Bombyx mori silk fibroin molecule is known to exist in two distinct structural forms: silk I (unprocessed silk fibroin) and silk II (processed silk fibroin). Using synthetic peptides, we attempt to explore the structural role played by Ser and Tyr residues on the appearance of silk I structural form of the fibroin. Twelve selected peptides (1-12) incorporating Ser and Tyr residues in the (Ala-Gly)(n) copolypeptide, that is, the sequences mimicking the primary structure of B. mori silk fibroin molecule, have been investigated under the silk I state, employing high-resolution (13)C cross-polarization/magic-angle spinning (CP/MAS) NMR spectroscopy. To acquire the silk I structural form, all the peptides were dissolved in 9 M LiBr and then dialyzed extensively against water, as established previously for the synthetic (Ala-Gly)(15) copolypeptide and B. mori silk fibroin. The diagnostic line shape of the Ala C(beta) peaks and the conformation-dependent (13)C chemical shifts of Ala and Gly resonances are presented to analyze and characterize the structural features. The results indicate that the incorporation of one Ser and/or one Tyr residue(s) at selected position in the basic (Ala-Gly)(15) sequence tend to retain predominantly the silk I structure. Conversely, the repeat pentameric and octameric Ala-Gly-Ser-Gly-Ala-Gly sequences, for example, (Ala-Gly-Ser-Gly-Ala-Gly)(5) or (Ala-Gly-Ser-Gly-Ala-Gly)(8), preferred predominantly the silk II form. The peptide sequences incorporating Ser and Tyr residue(s) into repeat Ala-Gly-Ser-Gly-Ala-Gly sequences, however, adopted the silk II structure with certain content structural heterogeneity or randomness, more pronounced for specific peptides studied. Interestingly, the crystalline Cp fraction of B. mori silk fibroin, when mixed with (Ala-Gly-Ser-Gly-Ala-Gly)(5) sequence in a 5:1 molar ratio, dissolved in 9 M LiBr, and dialyzed against distilled water, favor the silk I form. The finding tends to suggest that the less stable silk I form in (Ala-Gly-Ser-Gly-Ala-Gly)(n) sequences is likely to be induced and facilitated via intermolecular interactions with the Cp fraction, which predominantly prefers the silk I form under similar conditions; however, the hydrogen-bond formation involving O(gamma)H groups of the Ser residues may have some implications.