Lipophilicity values, HPLC measurements

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).

Technical details, including solid support, coating and column filling techniques, eluents, and factors affecting the reproducibility have been reviewed [173, 235, 236]. Lipophilicity values from HPLC measurements are not on a unique scale, but the log k values can be converted to -octanol/water partition coefficients with the help of a set of HPLC calibration standard compounds for which classical shake-flask partition coefficients are known. Experience shows once the HPLC method is successfully established in a laboratory, it will remain the method of choice for lipophilicity determinations. 

However, extrapolated capacity factors may be affected by the nature of the organic modifier. In a study concerning the measurement of lipophilicity indices for monosubstituted benzenes by HPLC, the log values of the more polar derivatives appeared to be lower when methanol was used as an organic modifier, compared to the log values obtained with acetonitrile. In contrast, for nonpolar derivatives, acetonitrile led to lower log fcw values. An analogous decrease in the log fcw values for a series of lipophilic 9//-xanthene and 9//-thioxan-thene derivatives was observed when tetrahydrofuran was... 

Especially when calibrated with standard compounds relative to the classical shake-flask method, rp-HPLC is the method of choice for the experimental determination of partition coefficients. The liability to large variation in measured log for the highly lipophilic compounds (log > 5) subjects the experimental data to major uncertainties and for such molecules calculation is generally preferable (Taylor, 1990). Reported log P >1 (e.g. for some chlorinated dioxins up to 11), corresponding to concentration differences between the aqueous and the 1-octanol phase of about 10 and more are not exact values, but reflect the accuracy of the experimental analytical techniques used they may be subject to major uncertainties of 2 log units. The respective data have to be treated with great caution in QS AR studies, because at best they indicate the order of magnitude of the compounds lipophilicity. 

Based on the (po concept, a fast gradient method RP-HPLC has been proposed to determine the chromatographic hydrophobicity index (CHI) as a high-throughput alternative to other lipophilicity measures.For this purpose, gradient retention times (tg) are measured and converted to CHI values by means of a calibration equation, derived by a set of standards with well-determined CHI (tpo) values ... 

Lipophilicity of a molecule is measured by its distribution behavior in a biphasic system either liquid-liquid (partition coefficient in 1-octanol-water) or solid-liquid (retention in RP-TLC or RP-HPLC) systems. According to definition suggested by lUPAC lipophilicity expresses the affinity of a molecule for a lipophilic environment. A reference scale representing lipophilicity appears to be the solute distribution between octanol and water. Berthod Carda-Broch (Berthod Carda-Broch) proposed another lipophilicity scale measuring ionic liquid BMIM PFg-water distribution constants. Relationship between the obtained values and respective octanol-water coefficients for a series of aromatic compounds differing in acid-base properties revealed that only the neutral compounds or ionizable ones with zwitterionic properties showed similar distribution behavior in the... 

Measured using the HPLC method (see Ref 19). Higher values indicate higher lipophilicity. 

---