Proinsulin derivatives

A process for preparing insulin, or a derivative thereof, said process comprising the steps of:

• (a) preparing by solid phase synthesis a polypeptide compound of formula (I):
• A-C-B (I)
• wherein:
• A is the A chain of insulin or a functional derivative or variant thereof;
• B is the B chain of insulin or a functional derivative or variant thereof;
• C is a peptide of the formula:
• (X1)p-(X2)n-(X3)q
• wherein:
• each X1 and X3 are both Arg;
• each X2 is selected from the group consisting of a basic amino acid, Gly, β-Ala, Pro,
• Hyp, pseudoproline, and an acidic or hydrophilic amino acid;
• p is 2;
• q is 2;
• and
• n is 1 and
• (b) subjecting the polypeptide compound of formula (I) formed in step (a) to stepwise oxidative folding, wherein said stepwise oxidative folding comprises the steps of:
• (i) treating the polypeptide compound of formula (I) with DMSO to form a first intermediate;
• (ii) treating said first intermediate with iodine in acetic acid/water to form a second intermediate;
• and
• (iii) cleaving said second intermediate to excise the C-peptide.