The influence of physicochemical properties, including lipophilicity, H-bonding capacity and molecular size and shape descriptors on brain uptake has been investigated using a selection of marketed CNS and CNS-inactive drugs. It is demonstrated that the polar surface area of a drug can be used as a suitable descriptor for the drugs' H-bonding potential. A combination of a H-bonding and a molecular size descriptor, i.e., the major components of lipophilicity and permeability, avoiding knowledge of distribution coefficients, is proposed to estimate brain penetration potential of new drug candidates. Previously reported experimental surface activity data appear to be strongly correlated to molecular size of the drug compounds. Present analysis offers a modern basis for property-based design and targeting of CNS drugs.

Permeability of a compound by passive diffusion through a biological membrane such as Caco-2 cell monolayers depends mainly on lipophilicity, molecular size and hydrogen bonding capacity of the solute. Since these properties are often used in structure-permeability correlations, we have evaluated different calculated descriptors for size and hydrogen bonding, as well as their intercorrelations. A new descriptor for hydrogen bonding potential is introduced. It is demonstrated that a combination of appropriate size and H-bonding descriptors using graphical or Eq.-based approaches may be of potential use for membrane permeation estimation.

Key indicatorsSingle-crystal X-ray study T = 293 K Mean (C-C) = 0.003 Å R factor = 0.035 wR factor = 0.100 Data-to-parameter ratio = 10.9For details of how these key indicators were automatically derived from the article, see

A complete thermodynamic description of H‐bonding for drug design is proposed. The approach is based on applicability of multiplicative principle to enthalpy (ΔH) and free energy (ΔG) of complex formation by means of Ed(Ea) and Cd(Ca) factors: Where Ed(Ea) and Cd(Ca) characterize the proton donor (acceptor) ability of compounds. The estimation of E and C values for 163 proton donors and 195 proton acceptors was done, making it possible to predict ΔH and ΔG for 31785 reactions. The comparison of these data with experimental ones for 936 reactions was carried out: The substituent effect on H‐bonding ability of compounds was considered. It was established that varying of substituents at active sites can alter its proton donor (acceptor) properties over a wide range of Ed(E3, cd(ca) with sufficient overlap between values of factors corresponding to different types of active sites. One could propose this approach to be applied to quantitative estimation of thermodynamic functions of H‐bonding in substrate‐receptor complexes; also for QSAR modelling, where E and C or its linear combination might be used as molecular descriptors characterizing quantitatively the relative H‐bonding ability of biologically active compounds. The second type of applications is used by the authors when modelling QSAR by means of the program package CLARAS (Classification and Recognition of Active Structures).

Thermodynamics of 18‐crown‐6 complexes with ammonium cations (NH4, MeNH3, Me2NH2, Me3NH, Me4N, Et4N, PhNH3, and PhCH2NH3 ) in methanol were determined by titration calorimetry. The results show strong contributions from entropy terms counteracting the enthalpy of complexation, and a linear decrease of the complexation free energy ΔG with the number of available N–H hydrogen bonds. In several cases formation of relatively strong complexes containing two ammonium ions per crown unit was observed. Tetramethylammonium ions show no detectable association with the crown ether, demonstrating the absence of significant Coulomb‐type interaction between the partial charges at the crown ether oxygen and the N+–C–H atoms. Ammonium ions bind to aza crown ethers with almost equal affinity as to the all‐oxygen anologs only, if methyl groups at the nitrogen atoms force the lone pairs into equatorial position. Molecular mechanics calculations (CHARMm) of corresponding gas‐phase complexes yield geometries and energies in agreement with this, with energetically equally good conformations of an essentially undistorted D3d crown accepting either 3 linear hydrogen bonds, or 6 bifurcated bonds from the primary ammonium cations. Complexation equilibria were measured with PhNH3, and PhCH2NH3 in water, 2‐propanol, tert‐butyl alcohol, n‐octanol, DMF, DMSO, pyridine, HMPT and acetone mostly by calorimetry, in some cases by potentiometry. The observed association constants varied by factors of up to 1000; the solvent effects can be described generally as a linear function of the hydrogen bond accepting power of the solvent molecules, in line with the mechanisms derived above. The lgK and ΔH values of the complexation of the PhNH3 or PHCH2NH3 cation with 18‐crown‐6 ligand are compared with a large range of available solvent properties. The best correlations (R ≈ 0.9) for lgK (or ΔG) are obtained with values characterizing the electron donor capacity of the solvent (Ca, β*, DN) for lgK, as found earlier for complexes between K+ and 18C6.