Ionic Liquid Matrixes Optimized for MALDI-MS of Sulfated/Sialylated/Neutral Oligosaccharides and Glycopeptides

Abstract

1,1,3,3-tetramethylguanidium (TMG) salt of alpha-cyano-4-hydroxycinnamic acid (CHCA) (G(2)CHCA) was reported by Tatiana et al. as a useful ionic liquid matrix (ILM) for sulfated oligosaccharides to suppress the loss of sulfate groups. However, the report mainly referred to positive ion spectra only and amounts of 10 pmol or more of the analyte were used. Herein, we demonstrated highly sensitive detection of sulfated/sialylated/neutral oligosaccharides and preferential ionization of glycopeptides by optimizing a newly synthesized ILM: TMG salt of p-coumaric acid (G(3)CA) and the existing G(2)CHCA in both positive and negative ion extraction modes. Sulfated oligosaccharides were detected with high sensitivity (e.g., 1 fmol) in both ion extraction modes, and the dissociation of sulfate groups was suppressed especially using G(3)CA. Sialylated and neutral oligosaccharides were also detected with high sensitivity (e.g., 1 fmol) with positive ion extraction while the dissociation of sialic acids was suppressed especially using G(3)CA. Additionally, glycopeptide ions were detected preferentially using the ILMs among the digest of a glycoprotein, ribonuclease B, in both ion extraction modes but particularly in the negative ion mode. As a result, the use of optimized ILMs provides an effective method for carbohydrate analysis due to the highly sensitive soft-ionization achieved in both ion extraction modes as well as the homogeneity of analyte-matrix mixtures.