Table 4.
Racemization of Cys during synthesis of the model peptide, H–Gly–Cys–Phe–NH2, as a function of the Cys protecting group
| Racemization (%)a | ||||
|---|---|---|---|---|
| Conditions | Trt | MBom | Dpm | Ddm |
| Conventional SPPSb | 8.0 | 0.4 | 1.2 | 0.8 |
| MW-assisted SPPSc | ||||
| 50 °C | 10.9 | 0.8 | 3.0 | 1.8 |
| 80 °C | 26.6 | 1.3 | 4.5 | 2.5 |
a
Defined as (H–Gly–d-Cys–Phe–NH2)/(H–Gly–l-Cys–Phe–NH2) × 100.
b
The HCTU-mediated reactions were performed using a 1 min preactivation procedure of coupling with Fmoc amino acid/HCTU/6-Cl-HOBt/DIEA (4/4/4/8 equiv) in DMF. The coupling time was 30 min, and concentrations of the active species were 0.20 M.
c
MW heating was performed using the same coupling procedure as the conventional SPPS in a 25 ml polypropylene open vessel placed into the MW cavity of a 300 W single-mode manual MW peptide synthesizer (CEM, Discover SPS) set at 50 or 80 °C; power pulsing sequences of 30 W were used for Cys coupling steps (5 min).