Laser Photodissociation Action Spectroscopy for the Wavelength-Dependent Evaluation of Photoligation Reactions

Abstract

The nitrile imine-mediated tetrazole–ene cycloaddition is a widely used class of photoligation. Optimizing the reaction outcome requires detailed knowledge of the tetrazole photoactivation profile, which can only partially be ascertained from absorption spectroscopy, or otherwise involves laborious reaction monitoring in solution. Photodissociation action spectroscopy (PDAS) combines the advantages of optical spectroscopy and mass spectrometry in that only absorption events resulting in a mass change are recorded, thus revealing the desired wavelength dependence of product formation. Moreover, the sensitivity and selectivity afforded by the mass spectrometer enable reliable assessment of the photodissociation profile even on small amounts of crude material, thus accelerating the design and synthesis of next-generation substrates. Using this workflow, we demonstrate that the photodissociation onset for nitrile imine formation is red-shifted by ca. 50 nm with a novel N-ethylcarbazole derivative relative to a phenyl-substituted archetype. Benchmarked against solution-phase tunable laser experiments and supported by quantum chemical calculations, these discoveries demonstrate that PDAS is a powerful tool for rapidly screening the efficacy of new substrates in the quest toward efficient visible light-triggered ligation for biological applications.

Nitrile Imines as Peptide and Oligonucleotide Photo-Cross-Linkers in Gas-Phase Ions

“…Similar to previous studies in solution, UVPD-MS 2 primarily resulted in N 2 loss, forming the transient nitrile imine intermediate at m / z 727. Loss of N 2 upon photodissociation of gas-phase 2,5-diaryltetrazole ions has been reported by Blanksby, Trevitt, and coauthors who measured the extinction coefficients as ε ≅ 2 × 10 4 L mol –1 cm –1 at λ max = 255 nm . Our UVPD efficiencies at 250 nm for tet -tagged peptides were consistent with the previous measurements.…”

“…Loss of N 2 upon photodissociation of gas-phase 2,5-diaryltetrazole ions has been reported by Blanksby, Trevitt, and coauthors who measured the extinction coefficients as ε ≅ 2 × 10 4 L mol −1 cm −1 at λ max = 255 nm. 37 Our UVPD efficiencies at 250 nm for tet-tagged peptides were consistent with the previous measurements. In addition to loss of N 2 , the UVPD-MS 2 spectrum showed consecutive loss of C 2 H 5 N (m/z 684), which is an atypical peptide ion dissociation.…”

“…35 The laser-ion trap mass spectrometer system was described in detail previously. 36 Following the previous report by Blanksby, Trevitt, and coauthors on the photodissociation of 2,5-diaryltetrazoles, 37 the excitation wavelength was tuned to 250 nm. Accurate mass measurements were performed on an Orbitrap Ascend Tribrid instrument (ThermoFisher, San Jose, CA) at a 100 000 resolving power.…”

Covalent crosslinking in gas-phase biomolecular ions. An account and perspective

“…88 2,5-Diphenyltetrazole has a major absorption band with a λ max at 255 nm and ε ≅ 2 × 10 4 L mol −1 cm −1 that was measured by Blanksby, Trevitt, et al in gas-phase ions. 90 This makes tetrazole a strong chromophore for photodissociation. Preliminary results with photodissociation of tetrazole-tagged peptides have shown a potential for crosslinking and structure elucidation.…”

Nitrile Imine Cyclizations and Rearrangements: N-Phenyl-C-styrylnitrile Imine and N-Phenyl-C-phenylethynylnitrile Imine

“…Nitrile imines are valuable precursors of pyrazoles, pyrazolines, and triazoles in 1,3-dipolar cycloaddition reactions. , Consequently, their formation by pyrolysis or photolysis of tetrazoles − and their reactivities and structures are prominent subjects of both experimental and computational research. − …”