MASS-SPECTROMETRIC MOLECULAR-WEIGHT DETERMINATION OF HIGHLY ACIDIC COMPOUNDS OF BIOLOGICAL SIGNIFICANCE VIA THEIR COMPLEXES WITH BASIC POLYPEPTIDES

Highly acidic compounds that are difficult to ionize by matrix-assiste d laser desorption ionization give excellent spectra when mixed with a basic peptide or protein to form a noncovalent complex. This phenomen on makes it possible to determine the molecular weights of polysulfate d, -sulfonated, and -phosphorylated biomolecules such as cysteic acid containing peptides, oligonucleotides, heparin-derived oligosaccharide s, and suramin (a drug containing two trisulfonated naphthalene moieti es). Peptides and small proteins rich in arginine were used as the bas ic components. The extent of complex formation correlates with the num ber of phosphate and sulfate groups in the acidic component and with t he number of arginines in the basic component. Neither the acidic amin o acid residue aspartic and glutamic acid nor the basic lysine and his tidine contribute to complex formation. For oligonucleotides, histone H4 was found to be the best complexing agent investigated. The analyti cal utility of the complex formation is demonstrated by the molecular- mass determination of acidic compounds from 500 to 6000 Ha at the pico mole or sub-picomole level with an accuracy of +/-+ 0.1% or better and by the absence of alkali cation adducts.