Mass spectrometry library characterization

WL Fitch, A Lu, K Tsutsui, N Shah. Single bead mass spectrometry for characterization of combinatorial libraries. Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portlan, OR, 1996, p. 1043. 

JA Loo, DE De-John, RRO Loo, PC Andrews. Application of mass spectrometry for characterizing and identifying ligands from combinatorial libraries. Ann Rep Medic Chem 31 319-325, 1996. 

Two groups have reported on the advantages of high resolution Fourier transform mass spectrometry for characterizing libraries [42, 43]. This technique can determine the accurate mass of a compound and thus distinguish between isobars which have the same nominal mass. However, high resolution MS does not solve the portion of the mass redundancy in libraries which is due to true structural isomers. 

METHOD REPRODUCIBILITY AND SPECTRAL LIBRARY ASSEMBLY FOR RAPID BACTERIAL CHARACTERIZATION BY METASTABLE ATOM BOMBARDMENT PYROLYSIS MASS SPECTROMETRY... 

For solution-phase libraries that are composed of mixtures of compounds, the difficulty of analysis escalates with increasing numbers of compounds. Typically, large mixtures of compounds are not analyzed before screening, whereas small ones may be analyzed for reaction completeness using mass spectrometry, HPLC, NMR, or combinations thereof. The identification and analysis of active compounds from these mixtures is painstakingly tedious, and often complete characterization is possible only after deconvolution procedures and resynthesis of the active compound. For solid-phase libraries, the methods currendy developed are discussed below. 

The development glycopeptide libraries obtained by the split-mix method is severely hampered by the lack of concurrent development of a general, facile separation and characterization technology. Some headway has been made with chemical coding of the libraries, but very few direct methods of analysis exist. One promising method that could be applied to the direct characterization of both types of libraries is mass spectrometry. More specifically, post-source-decay matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (PSD-MALDI-TOF-MS) and CID-FAB/MS/MS have been used to characterize glycopeptides.53-55... 

An additional problem arises with the difficult analytical characterization of the cyclic peptide libraries. This is preferably performed by mass spectrometry, although there are limitations with the ionization method. In fact, FAB-MS leads to overexpression of hydro-phobic components by ion suppression, whereas in MALDI-MS preferentially hydrophilic... 

Dunayevskiy, Y. Vouros, P. Carell, T. Wintner, E. A. Rebek, J. Jr. 1995. Characterization of the complexity of small-molecule libraries of electrospray ionization mass spectrometry. Anal. Chem., 67,2906-2915. 

Nedved, M. L. Habibi-Goudarzi, S. Ganem, B. Henion, J. D. 1996. Characterization of benzodiazepine combinatorial chemical libraries by on-line immunoaffinity extraction, coupled column HPLC-ion spray mass spectrometry-tandem mass spectrometry. Anal. Chem., 68, 4228-4236. 

Wang, T. Zeng, L. Strader, T. Burton, L. Kassel, D. B. 1998. A new ultra-high throughput method for characterizing combinatorial libraries incorporating a multiple probe autosampler coupled with flow injection mass spectrometry analysis. Rapid Commun. Mass Spectrom., 12, 1123-1129. 

Zeng, L. Burton, L. Yung, K. Shushan, B. Kassel, D. B. 1998b. Automated analytical/preparative high-performance liquid chromatography-mass spectrometry system for the rapid characterization and purification of compound libraries. /. Chromatogr. A, 794,3-13. 

The new field of molecular diversity raises three issues which need to be addressed by the organic analytical chemistry community (i) What tools can we use for following solid-phase reactions (ii) How can we analyze all these samples (iii) How much characterization of libraries is possible or appropriate This chapter deals with these problems and reviews the literature since a similar review written in June 1995 [2] (earlier seminal publications are described where appropriate). Other analytical issues such as decoding of combinatorial libraries or the applications of affinity separations and single-bead mass spectrometry for library deconvolution are dealt with in other chapters of this book. 

Flow injection analysis mass spectrometry (FIA-MS) has been reported to be a fast method for the characterization of combinatorial libraries (55,56). The method verifies the presence of the molecular ions of the expected product and side products or impurities but does not provide information on the quality of the analyzed samples. Significant improvements related to the increased analytical throughput, obtained by reducing the time between each injection without increasing the intersample carry-over from each analysis, were recently reported (57, 58). When coupled with RP-HPLC, FIA-MS allows the separation and the determination of the molecular weight of the components of each sample. This is normally enough to unequivocally attribute the structure of the expected library component and of any side products from a library synthesis. 

The dimensions of the library prevented its full analytical characterization, but enough data were acquired to judge its overall quality. Twelve percent of the individuals, that is, 1056 randomly selected compiounds, were analyzed by electrospray mass spectrometry, and 83% of the analyzed samples showed the expected molecular ion as the principal MS peak. Eighty-five randomly selected compounds (corresponding to 1% of the total) were analyzed by HPLC-MS and the average purity was determined as 73% area/area. Finally, six discretes (6.35-6.40) were randomly picked and fully characterized. An authentic sample of each was prepared in solution, purified, and used as a standard for the quantitative determination of the six library components. The structures of the six compounds and the overall satisfactory results from their analytical characterization are reported in Fig. 6.16. 

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