Table 1. Study of the Transamidation Reaction^a.

Sec peptide 3	X	3 D-AA content for X (%)	initial reaction rateb	yieldc (%) of 7	7 D-Aa content for X (%)
3a	Phe	0.34	0.30	7a: 41	4.5
3b	Ser	d	0.17
3c	Thr		0.04
3d	Arg	0.19	0.97	7d: 34	1.1
3e	Ala	0.36	1	7e: 43	2.2
3f	Glu	0.95	0.66	7f: 36	1.4
3g	Cys		0.26
3h	Gly		0.27
5d	Arg		0.005
5e	Ala	0.36	0.03	7e: 42	0.86

^aDiselenide 3.5 mM, peptide 6 7 mM, 200 mM MPAA, 70 mM TCEP, 210 mM TCEP=Se, pH 5.5, 37 °C under nitrogen atmosphere.

^bRelative to peptide 3e (X = Ala), initial rate of peptide 7e formation is 1.17 mM/h (see the Supporting Information).

^cIsolated by HPLC. Identical by LC–MS to authentic samples obtained by SPPS.

^dNot determined.