Identification of single amino acid chiral and positional isomers using an electrostatically asymmetric nanopore

Chirality is essential in nearly all biological organization and chemical reaction but is rarely considered due to technical limitations in identifying L/D isomerization. Using OmpF, a membrane channel from E. coli with an electrostatically asymmetric constriction zone, allows discriminating chiral amino acids in a single peptide. The heterogeneous distribution of charged residues in OmpF causes a strong asymmetric electrostatic field at the constriction. This asymmetry forces the sidechains of the peptides to specific orientations within OmpF, causing distinct ion current fluctuations. Using a statistical analysis of the respective ion current variations allows distinguishing the presence and position of each chiral amino acid. To explore potential applications, the disease-related peptide β-Amyloid, and its D-Asp1 isoform, as well as a mixture of the Icatibant peptide drug (HOE 140) and its D-Ser7 mutant, have been discriminated. Both chiral isomers were not applicable to be distinguished by mass spectrum approaches. These findings highlight a novel sensing mechanism for identifying single amino acids in single peptides and even for achieving single-molecule protein sequencing.