ARTICLE pubs.acs.org/JPCB
Forced Unbinding of Individual UreaAminotriazine Supramolecular Polymers by Atomic Force Microscopy: A Closer Look at the Potential Energy Landscape and Binding Lengths at Fixed Loading Rates Anika Embrechts,† Holger Sch€onherr,*,‡ and G. Julius Vancso* Department of Materials Science and Technology of Polymers and MESA+ Institute for Nanotechnology, University of Twente, Post Office Box 217, 7500 AE Enschede, The Netherlands ABSTRACT: Atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) was used to study the forced unbinding of quadruple self-complementary hydrogen-bonded ureaaminotriazine (UAT) complexes in hexadecane (HD). To elucidate the bond strength of individual linkages the unbinding forces of UAT supramolecular polymers were investigated for the first time. The bond rupture was probed at three different, fixed piezo retraction rates in far from equilibrium conditions. The number of supramolecular bonds (N) between AFM tip and the surface was determined by independent knowledge of the linker length. The observed rupture force of ureaaminotriazine (UAT)-based supramolecular polymer chains was found to decrease with increasing rupture length. The dependence of the most probable rupture force on N was in quantitative agreement with the theory of uncooperative bond rupture for supramolecular linkages switched in series. Experiments with three different, fixed loading rates provided identical values (within the experimental error) for the characteristic bond length xβ and the off-rate constant in the absence of force koff(f = 0). The value of xβ was found to agree with literature data on the hydrogenbond distance obtained via crystallographic data of the hydrogen-bonded dimer. This work broadens the scope of our previous report showing that relevant parameters of the bond energy landscape can be derived from a single data set of rupture events at a fixed loading rate for supramolecular linkages switched in series.
’ INTRODUCTION Self-assembly of supramolecular structures into higher-order hierarchical assemblies, for instance, mediated by hydrogen bonding, has received tremendous attention in nanotechnology, sensing, biochemistry, and biophysics. Hydrogen-bonded polymers, which are abundant in nature, are also key to the development of synthetic self-healing materials.1 Such materials, which are based on the directional recognition of designed hydrogenbonded moieties, may provide an avenue toward long-lasting self-repairing materials. In the last two decades, advanced nanoscale characterization tools have emerged that enable one to study the underlying spontaneous self-assembly processes on the molecular level.2 These techniques comprise, among others, optical3/magnetic tweezers,4 the biomembrane force probe5 and atomic force microscopybased single-molecule force spectroscopy (AFM-SMFS).2,6,7 The corresponding studies provided astonishing new insights into the structure and function of many proteins, as well as their folding mechanisms, as was recently demonstrated by the groups of Gaub7 and Samori.8 To obtain a more thorough insight into the function and strength of hydrogen-bonded systems on a molecular level, we synthesized supramolecular model compounds and studied their bond strength by AFM-SMFS. In particular, we investigated the bond strength of 2-ureido-4[1H]-pyrimidinone (UPy)9 dimers as well as a r 2011 American Chemical Society
