsource decay and photodissociation at 193 and 266 nm were measured for some peptide ions without a basic amino acid residue ([Y6+ H]+, [F5 + H]+, and [YPFVEPI + H]+) generated by matrix-assisted laser desorption ionization (MALDI). Data indicated statistical nature for the dissociation processes. Assuming that peptide ions formed by MALDI are in thermal equilibrium at temperature T and that their dissociation rate constants are specified by the critical energy (E0) and entropy (ΔSN), a method based on kinetic analysis was devised to determine these parameters simultaneously. The matrix used was found to affect the effective temperature of peptide ions, 2,5-dihydroxybenzoic acid (400-430 K) < sinapinic acid (440 K) < α-cyano-4-hydroxycinnamic acid (460-510 K), in agreement with previous perceptions. E0 of around 0.6 eV and ΔSN of -24 eu were smaller than previous quantum chemical results for small model peptide ions.