Effect of the Surface on Charge Reduction and Desorption Kinetics of Soft Landed Peptide Ions

Charge reduction and desorption kinetics of ions and neutral molecules produced by soft-landing of mass-selected singly and doubly protonated Gramicidin S (GS) on different surfaces was studied using time dependant in situ secondary ion mass spectrometry (SIMS) integrated in a specially designed Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) research instrument. Soft-landing targets utilized in this study included inert self-assembled monolayers (SAMs) of 1-dodecane thiol (HSAM) and its fluorinated analog (FSAM) on gold and hydrophilic carboxyl-terminated (COOH-SAM) and amine-terminated (NH 2-SAM) surfaces. We observed efficient neutralization of soft-landed ions on the COOH-SAM surface, partial retention of only one proton on the HSAM surface, and efficient retention of two protons on the FSAM surface. Slow desorption rates measured experimentally indicate fairly strong binding between peptide molecules and SAM surfaces with the binding energy of 20–25 kcal/mol. Detailed study of charge reduction of peptide ions deposited on different surfaces showed that neutralization efficiency increases in the order FSAM