Synthetic chemoreceptive membranes. Sensing bitter or odorous substances on a synthetic lipid multibilayer film by using quartz-crystal microbalances and electric responses

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Abstract
Specific adsorptions of bitter or odorous substances on a synthetic lipid multibilayer matrix (2C18N+2C1/PSS-) were detected by observing frequency changes of a multibilayer-coated quartz-crystal microbalance (QCM). Partition coefficient (P) and diffusion constants (D) of these substances in the lipid matrix could be obtained quantitatively by using the QCM method. There were good correlations between partition coefficient of various bitter or odor substances to the synthetic multibilayer film on the QCM and the intensity of bitter tastes or olfactory receptions in humans: the stronger the intensity of a bitter substance or odorant, the greater the adsorption on the lipid matrix. This indicates that the lipid-coated QCM acts as a sensitive and selective sensor for bitter taste and odor. Electric responses (changes of membrane potential and membrane resistance) of the 2C18N+2C1/PSS- film occurred consecutive by the adsorption of these substances. The bitter or odor substance showing the stronger intensity induced membrane potential change in lower concentrations. It was found that bitter substances having sterically bulky molecular structures absorb on the surface of the lipid-matrix, and the phase-boundary potential of the membrane is thereby changed. On the contrary, odor substances with relatively small or slender structures can penetrate into the lipid matrix and cause reduction of the membrane resistance (the increase of ion permeability). The selective adsorption behavior of bitter and odor substances by molecular shapes was confirmed by adsorption studies of simple C9-10 hydrophobic alcohols having various molecular structures.