Self-assembly of hydrolysed α-lactalbumin into nanotubes

Abstract

Self-assembly of proteins, peptides and DNA is a powerful approach for fabricating novel supramolecular architectures. Via this "bottom-up" approach many new nanomaterials have been and will be produced. Building blocks that self-assemble into fibrous materials are of special interest, because linear structures have many advantages. Partial hydrolysis of α-lactalbumin, a 14.2 kDa whey protein, by a protease from Bacillus licheniformis, results in such building blocks that self-assemble into unique nanotubes in the presence of appropriate cations at neutral pH.

This PhD thesis describes structural and dynamic properties of α-lactalbumin nanotubes. Structural characterisation of the α-lactalbumin nanotubes was performed using scanning force microscopy, static light scattering, small angle X-ray scattering and (cryo) electron microscopy. The partially hydrolysed α-lactalbumin molecules self-assemble into regular right-handed helical structures. These micrometre-long hollow cylinders have a diameter of 20 nm, and a cavity diameter of 8 nm. The α-lactalbumin molecules are presumably assembled via β-sheet stacking. In addition, Ca2+ or another divalent ion is needed to form the nanotubes. Salt bridges between specific carboxyl groups play an important role. Dynamic features described are the kinetics of assembly and disassembly, and the stability of the nanotubes under various conditions. Disassembly can be induced by removal of Ca2+ in the solvent. Chemical or enzymatic cross-linking of the nanotubes prevents disassembly and makes them stable.

The results obtained in this research may help to develop nanometre-sized materials that have a variety of applications in foods, pharmaceutics and nanotechnology.