Colloidal Properties of Recombinant Spider Silk Protein Particles

Colloidal particles have been prepared from polyanionic and polycationic recombinant spider silk protein. The amino acid sequences of these spider silk proteins are identical except for 16 residues bearing either a cationic or an anionic ionizable group. Electrophoretic titration showed that protonation of the acidic and basic amino acids had significant impact on the electrophoretic mobility of the protein particles and, in particular, on their point of zero mobility (PZM). The experimentally determined PZMs are in good agreement with the theoretical values evaluated on the basis of the relevant amino acid sequences. A comprehensive description of the electrokinetic properties of the recombinant spider silk protein particles as a function of pH and solution ionic strength was provided from adequate application of electrokinetic theory for soft particles. Within the framework of this formalism, spider silk protein particles are viewed as porous colloids penetrable for ions and characterized by a finite penetration length for the electroosmotic flow. The differentiated electrokinetic properties of the particles were shown to be solely governed by the electrohydrodynamic features of their poorly charged outer peripheral layer with a thickness of about 10–20 nm. This finding was further corroborated experimentally by demonstrating that electrokinetics of particles bearing an additional outer layer consisting of oppositely charged spider silk proteins is entirely dominated thereby. The presence of a fuzzy, ion-permeable particle interface with an extension of several tenths of a nanometer was confirmed by direct measurement of the resulting steric forces using the colloidal probe atomic force microscopy (AFM) technique.