Directional Dependence of Hydrogen Bonding

Figure 4.1 Illustration of the density based energy decomposition analysis (DEDA) scheme. Reprinted with permission from Lu, Z., Zhou, N., Wu, Q. and Zhang, Y. Directional dependence of hydrogen bonds A density-based energy decomposition analysis and its implications on force field development. J Chem Theory Comput 7, 4038-4049 (2011). Copyright (2011) American Chemical Society.

Oral bioavailability of a drug is primarily dependent upon its rate and extent of drug absorption and systemic clearance. Systemic clearance is primarily composed of hepatic, renal and biliary clearance. The PK properties are in turn directly impacted by the drug s physical properties, such as, log P, log D and pKa. The physical properties are in turn a function of the compound s structure, molecular weight, number of hydrogen bond donors and acceptors, and number of rotatable bonds. Oral bioavailability is the outcome from the dynamic interplay of these factors in the biological system. 

The characterization of a solvent by means of its polarity is an unsolved problem since the polarity itself has, until now, not been precisely defined. Polarity can be understood to mean (a) the permanent dipole moment of a compound, (b) its dielectric constant, or (c) the sum of all those molecular properties responsible for all the interaction forces between solvent and solute molecules (e.g., Coulombic, directional, inductive, dispersion, hydrogen bonding, and EPD/EPA interaction forces) (Kovats, 1968). The important thing concerning the so-called polarity of a solvent is its overall solvation ability. This in turn depends on the sum of all-specific as well as nonspecific interactions between solvent and solute. 

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