Electrical Conductivity of Proteins. II. Semiconduction in Crystalline Bovine Hemoglobin

Evidence has been presented to show that crystalline bovine hemoglobin is a semiconductor in the dry state. The semiconduction activation energy is 2.3 ev. This is similar to the values obtained for a number of other proteins, chromoproteins, and biological structures which have proteins as the main constituent. The effect of adsorbed water is to increase the conductivity enormously. The dependence on the amount of adsorbed water is exponential, in agreement with the results of Baxter and Spivey. In the ``wet'' state, the protein is still a semiconductor, but the semiconduction activation energy is lower. The dependence of the activation energy on the amount of adsorbed water is given by $$E_W{\mathrm{=E}}_D{\mathrm{-\delta }}_m.$$ I t has been proved that this is the only effect of water in causing the increase in conductivity. The value of σ₀ in the semiconduction formula is 7×10³[Ω—cm]^—1 and is independent of the adsorbed water content. The effect of water adsorption on the conductivity is reversible when the water is desorbed. Experimental evidence, which strongly indicates that the conductivity process in the wet protein is electronic rather than ionic, has been given previously by the writer. A theory is developed which shows that the effect of adsorbed water in increasing the dielectric constant of the protein lowers the work necessary to separate the charges due to the increase in the polarizability of the medium. This leads to a prediction that the dependence of the dielectric constant on the adsorbed water content should be of the form $${\kappa }^{\prime }\text{=κ/(1−κχm).}$$ Some previous measurements of Medley and King are in semiquantitative agreement with this. The theory also predicts that for higher adsorbate concentrations, the conductivity should become constant (saturation effect). Again, the measurements of Medley and King confirm this. This saturation effect allows an estimate to be made of the radius of the polarization (R) and the ionization energy minus the electron affinity (I — A) for the protein. R≈2.8×10^—8 cm and I — A≅10 ev. Both values are in reasonable agreement with expectations.