title: Formation of gas-phase peptide ions and their dissociation in MALDI: insights from kinetic and ion yield studies.
abstract: Insights on mechanisms for the generation of gas-phase peptide ions and their dissociation in matrix-assisted laser desorption ionization (MALDI) gained from the kinetic and ion yield studies are presented. Even though the time-resolved photodissociation technique was initially used to determine the dissociation kinetics of peptide ions and their effective temperature, it was replaced by a simpler method utilizing dissociation yields from in-source decay (ISD) and post-source decay (PSD). The ion yields for a matrix and a peptide were measured by repeatedly irradiating a region on a sample and collecting ion signals until the sample in the region was completely depleted. Matrix- and peptide-derived gas-phase cations were found to be generated by pre-formed ion emission or by ion-pair emission followed by anion loss, but not by laser-induced ionization. The total number of ions, that is, matrix plus peptide, was found to be equal to the number of ions emitted from a pure matrix. A matrix plume was found to cool as it expanded, from around 800-1,000 K to 400-500 K. Dissociation of peptide ions along b/y channels was found to occur statistically, that is, following RRKM behavior. Small critical energy (E0  = 0.6-0.7 eV) and highly negative critical entropy (ΔS(‡)  = -30 to -25 eu) suggested that the transition structure was stabilized by multiple intramolecular interactions.
authors.affiliation: Medical Proteomics Research Center, KRIBB, Daejeon, 305-806, Korea.
authors.forename: Jeong Hee
authors.initials: JH
authors.lastname: Moon
authors.forename: Sohee
authors.initials: S
authors.lastname: Yoon
authors.forename: Yong Jin
authors.initials: YJ
authors.lastname: Bae
authors.forename: Myung Soo
authors.initials: MS
authors.lastname: Kim
mesh_terms: D005740:Gases / Q000032:analysis* / Q000737:chemistry; D006801:Humans; D007477:Ions / Q000737:chemistry*; D007700:Kinetics; D010455:Peptides / Q000032:analysis* / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000295:instrumentation / Q000379:methods*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D005740:Gases; D007477:Ions; D010455:Peptides
keywords: MALDI; expansion cooling; ion yield; peptide ion dissociation; peptide ion formation; thermal model