Adhesion through single peptide aptamers

Abstract

Aptamer and antibody mediated adhesion is central to biological function and is valuable in the engineering of "lab on a chip" devices. Single molecule force spectroscopy using optical tweezers enables direct nonequilibrium measurement of these noncovalent interactions for three peptide aptamers selected for glass, polystyrene, and carbon nanotubes. A comprehensive examination of the strong attachment between antifluorescein 4-4-20 and fluorescein was also carried out using the same assay. Bond lifetime, barrier width, and free energy of activation are extracted from unbinding histogram data using three single molecule pulling models. The evaluated aptamers appear to adhere stronger than the fluorescein antibody under no- and low-load conditions, yet weaker than antibodies at loads above ∼25 pN. Comparison to force spectroscopy data of other biological linkages shows the diversity of load dependent binding and provides insight into linkages used in biological processes and those designed for engineered systems.

Figure 1

Schematics of optical tweezers pulling on a single peptide aptamer molecule linked to a carbon nanotube. The optical trap (red cone) captures a bead (A) which is linked to an aptamer (D) via a DNA molecule (C) and a biotin/streptavidin linkage (B).

Figure 2

Representative force curve showing a rupture event. Rupture forces and loading rates, dashed line, are directly measured from rupture curves.

Figure 3

Rupture-force probability distributions for peptide aptamer binding to a) glass, b) polystyrene and c) carbon nanotubes. Histograms are fit to the model of Evans-Ritchie (dotted line), Hummer-Szabo (gray) and Dudko et al. assuming a cusp shaped (ν=1/2, black solid line) or a linear-cubic (ν=2/3, black dashed line) energy barrier.

Figure 4

Rupture-force probability distributions for fluorescein binding to antibody 4-4-20 at average loading rates of a) 6.1 pN·s⁻¹, b) 11.8 pN·s⁻¹ and c) 24.7 pN·s⁻¹. Histograms are fit to the model of Evans-Ritchie (dotted line), Hummer-Szabo (gray) and Dudko et al. assuming a cusp shaped (ν=1/2, black solid line) or a linear-cubic (ν=2/3, black dashed line) energy barrier.

Figure 5

Energy landscape from parameters fitted using the model of Dudko et al. assuming a cusp shaped (ν=1/2) barrier, with energy wells represented as harmonic potentials.

Figure 6

Lifetime τ(F) as a function of the applied force (F) compared to curves derived from parameters obtained from literature (Table 4) for a range of biological interactions. Kinesin-microtubule interaction is for single headed kinesin in ADP state with plus-end loading. τ(F) is obtained assuming a cusp-like barrier when ΔG^‡ available solid lines; otherwise, Evans-Ritchie model is used with ν=1, dashed lines.