Elucidation of silk fiber formation by multi-modal SPM

S. Cohen
Weizmann Institute of Science,

Keywords: silk, nanomechanics, nano-IR, multimodal SPM


Silk is characterized by useful qualities, which make it a fascinating biomaterial – it combines high strength with flexibility, while being naturally biocompatible and biodegradable. The composition is not complex – being comprised from simple protein building blocks whereby the assembly of these components governs the fiber properties. Years of study have led to a basic understanding of the processing steps by which the proteins form the final product. Nonetheless, these studies have not led to synthetic routes to biomimetic silk fibers with competing properties. Full understanding of the silk structure and function requires comprehensive nano-scale elucidation. In this talk, I will present SPM imaging, nanomechanics, and nano-IR modes showing the chemico-physical changes occurring as the monomeric material is spun into fibrils which eventually fully develop into fibers. The multimodal SPM approach adapted here is used to study silk at different stages of the maturation process. These nanoscale studies are combined with modelling, structural studies, and bulk analytical work which, together with basic knowledge of the silk-spinning mechanism in the organism, provide a full understanding of the mechanistic route. During the assembly, the proteins undergo a structural transition from disordered alpha-helical and random coil states into ordered, crystalline beta-sheets. This process takes place by way of nanocompartments which are themselves encapsulated in microcompartments and provide a closed environment allowing the changes to take place reversibly. Key observations from these studies include: increasing beta-sheet nature (amide I bands) as processing encloses the protein monomers into nanocompartments, and then aligns them into fibers; the formation of nanofibrils proceeds through assembly of small spheres corresponding to the nanocompartments; elastic modulus of the silk monomers is similar to that of the nanocompartments, and that of the silk microfiber similar to that of the nanofibrils. This work reveals how the required tight control of the assembly process is achieved by regulating the stage at which fibrils are formed and fibers spun. The protein monomers produced in the gland are subject to concentration and pH changes, as well as to shear forces. The fiber formation and spinning must occur at the right stage. The microcompartments form only at the critical micelle concentration, and due to their exposed hydrophilic nature, prevent premature organization into β-sheet structure. Nanocompartmentalization is accompanied by changes in local surface charge which drive the assembly, and fibril formation is driven by pH changes which further affect charged interactions, as well as by shear forces which force water from the nanoassemblies as they extend into elongated strings. The SPM techniques follow the change in mechanical and chemical properties associated with the different stages at the single particle level, giving a complete molecular-based picture of the silk formation process.