MOBILE PHASE EFFECTS IN REVERSED-PHASE CHROMATOGRAPHY .3. CHANGES IN CONFORMATION AND RETENTION OF OLIGO(ETHYLENEGLYCOL)DERIVATIVES WITH TEMPERATURE AND ELUENT COMPOSITION

The rentention behavior of homologous oligo(ethylene glycols)s and their derivatives on octadecyl-silica with aqueous eluents was investigated under a wide range of conditions as far as the chemical nature and concentration of organic co-solvent as well as the temperature are concerned. The data revealed numerous irregularities concerning the dependence of retention factors temperature, eluent composition and the carbon number of eluite molecules. The observed behavior is explained by postulating that each eluite can have the extended zigzag o the compact meandering conformation, that the two conformers of an eluite have significantly different retention factors in the same chromatographic system and that the relative concentration of the conformers, i.e., the magnitude of the equilibrium constant for the transition, depends on the eluent composition, temperature and the number of ethylene oxide residues in the molecules. The two-state theory allows a separate treatment of the effect of temperature and retention on the equilibrium between the conformers and the predictions of the theory are in qualitative agreement with the experimental results. Thus secondary equilibra which do not involve a complexing agent, but the solvent proper, can give rise to non-linear Van'T Hoff plots, to increase of retention with temperature, to retention decreasing with the carbon number of homologues and to non-linear depedence of the logarithm of retention factor on the volume percent of organic co-solvent in acqueous eluents commonly used in reversed-phase chromatography. Similar phenomena are likely to be encountered in high-performance liquid chromatography when complex molecules, which can exist in two or more significantly different conformations, are chromatographed. Thus, the results of the present work not only can be useful to solve various analytical problems involved in the separation and identification of compounds having a poly(ethylene glycol) moiety, but also can serve as model for study of the chromatographic behavior of biological substances like peptides and oligonucleotides. © 1979.