Influence of temperature on the behaviour of small linear peptides in capillary electrochromatography

The influence of temperature, T, on the retention times, peak widths, peak symmetry coefficients and theoretical plate numbers of two small linear pep tides, [Met(5)]enkephalin and [Leu(5)]enkephalin, has been studied with cap illary electrochromatography (CEC) capillary columns of 100 mu m I.D. and 2 50 mm packed length with a total length of 335 mm, containing 3 mu m Hypers il n-octadecyl bonded silica. With increasing column temperature from 15 to 60 degrees C, the electroosmotic how (EOF) and the column efficiencies inc reased, whereas the retention coefficients (kappa(cec)) of both peptides de creased. A linear relationship was found between the EOF value and the squa re root of the temperature over this temperature range, with a linear regre ssion correlation of 0.998. Non linear Van 't Hoff plots (ln kappa(cec) ver sus 1/T) were observed for these peptides between 15 and 60 degrees C, sugg esting that a phase-transition occurred with the n-octadecyl chains bonded on the silica surface, affecting the CEC retention behaviour of these pepti des. In CEC systems, the kappa(cec) values of peptides incorporate contribu tions from both electrophoretic migration and chromatographic retention. Po sitive and negative kappa(cec) values can, in principle, thus arise with th ese charged analytes. However, the kappa(cec) values of the enkephalin pept ides under all temperature conditions studied were positive with an eluent composed of water-50 mM NH4OAc/AcOH, pH 5.2-acetonitrile (5:2:3, v/v) and t herefore the chromatographic component dominates the retention process with these small peptides under these conditions. (C) 2000 Elsevier Science BN. All rights reserved.