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A density functional theory (DFT) study on gas-phase proton transfer reactions of derivatized and underivatized peptide ions generated by matrix-assisted laser desorption ionization
Authors:Francesco L. Brancia  Mauro Stener  Alessandra Magistrato
Affiliation:1. Shimadzu Research Laboratory, Trafford Wharf Road, Wharfside, M17 1GP, Manchester, UK
2. Dipartimento di Scienze Chimiche, Università di Trieste, Trieste, Italy
3. Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Trieste, Italy
4. CNR-INFM DEMOCRITOS National Simulation Center, Trieste, Italy
5. SISSA/ISAS, Trieste, Italy
Abstract:In this study, classic molecular dynamics (MD) simulations followed by density functional theory (DFT) calculations are employed to calculate the proton transfer reaction enthalpy shifts for native and derivatized peptide ions in the MALDI plume. First, absolute protonation and deprotonation enthalpies are calculated for native peptides (RPPGF and AFLDASR), the corresponding hexyl esters and three common matrices α-cyano-4-hydroxycinnamic acid (4HCCA), 2,5-dihydroxybenzoic acid (DHB), and 6 aza-2-thiothymine (ATT). From the proton exchange reaction calculations, protonation and deprotonation of the neutral peptides are thermodynamically favorable in the gas phase as long as the corresponding protonated/deprotonated matrix ions are present in the plume. Moreover, the gain in proton affinity shown by the ester ions suggests that the increase in ion yield is likely to be related to an easier proton transfer from the matrix to the peptide.
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