Piezoelectricity in the globular protein lysozyme

Piezoelectricity is a particular type of electromechanical coupling in which a mechanical stress induces an electrical polarisation in materials that have a non-centrosymmetric crystal structure. A converse piezoelectric effect also exists in which an electrical field induces a mechanical strain. Although originally discovered in quartz, a solid-state material, the piezoelectric effect extends to include many biological materials such as wood, bone, viruses, amino acids and proteins. The classical theory of piezoelectricity describes the effect in solid-state materials fully; however, a comprehensive understanding of how piezoelectricity manifests in biological materials is lacking. Without this understanding, the physiological significance and potential applications of biological piezoelectricity cannot be realised. This thesis studied the electromechanical properties of lysozyme, a globular protein found abundantly in hen-egg whites. Crystals of the protein lysozyme were probed for piezoelectricity in order to understand whether the classical description of piezoelectricity applies to them. Films of lysozyme crystals were structurally characterised using synchrotron diffraction. The direct piezoelectric effect was quantified at the macroscale using static and quasi-static approaches based on the Berlincourt Method. The converse piezoelectric effect was also measured in crystals of lysozyme using Piezoresponse Force Microscopy. Furthermore, the related properties of ferroelectricity (spontaneous switchable polarisation) and pyroelectricity (changes of polarisation induced by a changing temperature) were investigated. Ferroelectricity was probed using Switching Spectroscopy Piezoresponse Force Microscopy and pyroelectricity was measured using a modified Byer-Roundy Method. A major finding of this thesis is that electromechanical coupling exists in crystals of lysozyme and can be described by the piezoelectric effect. This is the first example of piezoelectricity in a non-fibrous protein. Quantitative measurements show that the piezoelectric coefficients of monoclinic and tetragonal crystals of lysozyme are 1 pC N-1 and 6 pC N-1, respectively. This is significant; the piezoelectric coefficient of quart is 2 pC N-1. Unexpectedly, both longitudinal piezoelectricity and ferroelectricity was observed in tetragonal crystals of lysozyme whose symmetry should preclude these effects indicating that the crystals may be of lower symmetry. Observations of pyroelectricity and piezoelectricity in monoclinic lysozyme are consistent with classical theories. The findings contribute to our understanding of protein piezoelectricity and imply that the classical theory of piezoelectricity is applicable to biological materials.