Evaluation of Mass Spectrometric Methods Applicable to the Direct Analysis of Non-Peptide Bead-Bound Combinatorial Libraries

Abstract

Electrospray, matrix-assisted laser desorption, and time-of-flight secondary ion mass spectrometry have been explored as possible methods for the identification of active members of molecular combinatorial libraries. All three methods are found to yield accurate molecular weight information about a target molecule angiotensin II antagonist synthesized on a 40-microns polystyrene bead. Structural identification is also possible by accurate mass measurements to eliminate candidate structures with the same nominal mass and by analysis of the fragmentation patterns. In addition, the secondary ion mass spectrometry measurements yield spatially resolved spectra from a single bead after exposure to a suitable gas which clips the covalent bond at the linking position. All three approaches appear to offer a viable screening strategy of non-peptide libraries without the use of additional molecular tags.

Rapidly Measuring Reactivities of Carboxylic Acids to Generate Equireactive Building Block Mixtures: A Spectrometric Assay

“…Maybe reactivity tests of building blocks with different forms of activation 26 can also be accelerated by such automated tests. Expanding the set of equireactive mixtures beyond amino acid building blocks 4 may revive the interest in mixed couplings as the simplest approach for generating diversity in untagged libraries, 27,28 including those that are so large as to require recursive deconvolution. 29 Some reactivities determined were quite unexpected and may, therefore, be of interest to the chemical community beyond the practical application of generating combinatorial libraries more efficiently.…”

Imaging time-of-flight secondary ion mass spectrometry of solid-phase peptide syntheses

“…Such an edge effect was explained 12,33 as a consequence of superior conduction at the periphery of the bead leading to better charge neutralization. Finally, whereas the use of TFA vapors to cleave the covalent peptide-resin bond prior to imaging TOF-SIMS was found to provide some contamination as the ions were detected in part outside the beads, 18 well defined images were obtained in our case upon direct TOF-SIMS bombardment. The absence of physisorbed material was checked by selecting the protonated molecular ions of proline and phenylalanine at m/z 116 and 166, respectively, according to our previous study 32 (data not shown).…”

“…The structural information recovered from direct TOF-SIMS experiments was compared with the corresponding ESI and MALDI results obtained after compound release from the support. 18 Fitzgerald and co-workers 19,20 have described a method to analyze directly resin-bound peptides by MALDI mass spectrometry. The resin beads were derivatized by a photolabile linker which upon MALDI irradiation provided simultaneous peptide cleavage and ionization, thus not requiring chemical release in solution of the growing structures.…”

“…Finally, the unique feature of the SIMS method which has been compared with other desorption techniques such as liquid SIMS, FAB and MALDI by Busch,21 resides in the possibility of proceeding to imaging studies. 18,22 -24 Benkovic and Winograd 18 have applied the imaging TOF-SIMS technique to the determination of the molecular masses of some amino acids and tripeptides linked to polystyrene beads. The structures were released from the support by trifluoroacetic acid vapor before the SIMS assay.…”

“…For all imaging SIMS experiments, no vapor-phase cleavage of the anchored peptides was carried out to avoid sidereactions in particular with TFA 25 and more importantly to prevent ion leakage around the beads resulting in crosscontaminations, as observed by Benkovic and Winograd. 18 Besides, the controlled bead can be re-used for further synthetic steps.…”