A modified ubiquitin polypeptide, or a modified fragment thereof, said polypeptide or fragment thereof comprising a ubiquitin polypeptide, or a fragment thereof, attached to a N—S acyl transfer device at the C-terminus of said ubiquitin or said fragment thereof, wherein said modified fragment is selected from CKIQDKEGIPPDQQRLIF (Ub28-45) (SEQ ID NO:10), CGKQLEDGRTLSDYNIQKESTLHLVLRLRGG (Ub46-76) (SEQ ID NO:8) and CKIQDKEGIPPDQQRLIFAGKQLEDGRTLSDYNIQKESTLHLVLRLRGG (Ub46-76) (SEQ ID NO:8) and CKIQDKEGIPPDQQRLIFAGKQLEDGRTLSDYNIQKESTLHLVLRLRGG (Ub28-76) (SEQ ID NO:9);
- and wherein said N—S acyl transfer device has the general Formula I:
- wherein
- R=hydrogen or 2-nitrobenzyl;
- R1=methyl;
- R2=CONH2;
- R3=hydrogen.