title: Secondary Structures of Ubiquitin Ions Soft-Landed onto Self-Assembled Monolayer Surfaces.
abstract: The secondary structures of multiply charged ubiquitin ions soft-landed onto self-assembled monolayer (SAM) surfaces were studied using in situ infrared reflection-absorption spectroscopy (IRRAS). Two charge states of ubiquitin, 5+ and 13+, were mass selected separately from a mixture of different charge states produced by electrospray ionization (ESI). The low 5+ charge state represents a nativelike folded state of ubiquitin, while the high 13+ charge state assumes an extended, almost linear conformation. Each of the two charge states was soft-landed onto a CH3- and COOH-terminated SAM of alkanethiols on gold (HSAM and COOH-SAM). HSAM is a hydrophobic surface known to stabilize helical conformations of soft-landed protonated peptides, whereas COOH-SAM is a hydrophilic surface that preferentially stabilizes β-sheet conformations. IRRAS spectra of the soft-landed ubiquitin ions were acquired as a function of time during and after ion soft-landing. Similar to smaller peptide ions, helical conformations of ubiquitin are found to be more abundant on HSAM, while the relative abundance of β-sheet conformations increases on COOH-SAM. The initial charge state of ubiquitin also has a pronounced effect on its conformation on the surface. Specifically, on both surfaces, a higher relative abundance of helical conformations and a lower relative abundance of β-sheet conformations are observed for the 13+ charge state compared to the 5+ charge state. Time-resolved experiments indicate that the α-helical band in the spectrum of the 13+ charge state slowly increases with time on the HSAM surface and decreases in the spectrum of the 13+ charge state on COOH-SAM. These results further support the preference of the hydrophobic HSAM surface toward helical conformations and demonstrate that soft-landed protein ions may undergo slow conformational changes during and after deposition.
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Qichi
authors.initials: Q
authors.lastname: Hu
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Julia
authors.initials: J
authors.lastname: Laskin
mesh_terms: D006046:Gold / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D007477:Ions / Q000737:chemistry; D017433:Protein Structure, Secondary; D017550:Spectroscopy, Fourier Transform Infrared; D013499:Surface Properties; D025801:Ubiquitin / Q000737:chemistry*
chemical_list: D007477:Ions; D025801:Ubiquitin; D006046:Gold