Synthetic oligorotaxanes exert high forces when folding under mechanical load

Transcription

1 SUPPLEMENTARY INFORMATION Letters In the format provided by the authors and unedited. Synthetic oligorotaxanes exert high forces when folding under mechanical load Damien Sluysmans 1, Sandrine Hubert 1, Carson J. Bruns 2, Zhixue Zhu 2, J. Fraser Stoddart 2 and Anne-Sophie Duwez 1 * 1 UR Molecular Systems, Department of Chemistry, University of Liège, Liège, Belgium. 2 Department of Chemistry, Northwestern University, Evanston, IL, USA. * asduwez@uliege.be Nature Nanotechnology Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

2 Supplementary Information for Synthetic oligorotaxanes exert high forces when folding under mechanical load Damien Sluysmans, Sandrine Hubert, Carson J. Bruns, Zhixue Zhu, J. Fraser Stoddart, and Anne-Sophie Duwez* correspondence to: Materials and methods 1. Oligorotaxanes synthesis S2 2. Immobilization of the oligorotaxanes molecules on Au/Si substrates S2 3. AFM force experiments S2 4. Data analysis: Rupture force and L c distributions S5 5. Fluctuations at higher loading rates S7 References S8

3 Materials and Methods 1. Oligorotaxanes synthesis The [5]rotaxanes were synthesized according to a protocol described recently 1. Briefly, this one-pot synthesis uses a copper-catalyzed azide-alkyne cycloaddition to thioctic esterfunctionalized stoppers at both ends of the DNP-derived polyether chains in the presence of CBPQT 4+ rings. Using conventional chromatographic techniques, oligorotaxanes with half the DNP units encircled by a ring were isolated. The molecules were characterized by NMR spectroscopy and mass spectrometry Immobilization of the oligorotaxanes molecules on Au/Si substrates The oligorotaxanes were grafted onto gold-coated silicon substrates (Au (1nm) / Ti / Si wafer, Sigma-Aldrich) using our previously established method 2 to obtain a sparse regime of the molecule of interest. Substrates were cut (2 2 cm 2 ), cleaned by UV-ozone treatment for 15 min (UV-ozone cleaner, Model 42, Jelight Company Inc.) and dipped in EtOH for 15 min. Just after the cleaning procedure, they were dried by a N 2 flow and dipped for 1h at room temperature in a solution of the [5]rotaxane ( g, mol) and dodecyl sulfide (DDS) (8 1 5 g, mol) in Me 2 CO. DDS is used as passivation layer and to obtain a highly diluted regime of the oligorotaxane on the surface to favor single-molecule force experiments. The functionalized substrate was then rinsed in Me 2 CO to remove physisorbed molecules. 3. AFM force experiments Experiments were carried out with a PicoPlus 55 microscope (Agilent Technologies) equipped with a closed-loop scanner. Gold-coated tips (OBL-1 Biolever, Bruker; nominal spring constant k=.9.1 N m 1 ) were used for all the force experiments. The spring constant of each cantilever was calibrated by the thermal noise and Sader methods 3,4. Practically, before each experiment, we calibrate the cantilevers by the thermal noise method and after the experiment, we systematically use the Sader method, measure the cantilever dimensions by scanning electron microscopy and use the resonance frequency and quality factor to obtain the spring constant value. We only consider experiments for which the spring constant values obtained by both methods are consistent. Before use, the cantilevers were cleaned by UV-ozone (same treatment as for the substrates, see above). Before starting S2

