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. For example, protein adsorption on substrates is a highly dynamic process that is strongly influenced by Coulomb interactions, hydrogen bonding, and relatively weak noncovalent interactions. These processes are highly dependant on the primary and the highorder structure of the protein and on the properties of the surface and the solvent.It has recently been demonstrated that soft-landing (SL) of mass-selected ions on surfaces is a useful mass spectrometric approach for probing details of this intrinsic behavior of immobilized biological molecules that removes the strong effects of solvents [3][4][5]. SL is defined as intact capture of mass-selected polyatomic ions on substrates [6,7] and has already been successfully used to study charge retention by small closedshell ions [6 -9] and peptide and protein ions deposited onto self-assembled monolayer surfaces (SAMs) [10 -13]. Retention of biological activity (but not charge) by proteins deposited on liquid [10,14] and on plasma treated metal surfaces [15] has also been demonstrated.Recently we reported the first detailed study of the kinetics of desorption and charge reduction following SL of doubly protonated Gramicidin S (GS) onto the FSAM surface [16]. We utilized an in-line 8 keV Cs ϩ ion gun that allowed us to interrogate the surface in situ and in real time using secondary ion mass spectrometry (SIMS). We followed the evolution of the SIMS spectrum as a function of time during and after the deposition of [GS ϩ 2H] 2ϩ and obtained unique kinetics signatures for doubly protonated, singly protonated, and neutral peptides retained on the surface. Using the characteristic ion signatures and a kinetic model we measured rate coefficients for charge reduction and thermal desorption of ions and neutral GS molecules from the surface. An important process established in that work was the instantaneous loss of one or two protons by a fraction of ions colliding with the FSAM surface. Collision induced fast charge reduction is followed by very slow proton loss by ions trapped on the surface. Thermal desorption of ionic and neutral species efficiently competes with the charge reduction process.In this research, we applied the same in situ SIMS tec...
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