Column method octanol-water partition

At Sirius, a dedicated instmment (Profiler LDA) has been developed for the rapid measurement of log D by liquid-liquid partition chromatography. In this instrument the column is coated with a layer of octanol, and the retention times are therefore tmly related to octanol/water partitioning. Although the method used was first described 25 years ago [29], it has been difficult to apply in an automated system because of the tendency of octanol to be flushed from the column by the eluent, thus requiring frequent renewals of the octanol coating. In our method, the octanol coating is kept in place by reversing the direction of the eluent after each... 

Wasik, S. P., Miller, M. M., Tewari, Y. B., May, W. E., Sonnefeld, W. J., DeVoe, H., Zoller, W. H. (1983) Determination of the vapor pressure, aqueous solubihty, and octanol/water partition coefficient of hydrophobic substances by coupled generator column/ liquid chromatographic methods. Res. Rev. 85, 29 42. 

Donovan, S.F., Pescatore, M.C. (2002) Method for measuring the logarithm of the octanol-water partition coefficient by using short octadecyl-poly(vinyl alcohol) high-performance liquid chromatography columns. J. Chromatogr. A, 952, 47-61. 

Water solubility determined by the generator column method by van Haelst et al. [244]. n-Octanol/water partition coefficients determined by Van Haelst et al. using the slow stirring method [245]. 

Reverse-phase (RP) methods were initially established on the basis of similarity between octanol water partitioning and retention on an RP column for HPLC... 

Octanol-coated (ODS) columns are generally used. As noted vide supra, this method relies on correlation with reference compounds from the literature. References are ideally selected to span the range of lipophilicity estimated for the intended unknowns. Linearity should be demonstrated between the capacity factors (k ODS) and octanol water partition coefficients before values for the unknown compounds can be determined by interpolation. The hydrogenbonding character of the unknown should also be taken into account because inaccuracies can result from H-bonding of solute to residual silanol sites [29]. 

A drug is absorbed through diffusion across a series of separate barriers where the single layer of epithelial cells is the most significant barrier to absorption. Many in vitro methods have been developed for the study of this phenomenon. These methods include small animal gut studies, cell culture (i.e., Caco-2 cell culture model), octanol-water partition coefficients, measures of hydrogen bonding and desolvation energies, immobilized artificial membranes, and retention time on reversed-phase HPLC columns. 

Column and high performance liquid chromatography (HPLC) methods for measurement of solubility, octanol-water partition coefficient, and vapor pressure which are replacing the older equilibrium methods tend to underestimate aqueous solubility and vapor pressure and tend to overestimate the octanol-water partition coefficient. The standard deviation for both the equilibrium and dynamic systems are similar, but calibration between systems is necessary to insure that they agree. The range of errors for both types of measurement as mentioned in the literature are well within the range predicted by the computer-simulated error distributions generated in this report. The measurement error... 

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. 

The HPLC technique has certain advantages that allow partition coefficients to be determined relatively rapidly in comparison to the conventional shaking-flask method. The elution process also makes it feasible to run mixture of solutes and yield results relatively free of solute-solute or solute-impurity interactions with a column giving good separation. Basically, the precision of the method depends on whether the value of yw/7o solute in the octanol-water system [Eq. (5)] can be effectively simulated by the column composition. It is therefore important that efforts be expanded to ensure that the properties of the column environment accurately reflect the bulk solvent-water (e.g., octanol-water) characteristics. If the solute can truly be equilibrated between mobile and stationary phases during elution, its retention time is determined by the ratio of the activity coefficients (i.e., partition constant) in the mobile and stationary phases. When octanol and... 

The experimental approaches are similar to those for solubility, i.e., employing shake flask or generator-column techniques. Concentrations in both the water and octanol phases may be determined after equilibration. Both phases can then be analyzed by the instrumental methods discussed above and the partition coefficient is calculated from the concentration ratio Q/Cw. This is actually the ratio of solute concentration in octanol saturated with water to that in water saturated with octanol. 

Accurate values of Pow are clearly necessary for the ultimate calibration of all surrogate systems, but, in practice, direct measurements of Powby the traditional shake-flask method are seldom used. Particularly for compounds with low water solubility, experimental difficulties may arise from problems in phase separation without carryover, sorption to glass surfaces, or from formation of emulsions. All of these introduce serious uncertainties into the concentrations in the appropriate phases, and may consequently lead to substantial errors in the estimates of partition coefficients. The problem is particularly acute for compounds with extremely low solubility in water such as the chlorinated dibenzo[l,4]dioxins for which widely varying values have been reported (Marple et al. 1986 Shiu et al. 1988). For such compounds, use of a generator column has been advocated (De Voe et al. 1981 Woodburn et al. 1984). In essence, the following steps are carried out (1) a solution of the test substance in octanol is equilibrated with water and the concentration in the octanol phase is determined, (2) the octanol phase is... 

---