Abstract: Thymosin α1(Tα1) is an immunomodulating peptide isolated from thymic extracts containing 28 amino acid residues. It has been demonstrated that the C-terminal fragment of Tα1 is responsible for its activity. In order to increase bioactivity and improve stability to proteolytic enzymes of this fragment, a series of cyclic peptides based on the active fragment were designed, and synthesized through formation of thioesters with various amount of residues or a flexible linker. The resulting cyclic peptides were determined via ESI-MS and HPLC. The bioactivity was assayed in vitro through T-lymphocyte proliferation testing. Data from this test show that most of the cyclic analogs retained the immunoactivity, and some of them better than the original fragment. It can be concluded that cyclizations of this active fragment lead to the increasing of the bioactivity.

Key words: Thymosin α1, Cyclic peptide, Thioesters, Immunoactivity