Piezoelectric measurements of solid state biomolecular assemblies including globular and membrane proteins

Biomolecular assemblies are those that are composed of the various biological molecules that nature has crafted over millennia. These range from simple amino acid single crystals, to macro-molecule membrane protein crystals, all the way up to collections of fibrous proteins that form biological tissues. The form of electromechanical coupling known as piezoelectricity appears to manifest throughout the hierarchy of biological materials, appearing in tissues such as bone and tendon, amino acid single crystals, and almost everything in-between. Piezoelectricity is a linear form of electromechanical coupling in which an applied mechanical stress can induce an electrical polarisation, or vice versa, an applied electric field can induce a mechanical strain. Due to the intrinsic chirality of natural building blocks such as amino acids, biomolecular assemblies tend to adopt non centrosymmetric structures, endowing them with functional physical properties such as non-linear optical activity, piezoelectricity, pyroelectricity and ferroelectricity. To begin using these types of materials in technological applications, the issue of accurately and unambiguously measuring piezoelectricity in these materials must be addressed. This work focuses on the task of measurement, utilising molecular models to inform and benchmark the measurements. We use traditional measurement techniques such as the Berlincourt method to characterise crystal films of the proteinogenic amino acid L leucine, demonstrating its ability to generate up to 8 V under manual compression of 40 N. Following this, we outline a quantitative protocol for piezoresponse force microscopy before demonstrating its application to biomolecular racemic amino acid crystals. Subsequently, the techniques developed are applied to characterise the transmembrane protein ba3 cytochrome c oxidase, representing the first-time piezoelectricity has been measured in a membrane protein. The classical molecular models developed to support this work are a first for this research field, being able to predict piezoelectric coefficients of enormous molecular crystals, a feat previously unattainable with quantum mechanical models. Both longitudinal and shear piezoelectric coefficients were measured in ba3 cytochrome c oxidase, suggesting that the crystallographic symmetry is reduced during solid-state characterisation which allows for both a greater magnitude and greater number of non-zero coefficients. Furthermore, the magnitude of piezoelectricity experimentally measured in this work is comparable to that of widely used inorganic piezoelectric materials, suggesting that exotic biomaterials such as membrane protein crystals could be technologically viable alternatives as society moves towards a greener future