Electrostatic and Hydrogen Bonding in a Low-Dielectric Medium

Section 3.3.4 pointed out that cosolvents alter aqueous ionization constants as the dielectric constant of the mixture decreases, acids appear to have higher pKa values and bases appear (to a lesser extent than acids) to have lower values. A lower dielectric constant implies that the force between charged species increases, according to Coulomb s law. The equilibrium reaction in Eq. (3.1) is shifted to the left in a decreased dielectric medium, which is the same as saying that pKa increases. Numerous studies indicate that the dielectric constant in the region of the polar head groups of phospholipids is 32, the same as the value of methanol. [381,446-453] Table 5.2 summarizes many of the results. 

These and other values [381,406] allow us to depict the dielectric spectrum of a bilayer, shown in Fig. 5.2. Given this view, one can think of the phospholipid bilayer as a dielectric microlamellar structure as a solute molecule positions itself closer to the center of the hydrocarbon region, it experiences lower dielectric field (Fig. 5.2). At the very core, the value is near that of vacuum. A diatomic molecule of Na+Cl- in vacuum would require more energy to separate into two distinct ions than that required to break a single carbon-carbon bond  

An interesting hypothesis may be put forward. The interfacial pA lcm (Fig. 5.1) that a solute exhibits depends on the dielectric environment of its location in the bilayer. Simple isotropic water-miscible solvents may be used to approximate p mem pure methanol (e 32), may do well for the bilayer zone containing the phosphate groups pure 1,4-dioxane (e 2) may mimic some of the dielectric properties of the hydrocarbon region. It appears that psKa values of several weak bases, when extrapolated to 100% cosolvent, do approximate pvalues [119,162,172]. Fernandez and Fromherz made favorable comparisons using dioxane [448]. This idea is of considerable practical use, and has been largely neglected in the literature. 

TABLE 5.2 Dielectric Constants of Water-Lipid Interfaces (Expanded from Ref. 453)  

Unilamellar vesicles (PC, aT) Polar head/acyl core Chemical reaction, aT-DPPH 26 446 

---