Bombyx mori silk fibroin is a fibrous protein whose fiber is extremely strong and tough, although it is produced by the silkworm at room temperature and from an aqueous solution. The primary structure is mainly Ala-Gly alternative copolypeptide, but Gly-Ala-Ala-Ser units appear frequently and periodically. Thus, this study aims at elucidating the role of such Gly-Ala-Ala-Ser units on the secondary structure. The sequential model peptides containing Gly-Ala-Ala-Ser units selected from the primary structure of B. mori silk fibroin were synthesized, and their secondary structure was studied with (13)C CP/MAS NMR and wide-angle X-ray scattering. The (13)C isotope labeling of the peptides and the (13)C conformation-dependent chemical shifts were used for the purpose. The Ala-Ala units take antiparallel beta-sheet structure locally, and the introduction of one Ala-Ala unit in (Ala-Gly)(15) chain promotes dramatical structural changes from silk I (repeated beta-turn type II structure) to silk II (antiparallel beta-sheet structure). Thus, the presence of Ala-Ala units in B. mori silk fibroin chain will be one of the inducing factors of the structural transition for silk fiber formation. The role of Tyr residue in the peptide chain was also studied and clarified to induce "locally nonordered structure."