Rapid Method for the Estimation of Octanol / Water Partition Coefficient (Log Poct) from Gradient RP-HPLC Retention and a Hydrogen Bond Acidity Term (Sigma alpha2H)

We propose a rapid method for the measurement of octanol / water partition Coefficients (log Poct) via fast gradient reversed phase retention and the calculation of the hydrogen bond acidity of the compounds. The cycle time of the generic gradient HPLC method is 5 minutes. The general solvation equation obtained for the log Poct values and the fast gradient Chromatographic Hydrophobicity Indices with acetonitrile (CHIACN) and methanol (CHIMeOH) have been established. It has been revealed that the major difference between the log Poct and CHIACN lipophilicity scales is their sensitivity towards the hydrogen bond acidity sigma alpha2 H ) of the compounds. The CHIACN values of the uncharged (neutral) compounds are measured, then the H-bond acidity term sigma alpha2 H ) is calculated from the structures. The log Poct values are expressed using the following equation That was established from the data of 86 diverse compounds Log Poct = 0.054 CHIACN + 1.319sigma alpha2H -1.877 N=86 r=0.970 s=0.29 F=655 A hydrogen bond count (HBC) is not as good as sigma alpha2H as a measure of H-bond acidity but still gives an acceptable equation: Log Poct = 0.047 CHIACN + 0.36 HBC - 1.10 N=86 r=0.943 s=0.39 F=336 In order to improve the correlation between the log Poct and the CHIMeOH values several other terms, such as the H-bond basicity, polarisability and size term should also be considered. Therefore acetonitrile is suggested as the preferred organic modifier. This rapid method can be used as a high throughput lipophilicity screen for combinatorial libraries.