Alkali‐hydroxide‐doped matrices for structural characterization of neutral underivatized oligosaccharides by MALDI time‐of‐flight mass spectrometry

Abstract

We report new approaches using alkali-hydroxide-doped matrices to facilitate structural characterization of neutral underivatized oligosaccharides by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) MS. The approaches involved pretreatment of the analytes with NaOH or LiOH in aqueous solution, followed by mixing them with MALDI matrices prior to MS analysis. It was found that for open-ended neutral underivatized oligosaccharides partial alkaline degradation occurred upon laser desorption and ionization of the hydroxide-pretreated analytes in 2,5-dihydroxybenzoic acid (DHBA). The effect intensified when nonacidic compounds such as 2,4,6-trihydroxyacetophenone (THAP) and 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) were used as matrix. The degradation allowed facile identification of the reducing end residue of the analyte and facilitated its structural characterization by postsource decay TOF-MS. Applying the same technique using matrices composed of LiOH and THAP or AMT led to the production of singly as well as multiple lithiated ions of oligosaccharides containing hexoses with free 3-OH groups. Extensive lithiation through multiple hydrogen-lithium exchanges up to 6 Li atoms was observed for maltoheptaose, beta-cyclodextrin, and dextran 1500. Such a 'lithium tagging' technique makes it possible to differentiate positional isomers of milk-neutral oligosaccharides, lacto-N-difucohexaose I and II (LNDFH-I and LNDFH-II), without the need of chemical derivatization or tandem MS analysis.

Negative ion MALDI‐TOF MS, ISD and PSD of neutral underivatized oligosaccharides without anionic dopant strategies, using 2,5‐DHAP as a matrix

“…In this study, a general applicability of 2,5-DHAP as a MALDI matrix for underivatized neutral oligosaccharides in the negative ion mode, without addition of anion dopants, is described. We show that the general principles of previously published work with 2,5-DHB matrix, [13] anion [15,16] and hydroxide [30][31][32] dopants apply to our analyses as well. The deprotonated neutral oligosaccharide molecular ions can be produced.…”

“…We show that the general principles of previously published work with 2,5-DHB matrix, [13] anion [15,16] and hydroxide [30][31][32] dopants apply to our analyses as well. The deprotonated neutral oligosaccharide molecular ions can be produced.…”

Use of procaine and procainamide as derivatizing co‐matrices for the analysis of oligosaccharides by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

“…In fact, coordination to alkali metal cations tends to stabilize the [M + Na] + ions and complicate the fragmentation processes. [7] In order to improve sensitivity, alkali salts may be added as dopants [17] or the oligosaccharides derivatized. [8,[18][19][20][21][22][23][24][25] The main target of derivatization reactions is usually the carbonyl group (reducing end) and reductive amination is by far the commonest tagging reaction.…”

N-(1-Naphthyl) Ethylenediamine Dinitrate: A New Matrix for Negative Ion MALDI-TOF MS Analysis of Small Molecules

“…In this process, solvent plays a crucial role and profoundly modulates the analyte-matrix interactions, resulting in the increase or decrease of the MALDI ion signals [29][30][31]. For example, when using NEDN as a matrix, the mass spectrum of sucrose shows only a very weak signal if the matrix is dissolved in solvent such as pure water, methanol, or ethanol.…”

“…The matrixes, CCA and DHB, have long been regarded as the preferred matrixes for MALDI-MS analysis of carbohydrates [8,9,31,34]. To illustrate the advantage of using NEDN over CCA and DHB for MALDI-MS analysis of small molecules, we conducted a comparative study using sucrose as test compounds ( Figure 6S in Supplementary Material).…”