Low-frequency electrical properties of multilayer preparations of haemoglobin

Abstract

Multilayers of beef haemoglobin were deposited by the Langmuir–Blodgett method onto glass slides bearing an evaporated aluminium electrode. Evaporation of another electrode onto the surface of the topmost layer allowed measurement of the capacitance and conductance of the preparation and hence calculation of the loss factor, tan δ, and other properties. Preparations with 1, 3, 5, 7, 11, 21 and 29 layers have been measured in the frequency range 10^–3–10⁵ Hz. For a given preparation, the measured values depended on the presence of adsorbed water, and the frequency variation of the electrical properties depended on ambient temperature and the value of the measuring voltage. The capacitance measurements show that the thickness of the preparations increased linearly with the number of layers at the rate of ca. 5 nm per layer. When the preparations were measured in air at room temperature and humidity there was a loss feature centred at a frequency in the range 10^–2–10 Hz. This feature disappeared when measurements were made in a hard vacuum but at the same temperature. For measurements made in air, the position of the loss feature moved to higher frequencies as the temperature was increased and the activation enthalpy of the process was ca. 1.8 eV (41 kcal mol^–1). However, if the temperature was raised above ca. 328 K or if the measuring field strength was above ca. 1 × 10⁷ V m^–1, large persistent changes were brought about in the measured capacitance and loss. These may have been brought about by changes in protein structure; but afterwards, the measured values of capacitance and loss reverted towards the initial values over a period of some weeks.