Self-complementary recognition of supramolecular urea-aminotriazines in solution and on surfaces.

The recognition of self-complementary quadruple urea-aminotriazine (UAT)-based hydrogen-bonded arrays was investigated in solution and at surfaces. For this purpose, an UAT-based donor-acceptor-donor-acceptor (DADA) array and complementary receptors were synthesized. Two-dimensional proton nuclear magnetic resonance ((1)H NMR) measurements in CDCl(3) pointed at an intramolecular hydrogen-bond stabilization of the UAT, which promotes a planar molecular geometry and, thereby, results in a significant stabilization of the dimeric complex. The bond strength of the UAT dimers at surfaces was determined by atomic force microscopy-based single molecule force spectroscopy (AFM-SMFS) in hexadecane. The UAT receptor was immobilized on gold surfaces using an ultrathin layer of ethylene glycol terminated lipoic acid and isocyanate chemistry. The layers obtained and the reversible self-complementary recognition were thoroughly characterized with contact angle measurements, grazing angle Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and AFM. Loading rate-dependent SMFS measurements yielded a barrier width x(ß) and a bond lifetime at zero force t(off)(0) of 0.29 ± 0.02 nm and 100 ± 80 ms, respectively. The value of the corresponding off-rate constant k(off) suggests a substantially larger value of the dimerization constant compared to theoretical predictions, which is fully in line with the additional intramolecular hydrogen-bond stabilization detected in solution by (1)H NMR spectroscopy.

Rupture force of single supramolecular bonds in associative polymers by AFM at fixed loading rates.

Atomic-force-microscopy-based single-molecule force spectroscopy (AFM-SMFS) was used to study the bond strength of self-complementary hydrogen-bonded complexes based on the 2-ureido-4[1H]-pyrimidinone (UPy) quadruple H-bond motif in hexadecane (HD). The unbinding force corresponding to single UPy-UPy dimers was investigated at a fixed piezo retraction rate in the nonequilibrium loading rate regime. The rupture force of bridging supramolecular polymer chains formed between UPy-functionalized substrates and AFM tips in the presence of a bis-UPy derivative was found to decrease with increasing rupture length. The rupture length was identified as the chain length of single, associating polymers, which allowed us to determine the number of supramolecular bonds (N) at rupture. The rupture force observed as a function of N was in quantitative agreement with the theory on uncooperative bond rupture for supramolecular linkages switched in a series. Hence, the value of the dimer equilibrium constant Keq=(1.3+/-0.5) x 10(9) M(-1), which is in good agreement with previously estimated values, was obtained by SMFS of supramolecular polymers at a single loading rate.

Forced unbinding of individual urea-aminotriazine supramolecular polymers by atomic force microscopy: a closer look at the potential energy landscape and binding lengths at fixed loading rates.

Atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) was used to study the forced unbinding of quadruple self-complementary hydrogen-bonded urea-aminotriazine (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 urea-aminotriazine (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 x(ß) and the off-rate constant in the absence of force k(off)(f = 0). The value of x(ß) was found to agree with literature data on the hydrogen-bond 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.

Inverted microcontact printing on polystyrene-block-poly(tert-butyl acrylate) films: a versatile approach to fabricate structured biointerfaces across the length scales.

The combination of the recently introduced soft lithographic technique of inverted microcontact printing (i-muCP) and spin-coated films of polystyrene- block-poly( tert-butyl acrylate) (PS 690- b-P tBA 1210) as a reactive platform is shown to yield a versatile approach for the facile fabrication of topographically structured and chemically patterned biointerfaces with characteristic spacings and distances that cross many orders of magnitude. The shortcomings of conventional muCP in printing of small features with large spacings, due to the collapse of small or high aspect ratio stamp structures, are circumvented in i-muCP by printing reactants using a featureless elastomeric stamp onto a topographically structured reactive polymer film. Prior to molecular transfer, the substrate-supported PS 690- b-P tBA 1210 films were structured by imprint lithography resulting in lateral and vertical feature sizes between >50 microm-150 nm and >1.0 microm-18 nm, respectively. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and water contact angle measurements provided evidence for the absence of surface chemical transformations during the imprinting step. Following the previously established hydrolysis and activation protocol with trifluoroacetic acid and N-hydroxysuccinimide, amino end-functionalized poly(ethylene glycol) (PEG-NH 2), as well as bovine serum albumin and fibronectin as model proteins, were successfully transferred by i-muCP and coupled covalently. As shown, i-muCP yields increased PEG coverages and thus improved performance in suppressing nonspecific adsorption of proteins by exploiting the high local concentrations in the micro- and nanocontacts during molecular transfer. The i-muCP strategy provides access to versatile biointerface platforms patterned across the length scales, as shown for guided cancer cell adhesion, which opens the pathway for systematic cell-surface interaction studies.