Spontaneous oxidation of a short palindromic peptide with the sequence Ac-C-X-K-L-H-A-E-L-S-S-L-E-A-H-L-K-B-C-G-NH2 (X = Aib) provides up to three different cyclic products : a monomer with an intrachain disulfide bond, an antiparallel dimer, and a trimer with two parallel and an antiparallel chain. Control over the relative amount of the different products can be achieved by adding different amounts of TFE that modulate the population of the alpha-helix conformation during the reaction. Regioselective disulfide formation affords also the parallel dimer,which was not formed spontaneously, as well as the three possible noncyclic dimers with two protected cysteines and a single disulfide bond. CD spectroscopic studies of these dimers as well as NMR and CD studies of a peptide with-the two cysteines replaced by Leu provide the basis for ＇understanding the control of the spontaneous oxidation by TFE and the strong discrimination between parallel and antiparallel dimers. The special topological properties of this trimer and the fact that is formed spontaneously in high yield from the monomer provide a Potentially very useful building block for the construction of peptidic receptors as well as minimalist models for alpha-helical packing in globular proteins.