4 the experiments, the cantilever was dipped during 1 h in the solution of measurement for equilibration, away from the surface. The molecules were picked up by gently pressing the AFM tip with a maximum force of 25 pn against the substrate. Force measurements were performed at loading rates between 1 3 and 1 5 pn s 1. Pulling relaxing cycles were realized using a custom-made routine to guide the tip. Each force curve contains 2, data points. When a molecule attaches to the tip during the approach, one can follow its stretching in the retraction curve by observing the cantilever deflection. The deflection versus z-movement curves obtained were transformed into force versus distance curves using the Hooke s law as F = k x=k (z-d) with F the force experienced by the molecule, k the cantilever spring constant, x the distance between the tip and the substrate, z the vertical piezo-movement, and d the deflection of the cantilever. The force curves obtained reflect the restoring mechanical force of the stretched molecule against the cantilever during its extension. Most of the curves showed flat profiles, indicating that no molecule was stretched by the AFM tip. Three percent of total cycles show superimposable sawtooth profiles used for analysis. Short end-grafted molecules are very unlikely to interact through multiple attachments to the tip, or to the surface, in good solvent conditions for two reasons. The first one is related to the length (below 2 nm). It is too short to form loops and trains that interact with surfaces and which are at the origin of classical sawtooth profiles (with random ΔLc) observed for polymers or proteins 5,6. The second one is related to the quality of the solvents used here and the Hamaker-Lifshitz constant of the tip-molecule-substrate system. Molecules in good solvent conditions hardly interact with surfaces (and even long molecules, depending on the quality of the organic solvent and Hamaker-Lifshitz constant). Accordingly, the probability to observe a force profile with our system is only 3% at best. In 97 % of the cases, no molecule is caught by the AFM tip. When no molecule is caught, we frequently observe "escape transitions" in the approach profiles, a clear indication that the molecule has escaped from the tip-surface region when approaching the tip. This criterion is an important one providing evidence that the molecules hardly interact with the surface/tip, described in details by Butt and co-workers 6. If we had investigated much longer molecules and/or in theta or poor solvent conditions, the issue of multiple attachment would have been critical. But even S3

5 in that case, it remains possible to distinguish multiple attachments because in this case the peaks would appear at random ΔL c values. As a control experiment, we added a well-known π-competing compound, 1,5- dimethoxynaphthalene (DMNP), to the DMF solution during the force measurements. This competing compound intercalates between the DNP units and the CBPQT 4+ rings, disrupting the intramolecular interactions. The presence of DMNP has a profound effect on the characteristics of the force profiles, manifesting itself as the appearance of single-peak profiles instead of the characteristic sawtooth pattern. An example is shown in Fig. S1. This kind of profile is the signature of the stretching of a finitely-extensible chain, indicating the loss of the folded (co)-conformations of the oligorotaxane in solution. This tendency is even clearer when increasing the dipping time and concentration of the competing solution. The proportion of single-peak profiles reaches 43 % after 8 h in DMNP 5 µm, and increases to 54 % after 16 h in DMNP 5 µm and to 7 % after 4 h in DMNP 5 µm. 5 4 Force (pn) Distance (nm) 4 5 Figure S1. Stretching of a single oligorotaxane in π-competing conditions. Example of a single-peak pattern obtained on a single [5]rotaxane in a 1 2 M DMNP solution in DMF (loading rate of 1 4 pn s 1 (35 nm s -1 )). For pulling-relaxing experiments, before and after every cycle, a few curves in which no molecule has been stretched have been selected as references. From the comparison between these measurements possible drift could be identified. The baseline of the curve before the cycle is used as the zero force value for the first curve of the cycle; the stretching profiles of the successive curves are superimposed and the consistency of the proposed zero force value is tested on the last curve of the cycle, when the molecule is lost and the force drops to zero. The zero extension is the reference position of the piezo (contact point) and is evidenced by the change in the slope of the force extension curve that becomes vertical. The procedure to identify the zero extension during the cycle is analogous to the one used to identify the zero S4

6 force: the zero length position identified in the last curve before the cycle is assigned to the first curve of the cycle, and the stretching profiles of the curves of the cycle are superimposed and the consistency of the proposed position of the zero length is tested in the first curve after the cycle. 4. Data analysis: Rupture force and L c distributions The force curve analyses were performed with IgorPro (WaveMetrics) using customized routines. The selection of the force-distance curves was done manually in order to scan all possible features. To correlate the position of the peaks appearing in the multi-peak profiles with the structure of the molecules, worm-like chain (WLC) fits were used. The WLC model 5 predicts the relationship between the extension of an individual linear and flexible polymer chain and its entropic restoring force. The force required to extend a WLC with a persistence length l p and contour length L to a distance D is given by: F D = k %T l ( D L + 1 4(1 D 1 4 L ) with k B the Boltzmann constant and T the temperature. All the peaks were fitted with a persistence length of.3 nm. The contour length increment ( L c ) between successive peaks is related to the hidden length released after the breaking of one intramolecular interaction. Rupture forces of these multi-peaks were also collected. The rupture force and L c data obtained from the experiments were analyzed using customized routines with IgorPro (WaveMetrics) and MatLab (MathWorks). The rupture force data were also fitted using a Kernel smoothing function (N=1 points). The superimposition of this probability density function (PDF) with the histogram showed a distinct tail to the right (towards high force). This feature is common in rupture force measurements and it has been explained invoking the pulling geometry or heterogeneity in the chemical bonding and dynamical disorder 7,8. L c histograms and PDF were constructed. In order to obtain a deconvolution of these multiple populations in the distribution, we fit the recorded data with a Gaussian mixture model (GMM), that is, a weighted sum of M component Gaussian densities as given by: : P x λ = p 5 G x μ 5, σ 5 5;< S5

