This review is focused on what has been learned in recent research studies concerned with fundamental aspects of soft-landing and reactive landing of peptide ions on self-assembled monolayer surfaces (SAMs). Peptide ions are particularly attractive model systems that provide important insights on the behavior of soft landed proteins, while SAMs provide a convenient and flexible platform for tailoring the interfacial properties of metals and semiconductor surfaces. Deposition of mass-selected ions on surfaces is accompanied by a number of processes including charge reduction, neutralization, covalent and non-covalent binding, and thermal desorption of ions and molecules from the substrate. Factors that affect the competition between these processes are discussed.

Soft landing of mass-selected peptide ions onto reactive self-assembled monolayer surfaces (SAMs) was performed using a newly constructed ion deposition apparatus. SAM surfaces before and after soft landing were characterized ex situ using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and infrared reflection-absorption spectroscopy (IRRAS). We demonstrate that reactive landing (RL) results in efficient covalent linking of lysine-containing peptides onto the SAM of N-hydroxysuccinimidyl ester-terminated alkylthiol on gold (NHS-SAM). Systematic studies of the factors that affect the efficiency of RL revealed that the reaction takes place upon collision and is promoted by the kinetic energy of the ion. The efficiency of RL is maximized at ca. 40 eV collision energy. At high collision energies the RL efficiency decreases because of the competition with scattering of ions off the surface. The reaction yield is independent of the charge state of the projectile ions, suggesting that peptide ions undergo efficient neutralization upon collision. Chemical and physical properties of the SAM surface are also important factors that affect the outcome of RL. The presence of chemically reactive functional groups on the SAM surface significantly improves the reaction efficiency. RL of mass- and energy-selected peptide ions on surfaces provides a highly specific approach for covalent immobilization of biological molecules onto SAM surfaces.

The kinetics of charge reduction and desorption of different species produced by soft-landing of mass-selected ions was studied using in situ secondary ion mass spectrometry (SIMS) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The improved SIMS capability described in this work utilizes an in-line 8 keV Cs + ion gun and allows us to interrogate the surface both during the ion deposition and after the deposition is terminated. As a model system doubly protonated ions of Gramicidin S were deposited onto a fluorinated self-assembled monolayer (FSAM) surface. Our results demonstrate for the first time that various peptide-related peaks in FT-ICR SIMS spectra follow very different kinetics. We obtained unique kinetics signatures for doubly protonated, singly protonated and neutral peptides retained on the surface and followed their evolution as a function of time. The experimental results are in excellent agreement with a kinetic model that takes into account charge reduction and thermal desorption of different species from the surface.

Covalent immobilization of peptides on solid supports plays an important role in biochemistry with applications ranging from characterization of molecular recognition events at the amino acid level and identification of biologically active motifs in proteins to development of novel biosensors and substrates for improved cell adhesion. 1 Self-assembled monolayers (SAMs) provide a simple and convenient platform for tailoring chemical properties of surfaces. 2 Existing techniques for linking peptides to SAMs are based on solution-phase synthetic strategies and require relatively large quantities of purified material. 3 Here, we report a novel approach for highly selective covalent binding of peptides to SAMs using soft-landing (SL) of mass-selected ions. SL is defined as intact deposition of ions onto suitable substrates at hyperthermal (<100 eV) energies. 4,5 Recent studies have demonstrated that SAMs are excellent deposition targets for SL owing to their ability to dissipate kinetic energies of the projectiles and their efficiency in trapping captured species. 4,6,7 It has been proposed that SL could be utilized for controlled preparation of protein arrays. 5 This study is the first account of covalent immobilization of massselected peptides on SAM surfaces using SL. We demonstrate efficient reactive landing (RL) 8-10 of several model peptides onto the SAM of N-hydroxysuccinimidyl ester terminated alkylthiol on gold (NHS-SAM). This method introduces exceptional selectivity and specificity into the surface preparation step by eliminating the effect of solvent and sample contamination on the quality of the film. In addition, the ability to focus and direct an ion beam at selected spots on the surface with variable sizes allows controllable fabrication of peptide patterns. RGD sequence is by far the most effective and most frequently employed peptide motif for stimulated cell adhesion on synthetic surfaces. 11 It mediates cell adhesion by binding cellular integrin receptors. Covalent linking of RGD peptides to the substrate is essential for strong cell adhesion. Current strategies for preparation of such surfaces have been recently reviewed. 12 In this study we used cyclic pentapeptide c(-RGDfK-) (f ) D-phenylalanine) as a model system.NHS-SAM reacts with accessible primary amino groups in proteins or peptides by forming amide bonds. In the case of c(-RGDfK-), this reaction most likely involves the -amino group of the lysine side chain. 13 The experiment utilized the newly designed and constructed SL apparatus described elsewhere. 14 Briefly, doubly protonated, [M + 2H] 2+ , ions of c(-RGDfK-) (m/z ) 302.7) produced in an electrospray ionization (ESI) source are introduced into the vacuum system using an electrodynamic ion funnel, efficiently focused in a collisional quadrupole, mass selected using a quadrupole mass filter, and transferred to the surface through an electrostatic bending quadrupole and a series of einzel lenses. The optimized current of mass-selected ions delivered to the surface is 40 pA correspondin...

A new ion deposition apparatus was designed and constructed in our laboratory. Our research objectives were to investigate interactions of biomolecules with hydrophilic and hydrophobic surfaces and to carry out exploratory experiments aimed at highly selective deposition of spatially defined and uniquely selected biological molecules on surfaces. The apparatus includes a high-transmission electrospray ion source, a quadrupole mass filter, a bending quadrupole that deflects the ion beam and prevents neutral molecules originating in the ion source from impacting the surface, an ultrahigh vacuum (UHV) chamber for ion deposition by soft landing, and a vacuum lock system for introducing surfaces into the UHV chamber without breaking vacuum. Ex situ analysis of surfaces following soft landing of mass-selected peptide ions was performed using 15 keV Ga+ time-of-flight secondary ion mass spectrometry and grazing incidence infrared reflection-absorption spectroscopy. It is shown that these two techniques are highly complementary methods for characterization of surfaces prepared with a range of doses of mass-selected biomolecular ions. We also demonstrated that soft landing of peptide ions on surfaces can be utilized for controlled preparation of peptide films of known coverage for fundamental studies of matrix effects in SIMS.