Raman study of poly(alanine‐glycine)‐based peptides containing tyrosine, valine, and serine as model for the semicrystalline domains of Bombyx mori silk fibroin

For a deeper insight into the structure of Bombyx mori silk fibroin, some model peptides containing tyrosine (Y), valine (V), and serine (S) in the basic (AG) n sequence were synthesized by the solid‐phase method and analyzed by Raman spectroscopy in order to clarify their conformation and to evaluate the formation and/or disruption of the ordered structure typical of B. mori silk fibroin upon incorporation of Y, V, and S residues into the basic (AG) n sequence. The Raman results indicated that the silk I structure remains stable only when the Y residue is positioned near the chain terminus; otherwise, a silk I → silk II conformational transition occurs. The peptides AG V GAG Y GAG V GAG Y GAG V GAG Y G(AG) 3 and (AG) 3 Y G(AG) 2 V G Y G(AG) 3 Y G(AG) 3 treated with LiBr revealed a prevalent silk II conformation; moreover, the former contained a higher amount of random coil than the latter. This result was explained in relation to the different degrees of interruption of the (AG) n sequence. The Raman analysis of the AGSGAG‐containing samples confirmed that the AGSGAG hexapeptide is a good model for the silk II crystalline domain. As the number of AGSGAG repeating units decreased, the random coil content increased. The study of the Y domain (I 850 /I 830 intensity ratio) allowed us to hypothesize that in the packing characteristic of Silk I and Silk II conformations the Y residues experience different environments and hydrogen‐bonding arrangements; the packing typical of silk I structure traps the tyrosyl side chains in environments more unfavorable to phenoxyl hydrogen‐bonding interactions. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004