Multiply deprotonated polypeptide and protein molecules, (M - nH)n-, produced from pH approximately 11 aqueous solutions, are analyzed by electrospray ionization-mass spectrometry (ESI-MS). Aqueous ammonium hydroxide solutions of the analyte are shown to be preferable to sodium hydroxide solutions for negative-ion ESI due to the production of multiply sodiated protein species from the latter system. Proteins with Mr to 66,000 and having up to 57 negative charges have been detected. Multiply charged negative ions can be produced from ESI of the highly acidic protein pepsin (Mr approximately 34,600) because of its relatively large number of acidic residues, 42. In contrast, the small number of basic amino acid residues for pepsin (4) does not allow formation of highly protonated species essential for positive-ion detection, for mass spectrometers of limited m/z range. Similarly, negative-ion ESI-MS is extended to large oligosaccharide analysis. Preliminary tandem mass spectrometry experiments of multiply charged polypeptide anions demonstrate the utility and potential of negative-ion ESI-MS for structural elucidation.

The amp operon, which is located on the Escherichia coli chromosome, modulates the induction of plasmid-borne beta-lactamase genes by extracellular beta-lactam antibiotics. This suggests that the gene products AmpD and AmpE may function in the transduction of external signals. beta-Lactam antibiotics are analogs of cell wall components that can be released during cell wall morphogenesis of enterobacteria. The amp operon was studied to determine its importance in signal transduction during cell wall morphogenesis. The peptidoglycan compositions of amp mutants were determined by high-performance liquid chromatography and fast atom bombardment mass spectrometry. When a chromosomal or plasmid-borne copy of ampD was present, the amount of pentapeptide-containing muropeptides in the cell wall increased upon addition of the cell wall constituent diaminopimelic acid to the growth medium. These results suggest that beta-lactamase induction and modulation of the composition of the cell wall share elements of a regulatory circuit that involves AmpD. Escherichia coli requires AmpD to respond to extracellular signaling amino acids, such as diaminopimelic acid, and this signal transduction system may regulate peptidoglycan composition in response to cell wall turnover products.

The analysis of cardiac glycosides by the desorption/ionization (D/I) mass spectrometric technique potassium ion ionization of desorbed species (K+IDS) is presented. K+IDS mass spectra of digitonin, digoxin, digoxigenin, digitoxin and ouabain are discussed to demonstrate the capabilities of this D/I method. The K+IDS analysis consists of two steps: thermal desorption of neutral molecules representative of the analyte, followed by gas-phase addition of K+ ions to these species. Structural and molecular weight information of the cardiac glycosides is obtained with the K+IDS technique. The most intense peak in the K+IDS mass spectrum of an analyte, M, is frequently the [M]K+ ion. Interpretation of the K+IDS mass spectra is simple, since one thermal degradation mechanism dominates. This mechanism is a 1,2-elimination process. A variation of the original K+IDS technique, performed by changing the ionizing metal from K+ to Na+ (i.e. Na+IDS), is presented for the analysis of digoxin. The Na+IDS mass spectrum of digoxin contains more structural information than the K+IDS mass spectrum of that compound. This may lead to a means of controlling the types of information obtainable with this D/I technique by varying the cation that is thermionically generated. K+IDS analyses can be performed rapidly, no sample derivatization is necessary, no matrix is required and little instrument modification is necessary.

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High resolution mass spectrometry and tandem mass spectrometry analyses of the major ions of digoxin formed by fast atom bombardment are presented and discussed to investigate the mechanisms through which fragment ions are formed. Similar cardiac glycosides are also analyzed to provide support for the proposed fragment assignments. Remote site fragmentation with the charge localized on the aglycone portion of the molecule may provide an explanation for the fragment ions observed in these studies because the majority of these ions contain the aglycone portion of the molecule. The results obtained parallel previously reported results from an ammonia chemical ionization mass spectral study of cardiac glycosides. (J Am Soc Mass Spectrom 1990, 1, 455-472).