Figure 1: Electrical detection of the twenty proteinogenic amino acids.

(a) Schematic illustration of the peptide constructs used to probe the current blockade of the twenty amino acids. A cationic carrier of seven arginine amino acids (R7) is chemically linked at the C-terminus to the eighth amino acid, X, to form twenty X_R7 peptides. (b) Schematics of the experimental setup (not to scale). (c) Illustration of a typical current blockade. (d) Representative fragment of ionic current recording. (e) Typical histogram of the relative residual current, I_b/I₀ (left axis), and blockade duration, Δt (right axis), produced by the transport of S_R7 peptides through the aerolysin nanopore. (f-j) Superimposed histograms of I_b/I₀ obtained from nanopore experiments performed for each of the twenty X_R7 peptides, analyzed individually and grouped according to the properties of amino acid X: charged (f), hydrophobic aromatic (g), polar uncharged (h), hydrophobic non-aromatic (i), and when amino acid X is either cytosine, proline or glycine (j). In panel f, stars indicate blockade levels likely produced by R9, R7 and R6 polyarginine impurities (from left to right) present in the R_R7 sample. In panel j, C* and C indicate populations recorded before and after dithiothreitol treatment, see Supplementary Fig. 1. (k) Mean relative residual current and its standard deviation produced by the X_R7 probes versus volume of amino acid X. All data were acquired in 4 M KCl, 25 mM HEPES buffer, at 7.5 pH, 1 μM peptide concentration, 20.0 ± 0.5°C, and under a −50 mV bias applied to the trans compartment. For each histogram, at least 1000 events were analyzed.