Supporting Information Combinatorial Synthesis of Peptide Arrays with a Laser Printer**

1. Generation and characterization of amino-acid-toners In order to produce a toner mass, one of 20 different Fmoc-amino acid OPfp esters (Fluka, 10 % w/w), a polymer, e.g. SLEC PLT 7552 (Sekisui, 84.5 % w/w), pyrazolone orange (ABCR GmbH, 4.5 % w/w), and Fe(naphtol)2 complex (1 % w/w)[1] were melted, mixed and finally solidified (Table S1). The homogenized and crushed toner mass was then mixed with ~ 0,04 % w/w silica particles (Degussa, Aerosil 812, hydrophobic), and slowly fed into an air jet mill where grinded particles were collected within a narrow lattice (Hosokawa alpine 50AS). The physical parameters of amino-acid-particles are described by a mean diameter of ∼10µm, a small size distribution (Figure S1), a low tendency to aggregate, melting points of 60-80°C (Figure S2), a Q/m value of ~-4µC/g, and a similar toner transfer for the 20 different amino-acid-particles (Figure S3). These parameters were measured with a Mastersizer (Malvern, type 2000), by differential scanning calorimetry (Netzsch, type DSC 204 F1 Phoenix), and a Q/m meter (Trek, type 210HS-2), respectively. Light microscopy and scanning electron microscopy (SEM) pictures to determine particle morphology (Figure S4) were taken with an Axiovert 35 light microscope (Zeiss) and an SEM (LEO; Microscope type 1530; 10 kV electron volts). The stability of OPfp esters embedded into amino-acid-particles was analyzed by HPLC (Figure S5)