Matter-wave interferometry of large biomolecules demands efficient strategies for launching intact neutral particles into high vacuum and detecting them after traversal of the interferometer [1]. A key challenge is generating charged fragments from a neutral beam to enable mass-spectrometric detection.

In our previous work, a series of porphyrin-based photocleavable tags was developed and evaluated for this purpose [2]. These compounds demonstrated clean homolytic cleavage at 411 nm in the gas phase, producing neutral fragments and confirming the viability of laser desorption for neutral beam formation. However, because both fragments are released as neutral species, an additional vacuum ultraviolet postionization step at 157 nm is required for mass-spectrometric detection, adding complexity and further limitations to the overall detection scheme.

To probe an alternative approach to ionization, we present a rationally designed zwitterionic photocleavable tag in which charge separation is already a feature of the molecular architecture. A permanent quaternary ammonium cation and a sulfonate anion are held in fixed spatial relationship across a rigid [2.2]paracyclophane spacer, rendering the intact molecule net-neutral. DFT geometry optimization confirms a charge separation distance of 11.9 Å in the energy-minimized gas-phase conformation (Figure 1), reducing the Coulombic barrier for fragment separation well below the energy of a single 266 nm photon (4.66 eV). The nitrobenzyl ether photocage was selected on the basis of well-established photocleavage reactivity at 266 nm, and the [2.2]paracyclophane spacer was chosen for its rigidity, which enforces a defined charge separation geometry in the gas phase. Upon UV irradiation, intended homolytic cleavage of the nitrobenzyl ether linkage would liberate the pre-existing internal charges as spatially separated ionic fragments, directly detectable by mass spectrometry without any external ionization step.

The tag incorporates an azide handle for CuAAC bioconjugation, enabling modular conjugation to alkyne-functionalized biomolecules. Gas-phase photocleavage experiments are underway in collaboration with the Arndt group (University of Vienna).

[1] Y. Fein, P. Geyer, P. Zwick, F. Kiałka, S. Pedalino, M. Mayor, S. Gerlich, M. Arndt, Nature Physics, 2019, 15, 1242–1245.

[2] O. Rybakova, J. Reisinger, P. Rieser, P. Geyer, S. Gerlich, M. Arndt, A. S. Kumar, D. Häussinger, M. Mayor, V. Köhler, Helvetica Chimica Acta, 2025, 108, e202500022.