Laser desorption/Fourier transform ion cyclotron resonance vs. field desorption/magnetic sector mass spectrometry. Erythromycin, amoxicillin, digoxin, and daunorubicin

Structural elucidation studies on 14- and 16-membered macrolide aglycones by accurate-mass electrospray sequential mass spectrometry

Collision induced dissociation sequential mass spectrometry was used to investigate the fragmentation of the heptaketide macrolide aglycones, 6-deoxyerythronolide B (6-dEB), erythronolide B (EB), and acetate-starter EB (Ac-EB). The fragmentations of two previously reported octaketide analogs produced by "stuttering" of the erythromycin polyketide synthase, stuttered-6-dEB and acetate-starter stuttered-6-dEB were also studied. The accuracy with which the mass of each fragment was measured allowed it to be attributed to an unambiguous formula. Most of the experiments were repeated using samples dissolved in deuterated solvents. These data were then used to deduce plausible fragmentation pathways of the five compounds which were shown to have a high degree of similarity. Preliminary fragmentation analysis of a novel octaketide analog was performed and the structure was predicted as stuttered EB. Subsequent scale-up of the bacterial fermentations, followed by isolation and characterization by nuclear magnetic resonance spectroscopy confirmed this prediction. Further fragmentation experiments were then performed on this compound, which provided further evidence of the similarity of the fragmentation schemes. These results demonstrate the utility of collision induced dissociation sequential mass spectrometry analysis in the preliminary screening of bacterial fermentations for new polyketides. These studies were performed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

Alkali-metal ion/molecule association reactions and their applications to mass spectrometry

This review will be concerned primarily with thermal alkali-metal ion association reactions of the general type: A+ + M + N reversible (A + M)+ + N. Where A denotes a positively charged alkali metal ion, M is neutral species and N works as a third body. As indicated, most association reactions are reversible, and the A-M bond derives primarily from electrostatic forces whose energy (affinity) is typically 50 kcal/mol or less, often much less. The review includes reaction mechanism, instrumentation, and application to mass spectrometry, together with reaction rates and alkali ion affinities of the classified compounds.

Analysis of a commercial preparation of erythromycin estolates by tandem mass spectrometry and high performance liquid chromatography/electrospray ionization tandem mass spectrometry using an ion trap mass spectrometer

Because of the lack of a UV chromophore and their much smaller abundances in comparison with the major component, the minor components in erythromycin estolate preparations are difficult to analyze by high performance liquid chromatography ultraviolet (HPLC-UV). Tentative assignment of the major and minor components can be achieved with the combination of full scan and ZoomScan using an ion trap mass spectrometer. Tandem mass spectrometry (MS/MS) provided an effective method to quickly identify most components without chromatographic separation, and all the related compounds, except the isobaric pair ECE and PdMeEA, could be identified in this way. The best result was obtained by using liquid chromatography/tandem mass spectrometry (LC/MS/MS) operated in selected reaction monitoring mode. The major compound, the estolate of erythromycin A (EAE), and seven other minor components, could be separated and identified, with semiquantitative estimates of relative concentrations.

Structural elucidation studies of erythromycins by electrospray tandem mass spectrometry

Erythromycin A (EryA) was studied by electrospray ionisation tandem mass spectrometry (ESI-MS/MS) with the aim of developing a methodology for the structural elucidation of novel erythromycins developed by biological synthetic methods. Skimmer dissociation along with sequential mass spectrometry studies (up to MS5) have been employed in this study. In the low-resolution MS/MS analysis of the polyketides, there are several fragment ions that are easily assigned to various neutral losses. These have all been confirmed by accurate-mass measurements. There is also a series of peaks due to ring opening and fragmentation that can only be assigned by high-resolution MSn analysis. Further experiments were performed in deuterated media (D2O/CD3OD 50%) which, along with the high-resolution MSn of erythromycin analogues, has enabled us to identify some of the steps in the ring fragmentation, particularly the loss of the polyketide starter acid. This is an essential step for determining structural alterations in the novel polyketides, but further labelling experiments and studies on more erythromycin analogues are required before the complete fragmentation pathway can be confirmed.

