Abstract: This invention relates to a biotinylating reagent being used in purification process for synthesis mature peptide. The reagent has at one terminus a functional group capable of forming a bond with the N-terminus amino group of the mature peptide. The bond is stable to the final deprotection reaction under acidic condition at the post-synthesis step of the peptide synthesis process while the bond is specifically cleaved under a difined condition such as basic condition. The reagent also has at the other terminus a biotinyl group. The biotinyl-avidin coupling is used when the reagent modified mature peptide are immobilized selectively on avidin-agarose column. The compounds having following formula (II) are preferable as the reagent.