Dielectric Structure of Aqueous Interfaces: From Classical Non-Debye Electrostatics to a Quantum Theory of Interfacial Tension

Abstract

This chapter explores the dielectric structure of interfacial water from a classical and quantum electrostatics perspective. The protein-water interface is treated classically, while quantum effects are investigated only for simple featureless nonpolar interfaces due to the inherently higher complexities of the quantum approach. In a classical context, the most striking feature arising from the epistructural physics is the breakdown of the Debye ansatz that postulates the alignment of water polarization with the protein electrostatic field. The complexities of biological interfaces are shown to be in good measure due to this departure from the standard dielectric picture that has been historically—and incorrectly—extrapolated from a bulk interface. Accordingly, concepts like the dielectric permittivity coefficient are shown to be inadequate to describe interfacial electrostatics. The departure from bulk-like behavior is shown to enhance the physico-chemical inhomogeneity of protein surfaces and to enable the chemical functionality of the aqueous interface. Epistructural physics of the protein-water interface identifies a structural defect known as dehydron as causative of anomalous polarization effects causing a breakdown of the Debye standard ansatz. The previous chapter revealed that interfacial tension is a central thermodynamic factor driving biomolecular events and may be alternatively stored as electrostatic energy associated with the non-Debye component of water polarization. This chapter substantively supplements this picture by showing that dehydrons induce chemical basicity in interfacial water as a consequence of the breakdown of Debye dielectrics. Thus, the relevance of dehydrons as catalytic elements is highlighted. We anticipate that this discovery will prompt a re-writing of vast mechanistic chapters of biological chemistry.