We report a method to profile the torsional spring properties of proteins as a function of the angle of rotation. The torque is applied by superparamagnetic particles and has been calibrated while taking account of the magnetization dynamics of the particles. We record and compare the torsional profiles of single Protein G-Immunoglobulin G (IgG) and IgG-IgG complexes, sandwiched between a substrate and a superparamagnetic particle, for torques in the range between 0.5 × 10(3) and 5 × 10(3) pN·nm. Both molecular systems show torsional stiffening for increasing rotation angle, but the elastic and inelastic torsion stiffnesses are remarkably different. We interpret the results in terms of the structural properties of the molecules. The torsion profiling technique opens new dimensions for research on biomolecular characterization and for research on bio-nanomechanical structure-function relationships.
Control over the
placement and activity of biomolecules on solid
surfaces is a key challenge in bionanotechnology. While covalent approaches
excel in performance, physical attachment approaches excel in ease
of processing, which is equally important in many applications. We
show how the precision of recombinant protein engineering can be harnessed
to design and produce protein-based diblock polymers with a silica-binding
and highly hydrophilic elastin-like domain that self-assembles on
silica surfaces and nanoparticles to form stable polypeptide brushes
that can be used as a scaffold for later biofunctionalization. From
atomic force microscopy-based single-molecule force spectroscopy,
we find that individual silica-binding peptides have high unbinding
rates. Nevertheless, from quartz crystal microbalance measurements,
we find that the self-assembled polypeptide brushes cannot easily
be rinsed off. From atomic force microscopy imaging and bulk dynamic
light scattering, we find that the binding to silica induces fibrillar
self-assembly of the peptides. Hence, we conclude that the unexpected
stability of these self-assembled polypeptide brushes is at least
in part due to peptide–peptide interactions of the silica-binding
blocks at the silica surface.
Protein conformational changes are essential to biological function, and the heterogeneous nature of the corresponding protein states provokes an interest to measure conformational changes at the single molecule level.
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