α-Cyano-3-aminocinnamic acid: A novel reactive matrix for qualitative and quantitative analysis of plant N-glycans by MALDI-MS

N-glycans have a diversity of crucial biological roles in organisms. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has become an indispensable analytical instrument for biomolecules. However, due to the inherent low abundance, high structural heterogeneity, and poor ionization efficiency of N-glycans, as well as the extremely inhomogeneous co-crystal property using traditional matrices, the qualitation and quantitation of N-glycans by MALDI-MS remains challenging. In the present study, α-cyano-3-aminocinnamic acid (3-CACA) was reasonably designed and synthesized as a novel reactive matrix for N-glycan analysis. Combining with traditional matrix α-cyano-4-hydroxycinnamic acid (CHCA) as an acidic catalyst, a combinational matrix 3-CACA/CHCA was obtained with homogeneous co-crystallization and high derivatization efficiency, achieving the sensitive qualitation with the limits of detection low to femtomole and reproducible quantitation with good linearity (R2 > 0.998). As a result, the established method was successfully applied to the on-target derivatization and high-throughput quantification of N-glycans in eight varieties of the peach complex system, indicating that N-glycan has the potential to become a new biomarker for food allergy, and elucidating the prospective correlation between N-glycan epitopes and allergic reactions.

Enzyme-assisted ReMALDI-MS assay for quantification of cholesterol in food

Cholesterol is a vital building block for animal cell membranes and participates in the synthesis of various hormones. Accurate quantitation of cholesterol in food is crucial for healthy diets. Here, we describe an enzyme-assisted reactive matrix-assisted laser desorption/ionization mass spectrometry (ReMALDI-MS) assay for the quantification of cholesterol in food. First, cholesterol was converted to 4-cholesten-3-one using the cholesterol oxidase, and then reacted with a reactive matrix, 4-hydrazinoquinazoline (4-HQ), to form a hydrazone bond. Utilizing 4-HQ significantly improved the ionization efficiency of cholesterol, which possesses poor ionization efficiency in MALDI-MS, and no additional tedious derivatization/purification steps were needed. Thus, the proposed assay was successfully used for the quantification of cholesterol in bovine milk and cream. The standard recovery tests show a recovery range of 95.3-103.0% with a relative standard deviation of 0.3-3.1%. Therefore, the proposed enzyme-assisted ReMALDI-MS assay has great potential for quantification of cholesterol in other foods.

A dual-mode reactive matrix for sensitive and quantitative analysis of carbohydrates by MALDI-TOF MS

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a widely used tool for the analysis of carbohydrates. However, the detection of carbohydrates by MALDI-TOF MS is often limited by the unsatisfactory ionization efficiency, instability and the matrix interference in low molecular weight region. Here, we demonstrate that a reactive matrix, 2-hydrazinoquinoline (2-HQ), can be used to detect neutral, sialic and low molecular weight carbohydrates sensitively both in the positive and negative ion mode. Since 2-HQ reacts efficiently with the reducing end of carbohydrate to form stable hydrazone, the ionization efficiency of derived carbohydrates is significantly enhanced. Using 2-HQ, the sensitivity for analyzing glycans has been improved 10-fold and 100-fold compared with those using 3-aminquinoline (3-AQ) and 2,5-dihydroxybenzoic acid (DHB) as matrix, respectively. Moreover, quantitative analysis of neutral, acidic and low molecular weight carbohydrates has been achieved because of the good reproducibility by using 2-HQ as matrix. As a result, up to 50 glycans in a single sample spot of human fresh serum without any prior purification and enrichment have been successfully detected. Therefore, 2-HQ as a new reactive matrix has shown great potentials in widespread applications for sensitive, selective, quantitative, high speed and high throughput analysis of carbohydrates in complex samples by MALDI-TOF MS.

Rapid and accurate profiling of oligosaccharides in beer by using a reactive matrix via MALDI-TOF MS

Oligosaccharides analysis is crucial for brewing technology. Herein, we reported a rapid and highly reproducible method for profiling of oligosaccharides in beer using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) by employing a reasonably designed reactive-matrix, 2-phenyl-3-(p-aminophenyl) acrylonitrile (PAPAN). The PAPAN enhanced ionization efficiency of oligosaccharides and improved reproducibility comparing to the use of conventional matrix, 2,5-dihydroxybenzoic acid (DHB). After optimization of sample dilution factor and cationization agents, the distributions of maltooligosaccharides in different brands of beers were unambiguously identified. Since the PAPAN selectively reacts with the reducing end of oligosaccharides, the interferences from matrixes are effectively eliminated. Therefore, the method shows potentials for analysis of oligosaccharides in other foods.

Gold Nanoparticles (AuNPs) as Reactive Matrix for Detection of Trace Levels of HCN in Air by Laser Desorption/Ionization Mass Spectrometry (LDI-MS)

Under direct laser desorption/ionization mass spectrometric conditions, the irradiation of target spots made of gold nanoparticle residues generates a series of peaks at m/z 197, 394, 591… representing Au_n^- ions (n = 1-3). In contrast, spectra recorded from gold nanoparticles directly mixed with an alkali cyanide exhibited an additional peak at m/z 249, indicating an abundant generation of gaseous [Au(CN)₂]^- ions upon irradiation. The relative intensity of the m/z 249 peak surged when the amount of cyanide in the mixture was increased. Most remarkably, a peak at m/z 249 was observed even from neat AuNPs upon irradiation, if a nearby spot, which was not irradiated, happened to bear a cyanide sample. We postulated that traces of HCN emanating from the headspace of aqueous cyanide solution during the sample-plate preparation is sufficient to convert gold to AuCN, which is subsequently detected as [Au(CN)₂]^-. Further experiments demonstrated that the relative intensity of the m/z 249 peak diminishes exponentially as the AuNP spot becomes more distant from the putative HCN source. Eventually, the method was developed as an efficient procedure to detect HCN or alkali cyanides. Using KCN, the detection limits were determined to be below 10 pg of CN^- per spot. The method also demonstrated that, upon crushing, the seeds or roots of certain fruits and vegetables such as apple, peach, radish, and cassava, but not carrot, release HCN in amounts detectable by this method. Graphical Abstract.