ureaaminotriazine (UAT)-based complementary quadruple hydrogen-bonded motif in hexadecane (HD) on a molecular level,10 using the loading rate dependence of rupture forces described by the KramersBellEvans model.11 In general, the KramersBellEvans approach (eq 1) is applied to study the potential energy landscape via the loading rate dependence of the most probable rupture force f * of ligandreceptor interactions: ! rf ð1Þ f ¼ fβ ln rf 0 From the thermal scale force fβ, the loading rate rf, and the loading rate at zero force rf0, the bond lifetime toff(0) [or the offrate koff(0) = 1/toff(0)] can be determined: toff ð0Þ ¼
fβ rf 0
ð2Þ
More recently we reported on a new approach, which directly yields all parameters of the potential energy landscape on the basis of only one single data set acquired at one fixed loading rate.12 Received: September 16, 2011 Revised: November 13, 2011 Published: November 16, 2011 565
dx.doi.org/10.1021/jp2089752 | J. Phys. Chem. B 2012, 116, 565–570
The Journal of Physical Chemistry B
ARTICLE
Scheme 1. Synthesis of Bis(UAT)PEG DonorAcceptorDonorAcceptor Linker for Formation of Supramolecular HydrogenBonded Polymers Based on UreaAminotriazine Units
This approach, which was applied to supramolecular polymers of UPy in HD,9 relies on the extended Evans model for ligand receptor interactions switched in series.13 Williams and Evans13 predicted the decrease of f * as a function of the numbers of linkers N switched in series for uncooperative bond rupture: fβ ln N f ¼ fβ ðln rf ln N ¼ fsingle
ð3Þ
Using computer simulations, Fugman and Sokolov14 also reported the nonmonotonic dependence of polymer rupture force for small N. Here we report on the validation of this approach and the determination of the characteristic bond length in UAT complexes based on an extensive study of the unbinding forces of UAT-based self-complementary supramolecular hydrogen-bonding polymers in HD. In addition to probing UAT supramolecular polymers, the independence of the estimated parameters that characterize the corresponding energy landscape from the choice of the (fixed) loading rate is shown.
Figure 1. Schematic of AFM-SMFS of supramolecular UAT polymers in hexadecane. A gold-coated surface and a gold-coated AFM probe are functionalized with PEG 1 and PEGUAT 1 + 2 moieties. The bifunctional UATPEG linker 3 is added to the solution to form supramolecular polymers.
vacuum for several hours and stored under argon atmosphere. Isolated yield: 66.4% (1.40 g). 1H NMR (300 MHz, CDCl3): δ 10.29 (s, 2H, NH/N H-bond), 9.90 (s, 2H, NH/N intra H-bond), 9.33 [s, 2H, NH (from NH2)/N H-bond], 8.20 (d, 4H, ortho), 7.607.47 (t, 6H, meta, para), 5.63 (s, 2H, HNH), 4.21 [t, 4H, (CdO)-O-CH2], 3.693.58 (m, CH2 PEG), 3.42 [t, 4H, (CdO)-NH-CH2 PEG], 3.13 [t, 4H, (CdO)-NH-CH2 UAT], 1.71.3 [m, 16H, OdCdNCH2-(CH2)4-CH2-NH]. Mass (MALDI-TOF MS): m/z = 1961.9 (M, PEG Mw = 1251.5 g/mol, UAT Mw = 355 g/mol (2), PEG(UAT)2 Mw = 1961.5). FT-IR: 3500 (ν(OH) H‑bond, w), 3400 (νNH,1oamine, w), 33003200 (νNH,2oamine, w), 29592882 (νCH2, w), 1739 (νCdO ester, w), 1711 (νCdO, w), 1630 (νCdO amide, w), 1543 (νNH,amide II, w), 14541466 (νCH2, m), 1280 (νCO/CN, w), 1240 (νCO, w), 1239 (ν COC, w), 1101 (νCOC, s), 1060 (νCOC, s), 952 (νCH2, m), 841 (νCOC, νCH2, m), and 528 cm1 (νCH2, w). Sample Preparation. Gold substrates (200 nm gold on top of 3.5 nm Ti deposited onto glass substrates) were purchased from Ssens BV (Hengelo, The Netherlands). Prior to use, these substrates were cleaned in piranha solution [2:1 H2SO4 (Sigma Aldrich)/H2O2 (30%, Fluka) by volume], then rinsed three times with Milli-Q water and ethanol, and subsequently rinsed with dichloromethane and chloroform, respectively. The samples were directly transferred to the monolayer solution, preventing any direct contact with air. Caution: Piranha solution should be handled with extreme caution. It has been reported to detonate unexpectedly. On gold substrates, self-assembled monolayers (SAMs)
’ MATERIALS AND METHODS Materials. Anhydrous hexadecane (purity g99%, water
