Molecular origin of piezo- and pyroelectric properties in collagen investigated by molecular dynamics simulations

Molecular dynamics simulations were used to study the effect of mechanical and thermal stimuli on the electrostatic properties of collagen model helices. Our model sequences were based on glycine proline and hydroxyproline amino acids. We find that longitudinal mechanical strain induces significant variation of the polarization of the collagen fibril. Such a phenomenon is determined by reorientation of the backbone polar groups, which are free to respond to the mechanical solicitation. This non-negligible effect is facilitated by the peculiar folding structure of the collagen helix, which is characterized by the absence of an extended hydrogen-bond network. The stretching/compression of the helix requires a concomitant winding/unwinding motion of the global structure; therefore, the shear components of the stress tensor are the components that most effectively induce structural modification associated to the piezoelectric response. The present calculations also report a pyroelectric response to thermal activation. Model calculations indicate that the pyroelectric effect is dominated by secondary components associated with the piezoelectric tensor.