Synthesis of Barnase Site-Specifically Labelled with Two 13C Atoms Using Partially Protected Peptide Thioester Building Blocks

Barnase, site-specifically labelled with two 13C atoms, was synthesized using partially protected peptide thioesters as building blocks. Four partially protected peptide segments (Boc–[Lys(Boc)19,27]–barnase(1—34)–SC(CH3)2CH2CO–Nle–NH2, iNoc–[Lys(Boc)39,49]–barnase(35—52)–SC(CH3)2CH2CO–Nle–NH2, iNoc–[Lys(Boc)62,66]–barnase(53—81)–SC(CH3)2CH2CO–Nle–NH2, [Lys(Boc)98,108]–barnase(82—110)) were successively condensed in the presence of silver ions and N-hydroxysuccinimide (HONSu). Finally, barnase with full ribonuclease activity was obtained in a yield of 11% based on the carboxyl terminal peptide segment.