Distribution coefficients from hydrogen bonding

Repeated extractions may be required to recover the analyte sufficiently from the aqueous phase. Neutral compounds can have substantial values of Ko. However, organic compounds that form hydrogen bonds with water, are partially soluble in water, or are ionogenic (weak acid or bases) may have lower distribution coefficients and/or pH-dependent distribution coefficients. Additionally, the sample matrix itself (i.e., blood, urine, or wastewater) may contain impurities that shift the value of the distribution coefficient relative to that observed in purified water. 

Apart from the hydrophobic interactions provided by the alkyl part of the molecule, octanol has also hydrogen-bond acceptor and donor functions like lipid membranes have. This property of n-octanol made the octanol-water distribution coefficient that widely used. However, n-octanol or reversed phase materials cannot mimic the interfacial character of the bilayer structure. The ionic interactions between membrane phospholipids and solute are also not represented in the properties of octanol or reversed phase materials. To overcome this issue, alternative stationary phases... 

Lo used the three-term solubility parameter (Barton, 1983) and a graphical procedure to identify solvents for liquid-liquid extraction. In a 2D space constructed from the polar component of the solubility parameter 6p and the hydrogen bonding component of the solubility parameter 6, the distribution coefficient of the solute B, mg, is given by... 

Fig. 22.4 Molecular properties can be divided into experimental (subdivided into biological and physicochemical) and in silico (subdivided into structural and substructural) properties, physicochemical and biological properties. Examples of experimental data are IC50 (binding affinity), MIC (antibacterial minimum inhibition concentration), LD50 (lethal dose), Vd (volume of distribution), F% (bioavailability), pKg (ionization constant), log P (partition coefficient from shake flask determination), log kn,(lipophilicity from HPLC measurement), A (hydrogen bond capability), solubility. Examples of calculated properties, either for whole molecule or for substituents or buildings blocks, are MW (molecular weight), MR (molar refractivity), molecular volume, PSA (polar surface area), HA (number of H-bond acceptors), HD (number of H-bond donors), CLOGP (calculated log P values), L (substituent length), B5 (substituent width), cr (Hammett constant), F, R (field and resonance parameters), TT (Hansch constant), f (hydrophobic fragmental constant).

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