The Synthesis of Sub2.

We performed automated Fmoc-based solid phase synthesis on a Liberty Blue automated peptide synthesizer. Synthesis was conducted on a 0.1-mmol scale with Fmoc Rink amide 4-Methylbenzhydrylamine (MBHA) resin (0.52 mmol/g loading) and 3 equiv. of protected amino acids. Deprotection of the Fmoc group was carried out with 20% piperidine/DMF. Coupling was done using the desired Fmoc-protected amino acid (0.2 mmol, 2.0 equiv.), coupling reagent Oxyma (0.4 mmol, 4.0 equiv.), and N,N′-Diisopropylcarbodiimide (DIC) (0.4 mmol, 4.0 equiv.). After the final amino acid had been coupled on, the resin was washed twice with DMF and DCM. Cleavage from the resin was performed using trifluoroacetic acid (95%), triisopropylsilane (2.5%), and water (2.5%) with agitation for 4 h. The peptide was drained into cold methyl tert-butyl ether, where it precipitated out. We centrifuged the precipitate, decanted mother liquor, dissolved the pellet in DMF, and then purified the peptide by LC-MS.