Single Molecule Force Spectroscopy of self-complementary hydrogen-bonded supramolecular systems

Atomic Force Microscopy (AFM)-based techniques were used to control, manipulate and study supramolecular hydrogen-bonded dimers and polymers in organic solvents on a single molecule level. Hydrogen-bonded dimers and polymer units were synthesized and self‑assembled monolayer (SAMs) techniques were exploited to study the forces in supramolecular hydrogen-bonded host-guest interactions by single molecule AFM. Furthermore the mechanical properties of single supramolecular polymers were investigated and the relation of bond strength as a function of the number of polymer linkers N was established for supramolecular polymer units linked in series. 

In Chapter 2 the influence of non-covalent interactions and self-assembly in nature is discussed. Synthetic supramolecular systems and polymers are introduced as well as methods to study supramolecular hydrogen-bonded host-guest interactions on a single molecule level. Since examples from nature demonstrated its well-controlled and energy efficient use of non‑covalent interactions and self-assembly, a higher level of understanding is anticipated from the detailed investigation of these molecular forces. This in turn will lead to completely new approaches in the investigation, manipulation and design of nanostructures and self‑assembling systems. 

Chapter 3 describes the single molecule investigation of self-complementary host-guest interactions of ureidopyrimidinone (UPy) dimers and polymers in hexadecane. AFM-based Single Molecule Force Spectroscopy (SMFS) was used to investigate the energy landscape of the hydrogen-bonds and mechanical properties of supramolecular UPy-polymers on a single molecule level. The rupture forces atone fixed loading rate were observed as a function of the number of linkers Nand were in quantitative agreement with the theory on uncooperative bond rupture for supramolecular linkages switched in series. Based on these results, an estimate for the value of the dimer equilibrium constant Kdim = (1.3 ± 0.5) ´ 109M-1was obtained, which is in good agreement with previously estimated values based on loading rate‑dependent SMFS in hexadecane. 

In Chapter 4 an urea-aminotriazine (UAT)‑based donor-acceptor-donor-acceptor (DADA)‑array and complementary receptors were synthesized. The UAT-receptor was immobilized on gold surfaces using an ultrathin layer of ethylene glycol terminated lipoic acid and isocyanate chemistry. The surface chemistry and recognition of this hydrogen-bonded array was investigated as a prerequisite for AFM-based SMFS, usingcontact angle measurements, grazing angle Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and AFM. Reversible self‑complementary recognition of surface-immobilized UAT-moieties and solution borne UAT was confirmed by FTIR spectroscopy and AFM-based SMFS. Exploiting the Kramers-Bell-Evans approach,loading rate-dependent SMFS measurements yielded a value of the dimer binding constant of Kdim = (2 ± 1) x 107 M‑1 of this DADA-array in hexadecane. This value is significantly higher than predicted on the basis of additive primary and secondary hydrogen-bond interactions. The remarkable difference is rationalized by additional intramolecular hydrogen-bond stabilization, confirmed by 2D proton nuclear magnetic resonance (1H-NMR) measurements in CDCl3, which promotes a planar molecular geometry and stabilizes the dimeric complex. 

Chapter 5 describes the mechanical properties of UAT-based supramolecular polymers were studied.The unbinding forces of UAT complexes were investigated at different fixed piezo retraction rates in far-from-equilibrium conditions. The rupture forces of supramolecular UAT-polymer chains were found to decrease with increasing rupture lengths and their dependence on the number of linkers N was in quantitative agreement with the theory on uncooperative bond rupture for supramolecular linkages switched in series. In experiments with three different fixed loading rates, identical values for the characteristic bond length xb, to within the experimental error, were obtained. Thus a mean dimer equilibrium constant Kdim of (1.1 ± 0.3) ´ 107M-1was estimated. This value is in good agreement with the previously measured value for (UAT)2(Chapter 4). Hence the new approach to estimate the relevant parameters for the rupture of dimers in experiments on supramolecular linkages switched in series at one fixed loading rate, presented in Chapter 3 for the UPy-system, was validated. 

In Chapter 6 the influence of solvent polarity on the bond strength of UPy dimers was analyzed in 2‑propanol, 1-nonanol and hexadecane as well as in binary mixtures of 2‑propanol and hexadecane. The bond strength appears to scale with solvent polarity, which is consistent when solvent “competition” is assumed for hydrogen-bond formation. The corresponding distributions of the rupture forces were shown to deviate from a normal distribution. Whereas in pure hexadecane the rupture force distributions can be described with a Gaussian function, the distributions of rupture forces in polar 2-propanol, and to a lesser extent in 1-nonanol, deviate markedly from a Gaussian distribution, as confirmed e.g. by the Shapiro-Wilks test. The distributions in polar solvents are tentatively attributed to bond rupture in different local environments that may be related to molecular clustering observed by others in these solvents. In binary mixtures of 2-propanol and hexadecane similar bond strengths and bond strength distributions as in 2-propanol were observed. 

In Chapter 7 an outlook is presented on future developments which may create opportunities to enhance our understanding of the key contributors to hydrogen-bond stability and their influence on the stability and function of higher architectures or even bulk materials. Furthermore the feasibility of AFM-based SMFS as a tool for (single molecule) bond strength analyses is discussed. Ultimately AFM-based SMFS will contribute to a better understanding of the structure-property relationships that determine the bond strength and resulting bulk properties of these supramolecular systems.

Single Molecule Force Spectroscopy of self-complementary hydrogen-bonded supramolecular systems