Metallic Conductivity in Proteins: A New Paradigm for Biological Electron Transfer*

Is quantum mechanics driving some of the most elegant and inexplicable processes of life, such as respiration, metabolism and communication? In particular, electron transfer via proteins is fundamental to life but believed to occur only via tunneling or hopping. Quantum-mechanical electron delocalization and interference give rise to metallic conductivity in synthetic materials but has been considered impossible in proteins due to lack of periodicity, thermal fluctuations and low conductivity. Thus, it has been almost universally thought that the thermal noise of life would mask out any observable quantum phenomenon in proteins. A cornerstone of quantum physics is the interference of electron waves arising from the superposition principle. Metallic conductivity is an effect of interference of partial electron waves multiply scattered at the ion cores of the crystal lattice. But proteins are generally insulators. We have developed novel tools that have enabled for the first time direct visualization [1] and quantification [2] of metallic conductivity in protein filaments called pili. Soil bacteria Geobacter sulfurreducens ues pili to export electrons to extracellular electron acceptors, enabling bacteria to carry out respiration and energy sharing in environments that lack membrane-permeable electron acceptors such as oxygen. We have also shown that the conductivity of pili is essential for bacterial survival and growth. Our major enabling insight is that the metallic conductivity in pili arises from pi stacking of aromatic amino acids. With scanning probe microscopy, we are quantitatively imaging the quantum interference and entanglement in pili to determine electron coherence length and time. We are also evaluating the role of these quantum processes in bacterial growth, communication and community development.

[1] Malvankar et al. Nature Nanotechnology 9, 1012-1017 (2014)
[2] Malvankar et al. Nature Nanotechnology 6, 573-579 (2011)