Since the pH of the solution directly affects the concentration of neutral and ionic species, the equilibrium constant varies with pH, which also means that the lipophilicity of a compound is dependent on it. The partition coefficient as a function of pH is often called distribution coefficient (logDpH). The logDpH is often considered to be a more proper descriptor than logPN for human bioavailability due to the frequent pH-dependence of drugs. This property has been shown to be useful in QSAR models to explain how small molecules have human brain cell permeability or bind to human serum albumin . The logDpH has also been used as an effective predictor of pHdependent lipophilicity profiles for small molecules and to characterize structural properties in proteins and peptides, such as protein-folding and aggregation , solubility , and antimicrobial activity , through pH-dependent lipophilicity scales. As an alternative to experimentally determined logDpH values, theoretical lipophilicity profiles provide the opportunity to obtain this descriptor quickly and often with high accuracy. Equation 1 models logDpH as a function of pH for monoacidic and monobasic compounds. This equation is derived as the mass balance between the ionic and neutral species in thermodynamic equilibrium in the aqueous phase. This model assumes that the organic phase holds mostly neutral species, so that the acid-base dissociation is negligible, and it also assumes that there is not a partition equilibrium for the ionic species. log D pH = log P N -log $1 + 10 δ % [1] where d = pH -pKa for acids, and d = pKa -pH for bases.