Interaction of the Hydrophobic Tip of an Atomic Force Microscope with Oligopeptides Immobilized Using Short and Long Tethers

TitleInteraction of the Hydrophobic Tip of an Atomic Force Microscope with Oligopeptides Immobilized Using Short and Long Tethers
Publication TypeJournal Article
Year of Publication2016
AuthorsMa, CD, Acevedo-Velez, C, Wang, C, Gellman, SH, Abbott, NL
JournalLangmuir
Volume32
Pagination2985-95
Date PublishedMar 29
ISBN Number0743-7463
KeywordsAdhesiveness, Cyclohexanes/*chemistry, Mechanical Phenomena, Microscopy, Atomic Force/instrumentation, Oligopeptides/*chemistry, Polyethylene Glycols/*chemistry, Protein Structure, Secondary, Succinimides/*chemistry, Surface Properties
Abstract

We report an investigation of the adhesive force generated between the hydrophobic tip of an atomic force microscope (AFM) and surfaces presenting oligopeptides immobilized using either short ( approximately 1 nm) or long ( approximately 60 nm) tethers. Specifically, we used either sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SSMCC) or 10 kDa polyethylene glycol (PEG) end-functionalized with maleimide and N-hydroxysuccinimide groups to immobilize helical oligomers of beta-amino acids (beta-peptides) to mixed monolayers presenting tetraethylene glycol (EG4) and amine-terminated EG4 (EG4N) groups. When SSMCC was used to immobilize the beta-peptides, we measured the adhesive interaction between the AFM tip and surface to rupture through a single event with magnitude consistent with the interaction of a single beta-peptide with the AFM tip. Surprisingly, this occurred even when, on average, multiple beta-peptides were located within the interaction area between the AFM tip and surface. In contrast, when using the long 10 kDa PEG tether, we observed the magnitude of the adhesive interaction as well as the dynamics of the rupture events to unmask the presence of the multiple beta-peptides within the interaction area. To provide insight into these observations, we formulated a simple mechanical model of the interaction of the AFM tip with the immobilized beta-peptides and used the model to demonstrate that adhesion measurements performed using short tethers (but not long tethers) are dominated by the interaction of single beta-peptides because (i) the mechanical properties of the short tether are highly nonlinear, thus causing one beta-peptide to dominate the adhesion force at the point of rupture, and (ii) the AFM cantilever is mechanically unstable following the rupture of the adhesive interaction with a single beta-peptide. Overall, our study reveals that short tethers offer the basis of an approach that facilitates measurement of adhesive interactions with single molecules presented at surfaces.

DOI10.1021/acs.langmuir.5b04618