In modern drug discovery the screening of drug candidates against a biological target is often required. One technique suited for this task is native electrospray ionization mass spectrometry (ESI-MS). There, small amounts of molecule-target complexes can be detected in a label-free fashion. Since ESI-MS is often performed manually, Roche and our group collaborated to develop an ESI autosampler to speed up this analysis.^[1]Recent modifications^[2]allow the analysis of native molecule-target complexes. Based on those latest improvements, we developed a method to rapidly screen compounds against a target. First, a solution of a protein target is fed through an ESI capillary containing a hole. Next, a nanoliter droplet of a compound is picked up from a well plate and injected into this hole. Finally, the compound and the target are mixed in the capillary and binding can be observed via MS. Quantification of the injected ligand is achieved by in-line addition of an internal standard which is both in the droplet and the capillary. The capability of our method is demonstrated by screening 33 test compounds against carbonic anhydrase II as a model protein in 15 min. As a result, we found an excellent screening figure of merit^[3]: Z’ = 0.82. In Addition, binding constants can be determined either directly by our quantification strategy or by injecting the compound in different concentrations and fitting a titration curve. Compared to commercial autosamplers, our setup requires significantly less amount of compound (≈0.6 pmol/l) while operating at a similar throughput.

[1] a) P. F. Berndt, Christof; Steiner, Roger in System for dispensing a sample into a buffer liquid, Vol. US 9,446.408 B2(Ed. F. H.-L. R. AG), F. Hoffmann-La Roche AG, United States, 2016; b) V. Neu, R. Steiner, S. Muller, C. Fattinger and R. Zenobi,Anal Chem 2013, 85, 4628-4635.
[2] S. Ghiasikhou, A. Marchand and R. Zenobi, Microfluidics and Nanofluidics 2019, 23, 60.
[3] J.-H. Zhang, T. D. Y. Chung and K. R. Oldenburg, Journal of Biomolecular Screening 1999, 4, 67-73.