Characterisation of important interactions controlling retention behaviour of analytes in reversed-phase high-performance liquid chromatography

Retention behaviour of analytes in reversed phase high performance liquid chromatography - A review

The understanding of principles that drive the separation in reversed phase chromatography plays an important role in the prediction of elution of solutes in RP-HPLC. The separation in RP-HPLC is based on the principle of adsorption and partition. In addition, the log P value, pK_a value of the drug, chromatographic parameters like mobile phase pH, buffer concentration, organic modifier and mobile phase additives also influence the retention and selectivity of the analyte. It is found that hydrophobic, electrostatic, hydrogen bonding and other specific interactions between the stationary phase and the solutes along with the hydrophobicity of an analyte molecule (log P) modify the retention behaviour of the analytes. This article gives special attention to the influence of ionization and ion interaction on the separation of analytes. The drug molecules with different log P value containing protonated and deprotonated acids, bases and zwitterions are selected as examples and this article addresses various issues related to method development, relationships between analyte retention and mobile phase pH and pK_a value of the analyte. The advances in this regard, with highlights on topics such as mechanisms of retention and various factors that influence the retention behaviour of analytes is also updated with suitable examples.

A cyanopropyl reversed phase chromatography enables resolution of huperzine A and B on analytical and preparative scales

Huperzine A (HupA) and huperzine B (HupB) are two medically important components of Huperzia serrata. It is difficult to obtain high yields of the separation from the plant using conventional liquid extraction and chromatography. However, this study has found that RP chromatography with cyanopropyl (CN) medium was able to separate these two analogs simultaneously from the plant extract with higher resolution. Comparison between a CN medium and a popular C18 medium demonstrated the superiority of the CN over the C18 in resolution for both analytical and preparative separation of HupA and HupB. A preparative process was developed for simultaneous purification of HupA and HupB from H. serrata. The yields on the basis of the mass of the herbal powder for HupA and HupB were 0.019 and 0.008% respectively, which were about 1.9 and 10 times of those reported in the literature.

Microcalorimetric studies on the interaction mechanism between proteins and hydrophobic solid surfaces in hydrophobic interaction chromatography: effects of salts, hydrophobicity of the sorbent, and structure of the protein

This study examines the effects of different salts as well as the influence of the relative hydrophobicities of different sorbents on the adsorption processes of proteins in hydrophobic interaction chromatography (HIC). Comparative data acquired by the equilibrium binding analysis and by isothermal titration microcalorimetry (ITC) are presented. In particular, thermodynamic parameters, including the enthalpy changes, related to the interactions between several globular proteins and various Toyopearl 650 M sorbents under solvent conditions containing either 2.0 M ammonium sulfate or 2.0 M sodium sulfate at pH 7.0 and 298.15 K have been evaluated in terms of the molecular properties of these systems. The results reveal that the dependence of the free energy change, deltaGads, for protein adsorption to HIC sorbents on the salt composition can be mainly attributed to the enthalpy changes associated with protein and sorbent dehydration and hydrophobic interactions. Differences in binding mechanisms between the n-butyl- and phenyl-HIC sorbents were evident. In the latter case, the participation of pi-pi hydrophobic interactions leads to significant differences in the associated enthalpy and entropy changes. Furthermore, an increase in the hydrophobicity of either the sorbent or the protein resulted in more negative values for the free energy change, which arose mostly from dehydration processes. Entropic effects favoring HIC adsorption increased with an increase in the exposed nonpolar surface area of the protein. Consequently, an increased contribution from the entropy change to the respective change in free energy occurs when HIC sorbents or proteins of higher hydrophobicity are employed, with these larger entropy changes consistent with a change in the interaction mechanism from a binding event dominated by adsorption to a partitioning-like process. Data extracted from the ITC measurements also provided insight into the interaction mechanisms that occur between proteins and hydrophobic solid surfaces, yielding information that can be applied to the HIC purification of proteins according to the concept of critical hydrophobicity of the system and its thermodynamic consequences.