Scheme S1. Synthesis of linker 6

Figure S1. HPLC-MS of crude peptide 7 L.

Figure S2. HPLC-MS of crude peptide 10 L

Table S1. Conditions for nucleophilic release from 7 L

Figure S3. HPLC of entry 1 from Table S1.

Figure S4. HPLC of entry 2 from Table S1

Figure S5. HPLC and comparison between entry 7 and 8 from Table S1.

Figure S6. HPLC of crude peptide thioester epimers 14 L (Table SI1, entry 7) and 14D.

Figure S7. HPLC of crude peptide thioester epimers 15 L and 15D.

Figure S8. HPLC of crude peptide thioester epimers 16 L and 16D.

Figure S9. HPLC of crude peptide thioester epimers 17 L and 17D.

Figure S10. HPLC of crude peptide thioester epimers 18 L and 18D.

Figure S11. HPLC of crude peptide thioester epimers 19 L and 19D.

Figure S12. HPLC of crude peptide hydrazide epimers 21 L (Table SI1, entry 10) and 21D.

Figure S13. HPLC of crude peptide hydrazide epimers 22 L and 22D.

Figure S14. HPLC of crude peptide hydrazide epimer 23 L.

Figure S15. HPLC of crude peptide hydrazide epimer 24 L

Figure S16. HPLC of NCL of 14 L and 26 after 1 h

Figure S17. HPLC of NCL of 14 L and 26 after 16 h

Figure S18. HPLC chromatograms for non-activated pGlu peptide and activated pGlu peptide