The 2006 demonstration by Nishino, Huang and Shea that a polymer film imprinted with the C-terminal nonapeptide of cytochrome c selectively captures the native protein from a five-protein mixture founded the epitope-imprinting approach to synthetic protein receptors. Two decades of follow-up have resolved the methodological limitations the authors noted — imprinted epitopes now include N-terminal, internal, post-translationally modified and conformational sequences, and capacity, reusability and real-matrix performance are routinely measured. What remains open is the mechanistic question that demonstration left behind: the attribution of sequence-specific, nanomolar-affinity binding to cooperative hydrogen bonding and hydrophobic contacts rests on binding behavior alone and has never been directly demonstrated. No experiment or simulation has yet delivered the structure of an imprinted binding cavity, the identity, geometry and energetics of the polymer–protein contacts, or an energetic partition among interaction types. The problem is to establish this quantitative, molecular-level mechanism — or to refute the cooperative-interaction attribution with an alternative account of the observed affinity and sequence specificity.

Background

Molecular imprinting creates synthetic recognition sites by polymerizing functional and cross-linking monomers around a template molecule; removing the template leaves cavities that preferentially rebind it. The resulting molecularly imprinted polymers (MIPs) are robust, cheap 'plastic antibodies'. Imprinting whole proteins is difficult — proteins denature in the organic solvents of classical imprinting and the bulky template is hard to extract from a dense network — so the epitope approach imprints instead a short, surface-exposed peptide fragment of the target (an epitope, by analogy to the antigenic determinant recognized by an antibody), expecting the cavity molded on the fragment to recognize the same sequence displayed on the folded protein.

Nishino, Huang and Shea (Angew. Chem. Int. Ed. 2006, DOI 10.1002/anie.200503760) provided the founding demonstration for native proteins: they anchored the C-terminal nonapeptide of cytochrome c (sequence AYLKKATNE) to a glass slide, grew a thin hydrogel film around it, removed the peptide, and showed that the imprinted film selectively captured cytochrome c from a mixture of five proteins (each at 50 μg/mL) with a dissociation constant KdK_d = 72.3 nM and a saturation coverage of about 75% of a theoretical flat-surface monolayer estimated from an assumed 5.6 nm² protein footprint. Films imprinted with C-terminal nonapeptides of alcohol dehydrogenase and serum albumin captured their respective parent proteins, and single amino-acid substitutions at one position of a serum-albumin-derived heptapeptide template largely destroyed binding of serum albumin, establishing single-residue sequence specificity. The authors attributed this specificity to multiple cooperative hydrogen bonds and hydrophobic contacts between oriented functional groups in the imprinted cavity and the protein's terminal sequence. That attribution was inferred from the binding behavior rather than directly demonstrated: no experiment measured the hydrogen-bonding contribution, and no measurement resolved the cavity structure or the geometry and energetics of the polymer–protein contacts.

Later work has transformed the method's scope, so those restrictions are no longer the binding constraint: imprinted templates now include phosphorylated (Emgenbroich et al., Chem. Eur. J. 2008, DOI 10.1002/chem.200801046; İncel et al., Nat. Chem. Biol. 2026, DOI 10.1038/s41589-026-02274-2), acetylated (Yang et al., Chem. Commun. 2015, DOI 10.1039/c4cc09787d) and conformational epitopes (Zhang et al., Angew. Chem. Int. Ed. 2015, DOI 10.1002/anie.201412114), N-terminal and multiepitope combinations (Yang et al., Anal. Chem. 2016, DOI 10.1021/acs.analchem.6b01247), and internal sequences such as affinity tags and flexible interdomain linkers (Gómez-Arribas et al., Anal. Chem. 2019, DOI 10.1021/acs.analchem.8b05731; Ghosh et al., Biomacromolecules 2026, DOI 10.1021/acs.biomac.5c02567). The measurement tools needed for the mechanistic question also now exist and have been applied partially: saturation transfer difference (STD) NMR, which identifies which ligand residues are in closest contact with a receptor by watching magnetization transfer from selectively saturated receptor signals, and WaterLOGSY, a related NMR method detecting contacts through water-mediated transfer, have proven that specific binding sites exist in imprinted nanogels and mapped which epitope residues touch the polymer (Mier et al., Angew. Chem. Int. Ed. 2021, DOI 10.1002/anie.202106507; Herrera Leon et al., Anal. Chem. 2026, DOI 10.1021/acs.analchem.6c02629). At the same time, combined electrochemical, infrared-spectroscopic and atomic force microscopy (AFM) measurements revealed that a large part of apparent protein binding to epitope-imprinted films can be nonspecific 'out-of-pocket' adsorption to the non-imprinted polymer surface, in some systems with higher apparent affinity than the target peptide itself (Zhang et al., Chemosensors 2021, DOI 10.3390/chemosensors9060128). Reviews of the field continue to treat the recognition mechanism — how sequence selectivity in water actually arises in these amorphous, cross-linked networks — as an unfinished task (Liu et al., Prog. Polym. Sci. 2024, DOI 10.1016/j.progpolymsci.2024.101790).

Problem Statement

Establish a direct, quantitative mechanistic account of how the epitope-imprinted polymer films of Nishino, Huang and Shea (2006) capture their target proteins in aqueous solution, sufficient to explain the reported nanomolar equilibrium affinity and five-protein selectivity of the cytochrome c film and the single-residue sequence specificity demonstrated on serum-albumin-derived templates in the same study. The material class is that of the founding study: thin hydrogel films of acrylamide cross-linked with N,N′-ethylene-bis-acrylamide and polyethylene glycol 200-diacrylate, grown on a glass support around covalently anchored, surface-exposed C-terminal peptide templates — AYLKKATNE (residues 96–104) for cytochrome c, VVSTQTALA (residues 599–607) for serum albumin and its C-terminal heptapeptide STQTALA (residues 601–607) with the single-substitution variants used in the specificity test, and GRYVVDTSK (residues 339–347) for alcohol dehydrogenase. Concretely, this requires jointly determining: (i) the structure of the imprinted binding cavity in the cross-linked, solvent-swollen network — or the ensemble of cavity structures, since the sites are heterogeneous; (ii) the identity, geometry and energetics of the polymer–protein contacts, i.e. which polymer functional groups contact which epitope residues, at what distances and orientations, and with what energetic contributions; and (iii) the partition of the observed binding free energy among cooperative hydrogen bonding, hydrophobic contacts and any other interactions, thereby directly testing the attribution of specificity to cooperative hydrogen bonds and hydrophobic contacts; each term of the partition must be established independently — not left as an unconstrained residual of the measured total — and the resulting decomposition must be consistent with observations not used to construct it. These three deliverables serve one common scientific objective — a validated molecular-level mechanism of epitope-imprinted recognition — and are complete only together: a peptide-side contact map without energetics, or an energetic decomposition not tied to the structural assignment of the contacts, would not settle the question. The problem is also answerable by refutation: in place of the constructive account above, a rigorous demonstration that the cooperative-interaction attribution is quantitatively wrong, together with an alternative account of the observed affinity and sequence specificity at the same molecular level, is an admissible resolution.

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