2.2.1. Synthesis of Nim-Trityl-L-Histidine N-Carboxy Anhydride (Nim-Trityl-L-His-NCA)

For the synthesis of *Nim*-*Trityl*-L-His-NCA a previously reported method by our group [16] was mainly followed with some modifications. The synthesis was conducted in two steps. In the first step, the hydrochloric salt of *Nim*-*Trityl*-L-His-NCA was formed, and finally the desired pure *Nim*-*Trityl*-L-His-NCA was synthesized after the removal of HCl. The whole synthetic procedure is presented in detail in Supplementary Materials.

### 2.2.2. Synthesis of S-tert-Butyl-mercapto-L-Cysteine N-Carboxy Anhydride (tBM-L-Cys-NCA)

The *N*-carboxy anhydride of *t*BM-L-Cys-NCA was synthesized in a similar way to a previously described process [37]. The synthetic steps are provided in Supplementary Materials.

### 2.2.3. Synthesis of Sarcosine N-Carboxy Anhydride (Sar-NCA)

The synthesis of the Sarcosine *N*-Carboxy Anhydride was conducted following previously reported procedures [16,38]. The total synthetic route is described thoroughly in Supplementary Materials.
