Highly acidic compounds that are difficult to ionize by matrix-assisted laser desorption ionization give excellent spectra when mixed with a basic peptide or protein to form a noncovalent complex. This phenomenon makes it possible to determine the molecular weights of polysulfated, -sulfonated, and -phosphorylated biomolecules such as cysteic acid-containing peptides, oligonucleotides, heparin-derived oligosaccharides, and suramin (a drug containing two trisulfonated naphthalene moieties). Peptides and small proteins rich in arginine were used as the basic components. The extent of complex formation correlates with the number of phosphate and sulfate groups in the acidic component and with the number of arginines in the basic component. Neither the acidic amino acid residue aspartic and glutamic acid nor the basic lysine and histidine contribute to complex formation. For oligonucleotides, histone H4 was found to be the best complexing agent investigated. The analytical utility of the complex formation is demonstrated by the molecular-mass determination of acidic compounds from 500 to 6000 Da at the picomole or sub-picomole level with an accuracy of +/- 0.1% or better and by the absence of alkali cation adducts
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