Energy Bands
The electronic conductivity of a material is determined by the properties of its constituent atoms or molecules, and by the manner in which they are arranged in the lattice (1). Conductivity can be described in terms of a solid-state model that relates electronic processes to valance and conduction energy bands. The valance band consists of electrons that, because they have relatively low energy, are associated with individual atoms or molecules: the conduction band contains more energetic electrons that are free to move throughout the material in response to applied electromagnetic energy.
The number and mobility of conduction electrons determines the electronic conductivity of a material. If the valance and conduction bands are separated by a small gap, then, at typical temperatures, thermal activity will deplete the valance band and populate the conduction band; such a material is a conductor. If the bands are widely separated in energy, the conduction band will be vacant and the material will be an insulator. A semiconductor is a material whose band structure falls between that of a conductor and an insulator-it can be an insulator at one temperature and a conductor at a higher temperature. Semiconductors can contain impurity atoms whose energy states lie within the gap between the valance and conduction bands; such impurities strongly affect conductivity by donating or accepting electrons.
An important consequence of the existence of energy bands is that they permit electronic processes in one region of a material to affect not only the immediate area, but also the entire structure. Szent-Gyorgyi proposed that common energy levels existed over relatively large dimensions in biological structures, possibly with the cell wall itself as the boundary (2). Evans and Gergely (Szent-Gyorgyi's student) calculated the band gap in hydrogen-bonded models of biopolymers and showed that it would be so large that the biopolymers would behave electrically as insulators (3).
However, if impurity atoms were present, they could donate an electron to the conduction band, or remove one from the valance band, leading to mobile conduction electrons or mobile "holes" in the valance band (4, 5). Szent-Gyorgyi postulated that these electronic processes within the energy bands-electron mobility in the conduction band and charge transfer in the valance band-could give rise to biological phenomena and, indeed, to life itself (6, 7). Figures 4.1 and 4.2. depict his theory as applied to the bioelectrical role of ascorbate.
Fig. 4.1. A. A large protein molecule contains many electron pairs. In this state, a pair of electrons is very stable and unreactive; thus the molecule as a whole is very stable and unreactive. B. A pair of electrons with a negative charge. C. A methylglyoxal molecule with an uncoupled electron pair; i.e. an electron is missing from one of the orbital rings. In this state, the methylglyoxal molecule is a free radical and is highly reactive. It can now accept electrons from another molecule to fill its empty orbital ring. (Reproduced, by permission from Nutrition Today, P. O. Box 1829, Annapolis, Maryland 21404, September/October, 1979.)
For ordinary materials the question of their band structure could be resolved by a coordinated series of X-ray, chemical, and electrondynamics studies. But biological tissue is inhomogeneous and impure, and suitable techniques for carrying out many of the necessary studies on such materials have not yet been developed. Perhaps the most significant problem for the experimentalist is that posed by the universal presence of water in tissue. It is well established that the electrical conductivity of tissue increases sharply with water content (8,9). However, the nature of electrical conduction in tissue under physiological conditions of temperature and moisture-the relative contribution of electronic, protonic, and ionic processes-has not been established despite more than 30 years of study (10). Thus, no clear picture of the band structure in tissue has emerged. Other important solid-state techniques that have been used to study the electronic property of biological tissue include electron paramagnetic resonance (11-13), and photoconductivity (14-16). Again, although the results are consistent with a common-energy-band model proposed by Szent-Gyorgyi, they do not establish it as correct.