Membrane permeabilities for uncharged species plotted against n-octanol/water partition coefficients for indoleamines and model compounds. Compounds and partition coefficients are from Table 2. Red circles are measurements from Orbach and Finkelstein (1980) fitted with a unity slope regression line (Eq. 3). Red crosses are compounds of physiological interest, including indoleamines predicted using Eq. 3 for lecithin membranes and assuming that they are in a 100% uncharged form. The red triangle below is indole-3-ethanol from Bean et al. (1968) measured in tocopherol/cholesterol/brain lipid membranes. Green crosses below include the indoleamines, plotted with Eq. 1a, which gives permeabilities 1,322-fold lower than Eq. 3. The green relationship is our best guess for biological membranes using Pcell for melatonin set at 1.7 µm/s. Note that for all compounds that protonate, the practical permeability will be lowered further as a function of pH because the charged forms are nearly impermeant. Such reduced values are given as Pcell in Table 2 and Eq. 1b for pH 7.4.
