The possibility of structural elucidation of carbohydrates and oligonucleotides by matrix-assisted laser desorptionhonization followed by post-source decay (MALDI-PSD) is investigated. Spectra of both classes of compounds exhibit the better signal-to-noise ratios for the intact species in the negative-ion mode, but the most informative spectra for structural elucidation by PSD are obtained in the positive-ion mode, A novel fragmentation mode is demonstrated on a model sialylated complex type N-linked oligosaccharide. Prompt fragmentation in the positive-ion mode can remove the sialic acids at high laser irradiance if a suitable matrix is used. The remaining sugar backbone is then characterized by post-source decay. A modified oligonucleotide is analyzed with 3-hydroxypicolinic acid as the matrix and ammonium tartrate to displace sodium. Enough structural information is obtained to verify the modification.
Matrix-assisted laser desorptionhonization (MALDI) timeof-flight (TOF) mass spectrometry was introduced in 1988 by Karas and Hillenkamp' and has proved very useful in biological research? Proteins, carbohydrates3 and nucleo-tides4.' have all been analyzed by this method. Until recently, only molecular weight information, but no structural information, could be obtained by MALDI TOE However, very often the molecular weight alone cannot solve a given analytical problem and at least some structural information is desired. Kaufmann and Spengler have recently introduced the post-source decay (PSD) method to mass spectrometrically separate and analyze the fragments generated in the MALDI process.6s7 The major application of the technique has been to peptide sequencing. Martin et al. have described the site-specific analysis of the carbohydrates in glycopeptides' and Spengler et al. the analysis of small, uncharged carbohydrates?
Analysis of carbohydrates-usually derived from glycosylated proteins-is an important problem in biochemical analysis and therefore we were interested in investigating the suitability of PSD analysis for complex carbohydrate structures. In biochemical analysis of glycosylations, the glyco part is usually enzymatically cleaved off the protein and isolated by chromatographic means. Identification and structural elucidation can then be performed by nuclear magnetic resonance (NMR) or by digestion by exoglycosidases and comparison of measured retention times with retention times of standards in a high-performance liquid chromatographic (HPLC) analysis. It is also possible to sequence carbohydrates by a combination of exoglycosidase digests and analysis of the resulting truncated carbohydrates by MALDI." However, each of the former methods has limitations, such as the need for substantial amounts of sample (e.g. milligrams in the case of NMR) or the fact that only standard known structures can be determined (e.g. comparison with standard retention times). Therefore, methods which use low amounts of sample and yet yield general sequence information are needed. Mass spectrometry can, in principle, fulfil these requirements by producing