A LysLysLys-tag as trigger in polynorepinephrine epitope imprinting: The case study of soluble PD-L1 detection in serum by optical-based sensing

Polycatecholamines (pCAs)-based molecularly imprinted polymers (MIPs) represent the new performing generation of biocompatible ligand/receptor mimetics. In this context, dealing with MIPs synthesis for bio-macromolecules detection/extraction, one of the critical steps in ensuring effective binding affinity for the parent molecule is the selection of suitable epitopes for pCAs imprinting. To address this challenge, here we investigated the ability of lysine (K) residues to trigger the epitope imprinting process into a polynorepinephrine (PNE) matrix. To this aim, we first designed a set of model epitopes composed of three K and six alanine (A) residues to investigate the influence of each 'KA' combination on the imprinting process and the resulting binding performance by Surface Plasmon Resonance (SPR). Only the case of three flanking K residues in N-terminus arose as an excellent trigger for epitope imprinting. The efficacy of the 3K-tag strategy was then evaluated on two peptide templates belonging to soluble programmed cell death protein 1 ligand (PD-L1), which is of great interest as a cancer biomarker in liquid biopsies. These templates were selected due to their negligible natural ability to be imprinted into the PNE matrix and were modified with 3K-tags, in N-, C-, and N/C- positions, respectively. The SPR sensor developed by exploiting the N-3K tag strategy allowed us to achieve excellent sensitivity (0.31 ± 0.04 ng mL-1) and repeatability (avCV% = 4.5) in human serum samples. This strategy opens new insights both for epitopes' design for pCAs-based mimetics and as triggering tags when native epitopes display negligible imprinting capabilities.