No abstract

Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) allows detection of large molecules such as those present in synthetic and natural macromolecules. Until recently, it was reported that MALDI-TOF measurements can provide correct molecular weight (MW) averages only for samples with a narrow MW distribution (MJMn < 1.20). We have now developed a methodology for polydisperse samples. We recorded the GPC trace of two polydisperse polymeric samples, namely, poly(butylene adipate) (PBA) and poly(butylene adipate-co-butylene succinate) (PBAS), collecting about 40-50 fractions per run. Selected fractions were analyzed by MALDI-TOF, and the average MW of each fraction was determined, allowing calibration of the GPC curves against absolute MW. The two calibrated GPC traces were then used to compute average MW and molecular weight distributions (MWD) of the unfractionated samples. End group analysis from MALDI-TOF spectra revealed that the PBA sample is composed of seven different types of chains. For the copolymer sample, PBAS, analysis of the MALDI spectra established a random sequence distribution of comonomer units. The succinate/adipate molar ratio calculated from the MALDI spectra is in good agreement with the molar ratio found by NMR.

Negative Ion fast atom bombardment (FAB) mass spectra were found to allow the determination of the linkage position and sugar sequences In a series of (underivatized) dlsaccharkles and of linear oligosaccharides. Discrimination of 1-4, 1-6, 1-3, and 1-2 linkage type and determination of the reducing end and of the monosaccharide sequence Is made possible by the analysis of the negative metastable Ions produced In linked scans FAB MS. The peculiarity of negative Ionization Is believed to consist In a selective deprotonatlon of the anomeric hydroxyl (reducing end of the oligosaccharide chain), which Is more acidic with respect to the remaining OH groups. Once the negative charge Is localized at the oligosaccharide reducing end, the Ion fragmentation of this ring occurs rapidly and the mass losses observed are found to be diagnostic of the glycoside linkage type between adjacent sugar units. The overall negative Ions fragmentation process outlined above allows the simultaneous Identification of the reducing end of the chain, of the monosaccharide units sequence, and of the linkage type between adjacent units.

No abstract

The spectra recorded by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry (MALDI-TOF/TOF-MS/MS) of complex carbohydrates from human milk are presented. Besides ions originating from glycosidic cleavages and from sugar ring fragmentations, these spectra show intense peaks that may be assigned to ions produced by three new fragmentation pathways involving a six-atom rearrangement. These ions, together with the A fragments from sugar ring fragmentations, open the possibility of obtaining a complete mapping of the linkage positions present in the carbohydrates investigated by MALDI-TOF/TOF.