AN ELECTRONIC ANALOG FOR THE OLFACTORY PROCESSES * †

The adsorption of odorant molecules at the olfactory receptors is a prerequisite for olfactory information. The manner in which the biophysical mechanism at the receptor surface functions as a transducer for the information carried by the odorant molecule is still not resolved. Several theoretical approaches of merit relating to this mechanism have recently been advanced by Amoorel and Dravnieks2 The apparent energy input contributed by the odorant molecule is extremely small and may be in the range of 1 to 5 kcal. per mole, or equivalent to van der Waals forces. The theory advanced by Amoorel on the stereochemical configuration of receptor sites may also be envisioned as an odorant-receptor site compatability to produce the maximum energy exchange to effect rearrangement of the receptor structure. This change in energy levels of the receptor biochemical complex may result in either a more or less ordered configuration of the membrane structure so that the ionic potential gradient may become effective in the propagation of the nerve impulse. In line with present day knowledge of the neural mechanism, this would be possible if the assumption is made that the acetylcholine esterase system is operative in the receptor membrane. Any attempt to devise a qualitative and quantitative analog for the receptor surface can at this time be approached only by constructing detector mechanisms which display differential adsorption properties for various odorant compounds or detectors which depend on some physical o r chemical change at the detector surface which can be translated into an electrical signal. Such attempts in this field have been made by hart mat^,^ Hartman and T ~ l l e , ~ B e r t ~ n , ~ Moncrieff,6 and by Wilkens and Hartman.7 The electrochemical device developed in the early 1950's by J. D. Hartman has recently been refined to the state where this type of detector device shows excellent promise as a form of instrumentation for rapid and objective measurement of volatile flavors and other odors. The sensing element (FIGURE 1 ) consists of a polished metal wire microelectrode which contacts the surface of a porous rod saturated with a dilute electrolyte. This results in a small electrolyte-metal interface, to which is applied a low D.C. potential. A considerably larger electrode of platinum, in contact with the electrolyte, forms the complete detector cell circuit. When a D.C. potential is impressed on the detector cell, polarization occurs at the microelectrode. After several minutes, a nearly steady state current is obtained. As odorants in a moisture-saturated air carrier are introduced to the microelectrode-electrolyte juncture, a change in polarization is produced which is measured as current fluctuation. By the use of various combinations of metal microelectrodes, electrolytes, and impressed D.C. potentials, a system of several electrodes (FIGURES 2 and 3) may be assembled to operate almost simultaneously with differential response specificity to odorants, such as to make the system both qualitatively and quantitatively discriminating.