Specifically, the Ub mutant named avi-Ub(G76C) (
1) which carried an avi-tag was expressed in
E. coli. Then, the 0.7% perchloric acid was slowly added to cell lysate for precipitating unwanted protein (
Fig. 1c, Figs. S1 and S2 in Supporting information). After centrifuging the suspension, the supernatant was dialyzed into MES buffer (20 mmol/L MES, pH 7.0), and concentrated to a protein concentration of 10 mg/mL. In order to load the biotin tag onto
1, MgCl
2 (5 mmol/L), ATP (2 mmol/L), d-biotin (0.45 mmol/L) and BirA (1 µmol/L) were added to the supernatant and placed at 37 ℃ for enzymatic biotinylation [
17,
30]. Through monitoring the reaction using RP-HPLC, we found that
1 was completely disappeared after 1 h, and a new peak was emerged at the same time (
Fig. 1b). Electrospray ionization (ESI)-MS analysis further confirmed that the appeared product
2 was biotin-loaded avi-Ub(G76C) (
Fig. 1d). To convert the C-terminus of intermediate
2 to a hydrazide group, biotin-loaded avi-Ub (G76C) was reacted with hydrazinolysis reagent (MESNA 100 mg/mL, NH
2NH
2-HCl 50 mg/mL, tris(2-carboxyethyl)phosphine (TCEP) 5 mg/mL, pH 7.0) for 48 h at 50 ℃ as we previously developed protocol [
26]. As shown in
Figs. 1b and
e, the C-terminal of
2 was almost converted to hydrazide to afford
3 followed by
3 dialyzing into MES buffer (20 mmol/L MES, pH 3.0) and added NaNO
2 (7 equiv.) for 25 min at −15 ℃ [
31-
39]. Then, propargylamine (200 equiv.) was added to the above reaction (pH value was adjusted to 8.0), and the new aminolysis product was generated by stirring at room temperature for 1 h [
28,
29]. The identity and purity of avi-biotin-Ub-PA (probe
1) were confirmed by HPLC (
Fig. 1b, isolated yield 70%) and ESI-MS (
Fig. 1f and Fig. S3 in Supporting information). In this context, starting from 1 L of LB broth medium, about 25 mg of probe
1 was obtained within 5 days. Similarly, we were able to obtain avi-biotin-Ub-VME (probe
2) (
Fig. 1g and Fig. S4 in Supporting information) and avi-biotin-Ub-Cl (probe
3) (
Fig. 1h and Fig. S5 in Supporting information) through this sequential triple reaction without any purification steps. Next, we renatured probe
1 using the method previously reported [
14,
15,
25]. In addition, we measured the circular dichroism spectra (CD) of the probe
1, and used Ub-PA as comparison. As shown in Fig. S6 (Supporting information), the CD spectra of probe
1 and Ub-PA were nearly identical, indicating that synthesized probe
1 has the correct secondary structure. We also measured the circular dichroism spectra of probe
2 and probe
3, and the results were similar to those of probe
1 and Ub-PA (Fig. S6).