A comparison of folding techniques in the chemical synthesis of the epidermal growth factor‐like domain in neu differentiation factor α/β

Abstract: The 52‐residue α/β chimera of the epidermal growth factor‐like domain in neu differentiation factor (NDF_eα/β) has been synthesized and folded to form a three disulfide bridge (Cys¹⁸²–Cys¹⁹⁶, Cys¹⁹⁰–Cys²¹⁰, Cys²¹²–Cys²²¹) containing peptide. We investigated two general strategies for the formation of the intramolecular disulfide bridges including, the single‐step approach, which used fully deprotected and reduced peptide, and a sequential approach that relied on orthogonal cysteine protection in which specific pairs are excluded from the first oxidation step. Because there are 15 possible disulfide bridge arrangements in a peptide with six cysteines, the one‐step approach may not always provide the desired disulfide pairing. Here, we compare the single‐step approach with a systematic evaluation of the sequential approach. We employed the acetamidomethyl group to protect each pair of cysteines involved in disulfide bridges, i.e. Cys¹⁸² to Cys¹⁹⁶, Cys¹⁹⁰ to Cys²¹⁰ and Cys²¹² to Cys^221. This reduced the number of possible disulfide patterns from 15 to three in the first folding step. We compared the efficiencies of folding for each protected pair using RP‐HPLC, mapped the disulfide connectivity of the predominant product and then formed the final disulfide from the partially folded intermediate via I₂ oxidation. Only the peptide having the Cys¹⁸²–Cys¹⁹⁶ pair blocked with acetamidomethyl forms the desired disulfide isomer (Cys¹⁹⁰–Cys²¹⁰/Cys²¹²–Cys²²¹) as a single homogeneous product. By optimizing both approaches, as well as other steps in the synthesis, we can now rapidly provide large‐scale syntheses of NDF_eα/β and other novel EGF‐like peptides.