Fig. 1: Design of peptide libraries and validation by LC-MS/MS.

a Scheme of the experimental PLDMS workflow. b Phosphopeptide libraries used for substrate preference evaluation. c Amino acid distribution in the permutated positions (x1 and x2) of the Nterm_x1_1 (blue) and the Cterm library (red) after reference measurement (samples not treated with PP1 or PP2A, i.e. untreated) and filtering for expected sequences (see Supplementary Table 1 and the methods section). Perfectly random incorporation of all amino acids would result in 7% per amino acid. Source data are provided as a Source Data file. d Mascot Score (statistical value for how well detected data matches database sequences) distributions of the reference measurements for Nterm_x1_1 (left) and Cterm library (right). Empirically wrong peptides are peptides with sequences not matching expected sequences from the synthetic route. For the reference measurement, empirical filtering by Mascot Score cut-offs of 39 and 32 at a false discovery rate (FDR) of 5% for the Nterm_x1_1 and the Cterm library, respectively, allowed separation of correct peptides from wrong ones.