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We suggest a statistical thermodynamic theory for the conformational transition of a synthetic alanine (Ala), α-aminoisobutyric acid (Aib) alternative oligopeptide, Buo-(Ala-Aib)n-oMe, where the terminal groups Buo and oMe stand for t-butoxy and methoxy, respectively. Pure Aib homo-oligomers have always been found to adopt 310 helical conformations, while polyalanine has always α helical conformation. In an organic solvent (e.g. CD3CN) it shows that the length for the 310/α helix transitions of Buo-(Ala-Aib)n-oMe, is 8 at room temperature. In an aqueous solution oligopeptide has always coil conformation at room temperature. In an organic solution, helical structures of the oligopeptide are more stable than coil structure, so we studied the 310/α helix transitions, considering coiled-conformations, coiled and 310 helical conformations, and coiled and α helical conformations by using the zipper model. We determined the values of parameters (σA, σT, ξA, ξT) from the relating published data; σA = 0.00011, σT = 0.0060, ξA = 10.1, ξT = 3.90. The distributions of α helical length can be N-2, N-3, N-4, …, 3, 2, 1 (N = 2n) while those of 310 helical length, N-1, N-2, N-3, N-4, …, 3, 2, 1.
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