Solid-Phase Fluorescence and Ionization Efficiency in Negative-Ion Matrix-Assisted Laser Desorption/Ionization of Neutral Oligosaccharides: Interaction Between β-Carboline Matrix and Ammonium Salt

Abstract

Ammonium chloride (NH4Cl) with a beta-carboline harmine (7-methoxy-1-methyl-9H-pyrido[3,4-b]indole) Matrix promotes the generation of chloride-anionized molecules of neutral oligosaccharides in negative-ion ultraviolet matrix-assisted laser/desorption ionization mass spectrometry (MALDI MS). The relative abundances of anionized molecules and matrix ([Analyte + Cl]-/[matrix + Cl]-) correlate with the amount of NH4Cl added, and saturate at a level of NH4Cl approximately four times that of the matrix. Their solid-phase fluorescence spectra indicate that harmine and NH4Cl form a complex of the hydrochloride salt in the mixed crystal. The peak intensity at 448 nm from the harmine hydrochloride in the mixed crystal rises logarithmically with the amount of NH4Cl added, a result that quantitatively correlates with increases in the ion abundance ratios of the chloride-anionized molecules to that of harmine. The solid-phase spectroscopic method is useful for studying changes in the characteristics of the matrices and additives in the mixed crystal. Harmine hydrochloride, rather than harmine, works as an effective matrix. The attachment of the chloride to the matrix is essential for the generation of chlorinated-anionized molecules in MALDI. An N-acetyl glucosamine residue (GlcNAc) in lacto-N-tetraose promotes the generation of their chloride-anionized molecules, however, multi-GlcNAc residues in N-acetylchitooligosaccharides hinder it.