Nanopore Investigation of the Stereoselective Interactions between Cu2+ and D,L-Histidine Amino Acids Engineered into an Amyloidic Fragment Analogue

Stereochemistry is an essential theme for a number of industries and applications, constructed around discriminating various chiral enantiomers, including amino acids, chiral metal complexes, and drugs. In this work, we designed a set of peptide mutants of the human amyloidic A beta(1-16) sequence, known to display an effective Cu2+ coordinating pocket provided mainly by the intramolecular His-6, His-13, and His-14 residues, that were engineered to contain l- and d-His enantiomers in positions 6 and 13 and provide a local coordination environment with distinct Cu2+ binding geometries and affinities. We examined the mechanism of selective chiral recognition of Cu2+ by such mutant peptides, by quantifying their stochastic sensing in real time with a single a-hemolysin (alpha-HL) protein immobilized in a planar lipid membrane, while incubated in various concentrations of Cu2+. Our data reveal that the Cu2+-binding affinity lies within the micromolar range, and decreases by orders of magnitude as l-His is replaced with its d-enantiomer, with the effect being prevalent when such changes were inflicted on the His-6 residue. The presented results demonstrate the feasibility of tuning the metal selectivity in a relatively simple peptide substrate by enantiomeric replacement of key metal binding residues and illustrates the potential of the protein nanopores as a promising approach to quantify the chiral recognition of L/d amino acids by metals.