Structural elucidation studies of erythromycins by electrospray tandem mass spectrometry II

Erythromycin A (EryA), sec-butyl erythromycin B (SEryB), oleandomycin (Olean) and a synthetic derivative, roxithromycin (Rox), were used to investigate the fragmentation of polyketide macrolide antibiotics by collision induced dissociation (CID) tandem mass spectrometry (MS/MS). Analyses were performed with two commercially available mass spectrometers: a Q-TOF hybrid quadrupole time-of-flight instrument and a BioApex II (4.7 Tesla) Fourier transform ion cyclotron resonance (FTICR) instrument both equipped with electrospray ionisation (ESI) sources. One of the first fragmentation processes is the loss of an H(2)O molecule from the [M+H](+) ion. EryA has three hydroxyl groups on the polyketide ring and loses three H(2)O molecules during CID. This study indicates that these facts are not necessarily related. Deuterium exchange experiments were carried out in order to isotopically label free hydroxyl groups. (18)O-exchange experiments were also carried out in order to label the carbonyl group at the 9-position. In EryA and its analogue the first H(2)O loss shifts in mass from loss of 18 Da to loss of 20 Da in deuterated solvents. For both molecules the loss also shifts in mass from loss of 18 Da to loss of 20 Da during the (18)O-exchange experiments. This suggests that the first loss of H(2)O is from the 9-position carbonyl group, indicating that this, and not the nitrogen of the amino sugar, is the site of protonation of the activated MH(+) ions. For Rox the initial loss of H(2)O is replaced by loss of the 9-position oxime group, the rest of the fragmentation sequence being the same as for EryA. For Olean, there is no H(2)O loss from the parent ion. The results have allowed the proposal of a mechanism for the first loss of H(2)O in the EryA MS/MS fragmentation.

Potassium halide adducts as reagent ions in infrared laser desorption/ionization fourier transform ion cyclotron resonance mass spectrometry

Potassium halide adducts of the form K2X(+) (X = F, CI, Br, and I) desorbed from neutral salts by high power, pulsed, infrared laser radiation are detected in abundance by Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. FT-ICR detection of the K2X(+) adduct is favored at increased laser power densities (> 10(8) W/cm(2)) and at trapping potentials below 3 V, independent of X. In contrast, detection of K(+) is promoted at laser power densities below 10(8) W/cm(2) or at higher trapping potentials, with a threshold for trapping that is strongly dependent on X. When laser desorption/ionization (LDI)/FT-ICR is performed on 1:1 mixtures of KX and organic molecules, ejection pulses applied continuously at the cyclotron resonance frequency of K2X(+) inhibit formation of the cation-attached product, [M + K](+). Conversely, resonance ejection of K(+) enhances [M + K](+), apparently by reducing the matrix ion population trapped in the cell. In evaluating higher molecular weight adducts, only K3F 2 (+) formed in abundance by laser desorption of KF is found through double resonance experiments to contribute significantly to formation of [M + K](+). Finally, among the potassium halides, KI generates the highest ratio of detected K2X(+) to K(+) at low trapping potentials and is therefore best suited for cation-transfer reactions in infrared LDI/FT-ICR experiments performed at power densities in the 10(8) W/cm(2) range.

Laser desorption/fourier transform ion cyclotron resonance mass spectrometry: digoxin, digitoxin, and their reduced and sugar-hydrolyzed metabolites

Mass spectra of digoxin and digitoxin (the most widely prescribed drugs for treatment of congestive heart failure) and a complete set of their 14 dihydro- and sugar-hydrolyzed metabolites have been obtained via laser desorption/ionization with a Fourier transform ion cyclotron resonance (LD/FT/ICR) mass spectrometer. The most intense peak is typically the pseudomolecular [M + K]+ ion, but fragment ions corresponding to loss of 1-3 sugars and hydroxyls are also observed. LD/FT/ICR mass spectra for all 16 compounds were produced with a single set of sample and spectrometer parameters. No matrix peaks are present. Finally, LD/FT/ICR provides dynamic mass accuracy within approximately 5 ppm throughout a mass range of 404 less than m/z less than 819.