  Assessing the structural properties of large proteins is important to gain an
understanding of their function in, e.g., biological systems or biomedical
applications. We propose a method to examine the mechanical properties of
proteins subject to applied forces by means of multiscale simulation. We
consider both stretching and torsional forces, which can be applied
independently of each other. We apply torsional forces to a coarse-grained
continuum model of the antibody protein immunoglobulin G (IgG) using
Fluctuating Finite Element Analysis and identify the area of strongest
deformation. This region is essential to the torsional properties of the
molecule as a whole, as it represents the softest, most deformable domain. We
subject this part of the molecule to torques and stretching forces on an
atomistic level, using molecular dynamics simulations, in order to investigate
its torsional properties. We calculate the torsional resistance as a function
of the rotation of the domain, while subjecting it to various stretching
forces. We learn how these obtained torsion profiles evolve with increasing
stretching force and show that they exhibit torsion stiffening, which is in
qualitative agreement with experimental findings. We argue that combining the
torsion profiles for various stretching forces effectively results in a
combined force-torque spectroscopy analysis, which may serve as a mechanical
signature for the examined molecule.
