It is important to understand the conformational changes of both the crystalline and the noncrystalline domains of Bombyx mori silk fibroin as they are stretched from soluble Silk I to the mature Silk II fiber, to be able to design man-made strong and tough silk fibers. Here, the Ser, Tyr, and Ala Cβ of silk fibroin were labeled by 13 C to obtain domain-specific structural information, because Ser residues are present predominantly in the crystalline domains, Tyr predominantly in the noncrystalline domains and Ala residues in both domains. The conformational change was monitored by 13 C CP/MAS NMR using the conformation-dependent 13 C NMR chemical shift. With increasing stretching ratio, initially there is a change from Silk I* (a repeated type II β-turn structure) to β-sheet in the longer AGSGAG sequences, followed by shorter AGSGAG sequences, while the noncrystalline domains change gradually from random coil to β-sheet during the whole conformational change.

A better understanding of the correlation between the primary sequences and the conformations of silk fibroins (SF) is necessary in order to use silk more effectively as a functional material. In this paper, 13 C CP/MAS NMR was used to monitor separately the conformational transitions of [3-13 C]Ser-, [3-13 C]Tyr-, and [3-13 C]Alalabeled Samia cynthia ricini SF induced by stretching. The conformation was mainly α-helix with no β-sheet structure prior to stretching. At a stretching ratio of ×5, three peaks assigned to β-sheet structure were observed, and the fraction of these peaks increased rapidly upon further stretching. In particular, a rapid increase of the fraction of β-sheet at more than ×5 stretch was observed for the Ser residues that were associated with the transition of the polyalanine chain. For the Tyr residues, the transition from random coil to β-sheet occurs independently of the transition of the crystalline domain. These correlations were quantified and may be useful in future designs of artificial silk structure.

Genome-based discovery of two previously unreported fungal bifunctional terpene synthases (BFTSs) from phytopathogenic fungi are reported: FoFS catalyzing the formation of fusoxypenes A–C (1–3) and (−)-astellatene (4) and AtAS capable of synthesizing preaspterpenacid I (6). Interestingly, FoFS and AtAS catalyzed the formation of enantiomeric sesterterpenes with a 5–6–7–3–5 ring system. C22-oxidative modification of preaspterpenacid I by AtP450 was characterized as well. Plausible cyclization pathways of the fusoxypenes were illustrated by DFT calculations.