The chemical synthesis of 3-15 was achieved on a polystyrene-based Rink amide resin. Peptides 3-5 were synthesized on an automated microwave peptide synthesizer. Initial attempts using N,N′diisopropylcarbodiimide (DIC)/oxyma coupling strategy and 10% (w/v) piperazine in ethanol:NMP (1:9) as the deprotection cocktail yielded both the desired peptide and a truncated side product that lacks an N-terminal lysine residue (Supplementary Figure ). This issue was addressed by extending the incubation time of the N-terminal lysine coupling (see Experimental section). After the synthesis on the solid support, the peptide was cleaved by treatment with trifluoroacetic acid, precipitated in cold diethyl ether, purified by reversed-phase HPLC (RP-HPLC) and lyophilized.