Rational design in combination with a screening process was used to develop affinity polymers for a specific binding site on the surface of immunoglobulin G (IgG) proteins. The concept starts with the identification of critical amino acid residues on the protein interface and their topological arrangement. Appropriate binding monomers were subsequently synthesized. Together with a sugar monomer (2–5 equiv) for water solubility and a dansyl monomer (0.5 equiv) as a fluorescent label, they were subjected in aqueous solution to linear radical copolymerization in various compositions (e.g., azobisisobutyronitrile (AIBN), homogeneous water/DMF mixtures). After ultrafiltration and lyophilization, colorless dry water-soluble powders were obtained. NMR spectroscopic and gel permeation chromatography (GPC) characterization indicated molecular weights between 30 and 500 kD and confirmed retention of monomer composition as well as the absence of monomers. In a competitive enzyme-linked immunosorbent assay (ELISA) screen of the polymer libraries (20–50 members), few copolymers qualified as strong and selective binders for the protein A binding site on the Fc fragment of the antibody. Their monomer composition precisely reflected the critical amino acids found at the interface. The simple combination of a charged and a nonpolar binding monomer sufficed for selective submicromolar IgG recognition by the synthetic polymer. Affinities were confirmed by fluorescence titrations; they increased with decreasing salt load but remained largely unaltered at lowered pH. Other proteins, including those of similar size and isoelectric point (pI), were bound 10–1000 times less tightly. This example indicates that interaction domains in other proteins may also be targeted by synthetic polymers if their comonomer composition reflects the nature and arrangement of amino acid residues on the protein surface.

Transition from monotopic symmetrical to ditopic unsymmetrical molecular recognition frequently occurs when a general, powerful, but unspecific receptor molecule is transformed into a specific ditopic host. Especially in water, this endeavor is accompanied by great challenges, comprising, among other things, host–guest orientation, orthogonal recognition modes, and the nature of the linker unit. This work presents a case study on a powerful general host for basic amino acids and peptides. The symmetrical molecular tweezer skeleton was systematically desymmetrized and modified with various common linker units, and the profound influence of these changes on the molecular recognition profile was studied in detail by NMR spectroscopy, fluorescence titrations, X-ray crystallography, and molecular simulations. A number of diverse effects were revealed that could be attributed to the chemical nature of the different linkers. In general, long alkyl tethers block the cavity of the tweezers by van der Waals contacts to CH groups around its entrance; alkoxyalkyl tethers likewise lower tweezer affinities for basic amino acids by competing self-inclusion. As a general trend, affinities for linkers with ester and carboxylate moieties were substantially higher than those for tethers with ethers and alcohols, likely because the electron-rich carbonyl group keeps the cavity open. Additional hydrogen bonds between the linker unit and suitable amino acid or peptide guests greatly support the complexation process; finally, high solvent polarity and salt load shift the binding equilibrium from external ion pairing to guest inclusion.

The complete, entirely artificial, signal-transduction process was realized with a pair of tailored transmembrane units that were equipped with receptor- and reactive sites at both amphiphilic ends. Thus, docking of the primary messenger, transmission of the signal, and release of the secondary messenger could all be imitated in a single experimental setup. The system imitates the signaling principle of receptor tyrosine kinases and employs bisphosphonate head-groups for oligoamine-recognition and a pair of thiol nucleophiles and pyridine disulfide tail-groups for intravesicle S_N2 displacement. This system operates in a unidirectional fashion, does not suffer from intervesicle competition, and is highly sensitive towards the lipid composition of the membrane and the nature of the primary messenger.