Jean-Louis Aubagnac

Mass spectrometry in combinatorial chemistry

In the fast expanding field of combinatorial chemistry, profiling libraries has always been a matter of concern--as illustrated by the buoyant literature over the past seven years. Spectroscopic methods, including especially mass spectrometry and to a lesser extent IR and NMR, have been applied at different levels of combinatorial library synthesis: in the rehearsal phase to optimize the chemistry prior to library generation, to confirm library composition, and to characterize after screening each structure that exhibits positive response. Most of the efforts have been concentrated on library composition assessment. The difficulties of such analyses have evolved from the infancy of the combinatorial concept, where large mixtures were prepared, to the recent parallel syntheses of collections of discrete compounds. Whereas the complexity of the analyses has diminished, an increased degree of automation was simultaneously required to achieve efficient library component identification and quantification. In this respect, mass spectrometry has been found to be the method of choice, providing rapid, sensitive, and informative analyses, especially when coupled to chromatographic separation. Fully automated workstations able to cope with several hundreds of compounds per day have been designed. After a brief introduction to describe the combinatorial approach, library characterization will be discussed in detail, considering first the solution-based methodologies and secondly the support-bound material analyses.

Use of mike spectrum or cid/mike spectrum of a quasi-molecular or cationized ion obtained by the MBSA-FAB method in order to characterize a polar compound

Abstract CID/MIKE (or MIKE) spectrum of the quasi-molecular (or cationized) ion obtained by the MBSA-FAB method characterizes a polar compound.

Quantitative determination of rocuronium in human plasma by liquid chromatography–electrospray ionization mass spectrometry

Liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) was used for the quantification of the neuromuscular blocking agent rocuronium in human plasma. Verapamil was used as internal standard. The samples were subjected to a dichloromethane liquid-liquid extraction after ion pairing of the positively charged ammonium compound with iodide prior to LC-MS. Optimized conditions involved separation on a Symmetry Shield RP-18 column (50 x 2.1 mm, 3.5 microm) using a 15-min gradient from 10 to 90% acetonitrile in water containing 0.1% trifluoroacetic acid at 250 microl/min. Linear detector responses for standards were observed from 25 to 2,000 ng/ml. The extraction recovery averaged 59% for rocuronium and 83% for the internal standard. The limit of quantification (LOQ), using 500 microl of plasma, was 25 ng/ml. Precision ranged from 1.3 to 19% (LOQ), and accuracy was between 92 and 112%. In plasma samples, at 20 and 4 degrees C, rocuronium was stable at physiological pH for 4 h; frozen at -30 degrees C it was stable for at least 75 days. The method was found suitable for the analysis of samples collected during pharmacokinetic investigations in humans.

Tof-SIMS Analysis of Polymer Bound Fmoc-protected Peptides

Abstract We report the results obtained in the step by step control of a solid phase peptide synthesis carried out on Sheppard polyamide resin by Time of flight - Secondary Ion Mass Spectrometry.

Automated monitoring of phosphatidylcholine biosyntheses in Plasmodium falciparum by electrospray ionization mass spectrometry through stable isotope labeling experiments.

The metabolic pathways contributing to phosphatidylcholine biosyntheses in Plasmodium falciparum, the malaria-causing parasite, was explored by electrospray ionization mass spectrometry. Phosphatidylcholine produced by the CDP-choline pathway and by the methylation of phosphatidylethanolamine was identified and quantified through isotopic labeling experiments. A straightforward method based on cone voltage directed in-source fragmentations and relative abundance measurement of endogenous versus deuterated specific fragment ions was developed for simple and rapid automated data acquisition. Such high-throughput analytical protocol allowed us to measure the relative contribution of two different metabolic pathways leading to phosphatidylcholine without performing technically more demanding and time-consuming MS/MS or LC/MS experiments.

Impurity Detection in Solid-Phase Organic Chemistry: Scope and Limits of HR MAS NMR

Abstract We evaluate the possibilities and limits of HR MAS NMR to detect and quantify side products during solid-phase synthesis, using a model system where pyroglutamate and glutamate are coupled in a well defined ratio. Resins swollen in deuterated and protonated solvent are studied. Use of the LED sequence eliminates the peaks due to the use of protonated solvents, but sensitivity is decreased, and differential losses of magnetization might lead to a biased population estimation. However, as all sample workup steps are eliminated, this technique will be helpful in detecting minority species in solid-phase combinatorial chemistry, and its application at the different steps of the reaction might lead to the early detection of otherwise unidentifiable active components.

FAB mass spectra of alkali organobis-(benzene-1,2-diolato) silicates

Abstract Negative and positive ion FAB mas spectra of alkali silicates (C + A − where C = cation and A = anion) emphasize the facility with which it is possible to obtain gas-phase ions by the bombardment with high-energy neutral atoms of a matrix containing ionic substances. The negative ion FAB mass spectra permit the mass of the anion to be determined unambiguously while the adduct ions observed in the positive ion mass spectra allow the cation to be identified. Reduction processes including both one-electron reductions with the formation of AH − ions as well as two- and four-hydrogen additions are observed.

Step-by-step Control by Time-of-flight Secondary Ion Mass Spectrometry of a Peptide Synthesis Carried Out on Polymer Beads

The use of time-of-flight secondary ion mass spectrometry (Tof-SIMS) for the step-by-step control of a peptide synthesis on a solid support is illustrated. The positive secondary ions, issuing from energetic fragmentation, are characteristic of the terminal groups of the step under investigation. The negative secondary ions result from the cleavage of the benzyl ester link between the polymer support and the first amino acid, and provide direct identification of the growing sequence. Tof-SIMS offers this unique analytical possibility without any preliminary separation of the peptide from its support.

FAB mass spectra of alkali bis (benzene-1,2 diolato)phenyl silicates and tris (benzene-1,2 diolato) silicates and germanates: influence of the matrix

Abstract FAB spectra in the negative ion mode allow characterization of anionic pentacoordinated silicon complexes and dianionic hexacoordinated silicon and germanium complexes. With the dianionic derivatives, identification occurs from cluster ions which also allow characterization of the cation. With the three matrices used (glycerol, ortho-nitrobenzyl alcohol (NBA) and “magic bullet”), different spectra were obtained and compared. With glycerol, the reactivity described by Rose and co-workers (substitution of catechol by glycerol) (Int. J. Mass Spectrom. Ion Processes, 87 (1989) 225) has been confirmed for alkali silicates.

Structure determination of a cyclic tetrapeptide : tentoxine by mass spectrometry using fast atom bombardment for ionization

Abstract The molecular mass and the sequence of tentoxine, a cyclic tetrapeptide, are determined by the mass spectrum of this compound which was obtained by a mass spectrometer using the fast atom bombardment for ionization.