7 where x is the vector of the observed L c values, G x μ 5, σ 5 is the normalized Gaussian component with mean µ i and variance σ i and p i is the weight of the i th component. The : weights satisfy the normalization condition 5;< p 5 = 1, and λ represents the set of all the parameters λ = p 5, μ 5, σ 5 for i=1,2,3 or 4. In practice, we first determined the optimal number of Gaussian components to fit the data sets by calculating the Bayesian (BIC) and the Akaike Information Criterion (AIC) 9 for models with different numbers of components. For each estimated average rupture force or L c, the 95% confidence interval was computed as ±1.96 σ (p 5 N) where σ is the estimated variance of the i th Gaussian component while p 5 N represents the effective size of the population. Rupture force values were also compiled in histograms (Fig. S2) and show a most probable population between 1 and 15 pn, consistent with the breaking of weak interactions such as electrostatic interactions in a high dielectric medium, H-bonds and π-stacks. Probability density Force (pn) 4 5 Figure S2. Distribution of the rupture forces for the [5]rotaxane in DMF. Histogram (bin size = 2 pn) and Gaussian fit are represented. Loading rate: 1 4 pn s 1. The distribution is obtained from 24 force values. The theoretical length increment corresponding to the breaking of the interaction between a naphthalene unit and a bipyridinium dication can be estimated using the X-ray crystallographic data obtained from crystals prepared from a solution of oligopseudorotaxanes (Fig. S3) 1,11. The distance between two successive naphthalene units in the serpentine-like S6

8 folded conformation was shown to be.7 nm (two π-stacks on both sides of a bipyridinium ion). For an extended conformation (not folded), we estimate this distance at 1.9 nm (four ethylene oxide units (4.35 nm) plus four C C/C O bonds linking the PEO with naphthalene units (4.13 nm)) 12. The length increment between the folded and unfolded units thus comes to 1.2 nm ( = 1.9 nm.7 nm). Figure S3. Side-view (top) and top-view (bottom) of a ball-and-stick representation of a oligopseudorotaxane obtained from X-ray crystallographic data Fluctuations at higher loading rates Force (pn) 4 2 Force (pn) Distance (nm) Distance (nm) Figure S4. Fluctuations between folded and unfolded states during the pulling experiments. Force-distance curves obtained for the oligorotaxane in DMF at 75 pn s 1 (left) and 1, pn s 1 (right), showing fluctuations between two distinct co-conformations (grey circles). S7

9 Probability density x1 3 Number of fluctuations (s -1 ) Figure S5. Distribution of the number of fluctuations observed in the transition region during the pulling of the oligorotaxane in DMF at 1 5 pn s 1. The gaussian fit is centered on 4293 ± 596 fluctuations s 1 (N = 42). References 1. Zhu, Z., Bruns, C. J., Li, H., Lei, J., Ke, C., Liu, Z., Shafaie, S., Colquhoun, H. M. & Stoddart, J. F. Synthesis and solution-state dynamics of donor acceptor oligorotaxane foldamers. Chem. Sci. 4, (213) 2. Lussis, P., Svaldo-Lanero, T., Bertocco, A., Fustin, C.-A., Leigh, D. A. & Duwez, A.-S. A single synthetic small molecule that generates force against a load. Nat. Nanotechnol. 6, (211) 3. te Riet, J., Katan, A. J., Rankl, C., Stahl, S. W., van Buul, A. M., Phang, I. Y., Gomez- Casado, A., Schon, P., Gerritsen, J. W., Cambi, A., Rowan, A. E., Vancso, G. J., Jonkheijm, P., Huskens, J., Oosterkamp, T. H., Gaub, H., Hinterdorfer, P., Figdor, C. G. & Speller, S. Interlaboratory round robin on cantilever calibration for AFM force spectroscopy. Ultramicroscopy 111, (211) 4. Sader, J. E., Sanelli, J. A., Adamson, B. D., Monty, J. P., Wei, X., Crawford, S. A., Friend, J. R., Marusic, I., Mulvaney, P. & Bieske, E. J. Spring constant calibration of atomic force microscope cantilevers of arbitrary shape. Rev. Sci. Instrum. 83, 1375, 1 16 (212) S8

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