Interactions between shape-persistent macromolecules as probed by AFM

Interactions between shape-persistent macromolecules as probed by AFM Johanna Blass‡1,2, Jessica Brunke‡3, Franziska Emmerich1,2, Cédric Przybylski4, Vasil M. Garamus5, Artem Feoktystov6, Roland Bennewitz1,2, Gerhard Wenz3 and Marcel Albrecht*3 Full Research Paper Open Access Address: 1INM-Leibniz-Institute for New Materials, Saarland University, Campus D 2.2, D-66123 Saarbrücken, Germany, 2Physics Department, Saarland University, Campus D 2.2, D-66123 Saarbrücken, Germany, 3Organic Macromolecular Chemistry, Saarland University, Campus C 4.2, D-66123 Saarbrücken, Germany, 4UPMC, IPCM-CNRS UMR 8232, Sorbonne Universités, 75252 Paris Cedex 05, France, 5Helmholtz-Zentrum Geesthacht (HZG), Centre for Materials and Costal Research, Max-Planck-Str. 1, 21502 Geesthacht, Germany and 6Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstr. 1, 85748 Garching, Germany Beilstein J. Org. Chem. 2017, 13, 938–951. doi:10.3762/bjoc.13.95 Email: Marcel Albrecht* - © 2017 Blass et al.; licensee Beilstein-Institut. License and terms: see end of document. * Corresponding author Received: 23 October 2016 Accepted: 24 April 2017 Published: 18 May 2017 This article is part of the Thematic Series "Spatial effects in polymer chemistry". Dedicated to Gerhard Wegner and his work on shape-persistent polymers. Guest Editor: H. Ritter ‡ Equal contributors Keywords: AFM; cyclodextrin; inclusion complexes; molecular recognition; polyconjugated polymers; shape persistent polymers Abstract Water-soluble shape-persistent cyclodextrin (CD) polymers with amino-functionalized end groups were prepared starting from diacetylene-modified cyclodextrin monomers by a combined Glaser coupling/click chemistry approach. Structural perfection of the neutral CD polymers and inclusion complex formation with ditopic and monotopic guest molecules were proven by MALDI–TOF and UV–vis measurements. Small-angle neutron and X-ray (SANS/SAXS) scattering experiments confirm the stiffness of the polymer chains with an apparent contour length of about 130 Å. Surface modification of planar silicon wafers as well as AFM tips was realized by covalent bound formation between the terminal amino groups of the CD polymer and a reactive isothiocyanate–silane monolayer. Atomic force measurements of CD polymer decorated surfaces show enhanced supramolecular interaction energies which can be attributed to multiple inclusion complexes based on the rigidity of the polymer backbone and the regular configuration of the CD moieties. Depending on the geometrical configuration of attachment anisotropic adhesion characteristics of the polymer system can be distinguished between a peeling and a shearing mechanism. 938 Beilstein J. Org. Chem. 2017, 13, 938–951. Introduction Shape-persistence is an important key feature in self-organisation strategies of supramolecular building blocks resulting in high structural perfection of the obtained molecular assemblies [1], such as shape persistent macrocycles, cage compounds or rotaxanes [2-4]. Especially shape-persistent polymers are of significant scientific interest as their defined structural characteristics offer various applications as sensor materials, biomimetic filaments or organic electronics [5-7]. Furthermore, compared to polymers with flexible chains, shape persistent macromolecules with high structural rigidity are able to form stable aggregates based on multiple supramolecular interactions, which can be detected and quantified without the presence of side effects, such as self-passivation or coiling processes. Dendrimers, nanoparticles and shape-persistent polymers had been previously discussed as scaffolds for the design of multiple ligands of high affinity [8]. Nevertheless, well-defined model systems in which the influence of rigidity and regularity on cooperativity of binding was systematically investigated have not been reported so far. Rigid linear polymers have been considered as suitable scaffolds for the design of supramolecular systems showing multiple interactions. A high rigidity of the macromolecule is maintained by rigid, linear repeat units, such as trans-ethenylene, ethynylene, or p-phenylene moieties. The observed persistence lengths of polyconjugated polymers ranged from 6 to 16 nm, depending on the side groups and the method of determination [9-11]. Among many supramolecular interactions, such as hydrogen bonding, π–π-interactions or hydrophobic host–guest interactions [12-16], the interactions of cyclodextrins (CDs) with hydrophobic guest molecules are of special interest, since CDs are readily available bio-based materials and interactions take place under physiological conditions [17]. CDs are ideal candidates for the investigation of multivalent interactions as they combine high affinities with a versatile integrability in macromolecular systems [18]. CDs have already been employed for the construction of supramolecular polymers [19-21], supramolecular hydrogels [22,23], molecular printboards [24,25] or multivalent interfaces [26-28] with tunable chemical and physical properties. Herein, for the first time, we present studies concerning the synthesis of shape-persistent CD polymers to investigate multivalent binding with ditopic guest molecules on the molecular level (Figure 1). The ditopic guest (shown in red colour) should act as a connector between opposing CD moieties. Only a few examples of shape-persistent CD polymers have been reported so far, including CD-modified conjugated oligomers and polymers composed of rigid phenylene ethynylene (PPE) structure units which are able to form self-inclusion complexes with tunable electrochemical properties [29-35]. The synthesis of PPE, in which two β-CD rings were attached to every second phenylene group, was described by Ogoshi et al. [36] using a Sonogashira–Hagiwara coupling. We preferred a poly-phenylene-butadiynylene backbone, synthesized by a Figure 1: Interaction of a shape-persistent CD polymer with ditopic guests. 939 Beilstein J. Org. Chem. 2017, 13, 938–951. Glaser–Eglington coupling, since the repeating unit is long enough (l = 0.944 nm) to allow the connection of one CD moiety at each phenylene unit. Based on the stiffness of the polymer chain self-passivation of CD polymer modified surfaces is reduced to a minimum. Furthermore, the ethynyl end groups are easily functionalized by click chemistry. Isothermal titration calorimetry (ITC), fluorescence spectroscopy, quartz crystal microbalance (QCM), surface plasmon resonance (SPR) and atomic force microscopy (AFM) have been employed to quantify the strength of the multivalent interactions [8]. Because binding affinities can be very high for multivalent supramolecular systems, the constituents are commonly used in low equilibrium concentrations. Since AFM even allows the investigation of single molecules, such as DNA [37,38] or molecular self-assembling based on “Dip-Pen” n (...truncated)