A wonderful process of soluble silk fibroin transiting to solid silk catches great attentions for decades. In this paper, fluorescence and circular dichroism (CD) spectroscopy were used to monitor the conformational transition of regenerated Bombyx mori silk fibroin (RSF) in aqueous solutions under different pH values. It is shown that the secondary structure of RSF is very sensitive to pH value of solution. During acid driving conformational transition process, the silk protein aggregated near the isoelectric point of RSF (pI＝4.22), whereas its conformation is relatively stable as pH value is around 6. These changes are in accordance with the states and environments of fibroin along the silkworm silk gland, showing the essential way for silkworm controlling the state of fibroin by pH. On the other hand, the existence of intermediate during pH induced unfolding process implied that the conformational transition of RSF does not fit the simple two-state mechanism.

Key words: silk protein, conformation transition, circular dichroism, fluorescence spectrum