Temperature-Responsive Liquid Chromatography. 2. Effects of Hydrophobic Groups in N-Isopropylacrylamide Copolymer-Modified Silica

Abstract

We recently reported the new concept of temperature-responsive liquid chromatography using temperature-responsive poly(N-isopropylacrylamide)-modified surfaces as high-performance liquid chromatography media with aqueous mobile phases. Incorporation of hydrophobic sites is an important factor to improve the efficacy (selectivity and retention) of temperature-responsive chromatography. Toward this goal, we have synthesized semitelechelic copolymers of N-isopropylacrylamide (IPAAm) and butyl methacrylate (BMA) having reactive terminal functional groups using telomerization. The lower critical solution temperatures of the copolymers shift to lower temperatures with increasing hydrophobic BMA content in the poly(IPAAm-co-BMA) relative to that of the IPAAm homopolymer. This temperature-responsive semitelechelic copolymer was grafted to the surface of (aminopropyl)silica through the reaction of activated ester-amine coupling. The polymer-modified silica was used as a column packing material. Separation of a mixture of five steroids having various hydrophobicities was investigated. Retention of steroids on poly(IPAAm-co-BMA)-modified columns is increased with an increase in column temperature. The capacity factors for steroids on the copolymer-modified silica beads was much larger than that on homopolymer PIPAAm-modified columns. The capacity factor for testosterone at 50 °C was 33.8 for poly(IPAAm-co-BMA) containing 5 mol % BMA, while that for the PIPAAm homopolymer was 15.0 at the same temperature. The influence of column temperature on steroid retention behavior on copolymer-modified stationary phases was significant compared with the case of homopolymer-modified columns. Furthermore, retention times for steroids increased remarkably with increasing BMA composition. The temperature-responsive elution behavior for the steroids was strongly affected by the hydrophobicity of the grafted polymer chains on silica surfaces. Possible protein separation in temperature-responsive liquid chromatography was explored using insulin chains A and B, and β-endorphin fragment 1−27. On IBc-3.2-modified silica column, these three peptides were successfully separated at 30 °C with 0.5 M NaCl aqueous solution (pH 2.1) as mobile phase. The retention times of these peptides were related to the number of hydrophobic amino acid residues in the peptides. In the proposed chromatography system, elution of target substances is controlled only by a small change in column temperature without any further modification of the aqueous mobile phase.