Fluorescence Study On The Conformational Transition Of Regenerated Silk Fibroin In Aqueous Solution

Natural silks, from spiders and silkworms, have attracted extensive attention for their outstanding mechanical properties. The unique spinning process of natural fibers is proved to be crucial to their mechanical properties. Silkworm silk is formed under mild physiological conditions compared with industrial polymers which are extruded with high temperature, high pressure, toxic solvents and high draw down ratios. Many factors such as pH value, metallic ions and so on are involved in the process of silk forming, and have certain effects on the conformational transition of silk fibroin. Therefore it is very necessary to understand the spinning mechanism in the formation process of natural silks in order to develop biomimetic spinning for the production of artificial silk fibers and other artificial materials with excellent properties.In this work, fluorescence techniques such as fluorescence quenching, fluorescence kinetic, external probe binding and so on, were used to study the conformational transition mechanism of regenerated Bombyx mori silk fibroin induced by both ethanol and Cu(II).Both fluorescence kinetic study and Thioflavin T as an external fluorescent probe were used for the f?? time to investigate the mechanism of the ethanol-induced conformational transition of silk fibroin in aqueous solution. A three-step mechanism is found: the destruction of originally existedβ-sheet conformation, the rearrangement of chain segments, and finally the formation ofβ-sheet conformation. Chain segments get more freedom to rearrangement if the originally existedβ-sheet conformation is destroyed more thoroughly, and therefore the conformational transition goes on more quickly. This is a dynamic-controlled process. Otherwise, the conformational transition goes on relatively slowly and finally moreβ-sheet conformation is formed. This is a thermodynamic-controlled process.Cu(II) is a good fluorescence quencher to Trp residues. Fluorescence quenching was mainly used to study the copper effect on silk fibroin. Results shows that both dynamic and static quenching are included in the quenching effect on Cu(II) to Trp, and Trp is also the probable binding site of fibroin besides His residue to Cu(II). Trp residues which locate in the hydrophobic region of fibroin is more favorable to Cu(II) binding. Cu(II) ions bind with the hydrophobic regions of fibroin to cause a conformational change.Several kinds of interesting assembly abilities of silk fibroin with the influence of certain ions were found in the study of TEM and AFM. The assembly is mainly controlled by the pH value of solution. From neutral solution, fibroin is able to form both particles with "shell-core" structure and fibre-like struc?? with regular arrangement at certain direction.