Vitamin C for peptide ligation: Phosphine and thiol additives maintain reducing environments and increase the reactivities of peptide thioesters in native chemical ligation. Thiol additives such as thiophenol also act as radical scavengers that inhibit phosphine-induced desulfurization of cysteine. This role can be assumed by the odourless, nontoxic, highly water-soluble and inexpensive ascorbate, which can replace the usually added thiols.