The hexa- and pentapeptide extension of proalbumin I. Chemical synthesis of serum albumin propeptides

The proalbumin hexapeptide extension was synthesized beginning from the C-terminal end by stepwise N-terminal peptide chain elongation starting from $\text{N-tert-}\text{butyloxycarbonyl}\text{-(N}{}^{\mathrm{g}}\text{-}\text{nitro)arginyl}\text{-(N}{}^{\mathrm{g}}\text{-}\text{nitro)arginine}$ 4-nitrobenzyl ester; $\text{[α]}{}_{365}{}^{20}\text{-12°}\text{C}$ ($\text{c = 1}$; dimethylformamide). The other amino acids were incorporated by excess mixed anhydrides of Ddz-amino acids (Ddz; 3,5-dimethoxyphenylisopropyloxycarbonyl) yielding the fully protected hexapeptide in crystalline quality. After removal of the protective groups by acid treatment and hydrogenation the peptide was purified by Dowex ion-exchange and Sephadex chromatography. The purity was confirmed by thin-layer chromatography and amino acid analysis.