Sphingomyelins as Semi-permanent Capillary Coatings for Protein Separations in CE and Off-line Analysis with MALDI-MS
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Phosphatidylcholine (PC) is one of the major phospholipids that make up the biological cell membrane. It was previously reported to form capillary inner wall coatings for CE. The zwitterionic head group of PC produced a neutral net-charged bilayer, which was found effective in preventing wall adsorption of both cationic and anionic proteins [J. M. Cunliffe et al. Anal. Chem. 2002, 74, 776-783]. Another major membrane phospholipid that possesses a zwitterionic head group is sphingomyelin (SM). In this work, the novel characterization of SM on its effectiveness in capillary coating formation for CE separations of proteins and peptides was presented. Similar properties were observed between PC and SM, including their effects on the EOF, peak efficiencies, and migration time reproducibilities. SM appeared to be more readily soluble in aqueous solutions, and it was found equally effective as PC in facilitating protein separation. The main difference observed was their performances in delivering a peptide mixture for off-line analysis with MALDI-MS. Superior sample recovery was evident from the capillary coated with SM compared with that with PC. The number of peptides identified from a 1 ng/microL myglobin tryptic digest sample increased from 5 to 16 (42 and 69%, respectively in protein sequence coverage).