A dark intermediate in the fluorogenic reaction between tetrazine fluorophores and trans-cyclooctene

Felix Hild; Philipp Werther; Klaus Yserentant; Richard Wombacher; Dirk-Peter Herten

doi:10.1016/j.bpr.2022.100084

A dark intermediate in the fluorogenic reaction between tetrazine fluorophores and trans-cyclooctene

Biophys Rep (N Y). 2022 Nov 5;2(4):100084. doi: 10.1016/j.bpr.2022.100084. eCollection 2022 Dec 14.

Authors

Felix Hild¹, Philipp Werther², Klaus Yserentant^{1

3

4}, Richard Wombacher^{2

5}, Dirk-Peter Herten^{1

3

4}

Affiliations

¹ Physikalisch-Chemisches Institut, Heidelberg University, Heidelberg, Germany.
² Institute of Pharmacy and Molecular Biotechnology, Heidelberg University, Heidelberg, Germany.
³ Institute of Cardiovascular Sciences, College of Medical and Dental Sciences and School of Chemistry, University of Birmingham, Birmingham, United Kingdom.
⁴ Centre of Membrane Proteins and Receptors (COMPARE), The Universities of Birmingham and Nottingham, Birmingham, West Midlands, United Kingdom.
⁵ Max-Planck-Institut für Medizinische Forschung, Heidelberg, Germany.

Abstract

Fluorogenic labeling via bioorthogonal tetrazine chemistry has proven to be highly successful in fluorescence microscopy of living cells. To date, trans-cyclooctene (TCO) and bicyclonyne have been found to be the most useful substrates for live-cell labeling owing to their fast labeling kinetics, high biocompatibility, and bioorthogonality. Recent kinetic studies of fluorogenic click reactions with TCO derivatives showed a transient fluorogenic effect but could not explain the reaction sequence and the contributions of different intermediates. More recently, fluorescence quenching by potential intermediates has been investigated, suggesting their occurrence in the reaction sequence. However, in situ studies of the click reaction that directly relate these observations to the known reaction sequence are still missing. In this study, we developed a single-molecule fluorescence detection framework to investigate fluorogenic click reactions. In combination with data from ultra-performance liquid chromatography-tandem mass spectrometry, this explains the transient intensity increase by relating fluorescent intermediates to the known reaction sequence of TCO with fluorogenic tetrazine dyes. More specifically, we confirm that the reaction of TCO with tetrazine rapidly forms a fluorescent 4,5-dihydropyridazine species that slowly tautomerizes to a weakly fluorescent 1,4-dihydropyridazine, explaining the observed drop in fluorescence intensity. On a much slower timescale of hours/days, the fluorescence intensity may be recovered by oxidation of the intermediate to a pyridazine. Our findings are of importance for quantitative applications in fluorescence microscopy and spectroscopy as the achieved peak intensity with TCO depends on the specific experimental settings. They clearly indicate the requirement for more robust benchmarking of click reactions with tetrazine dyes and the need for alternative dienophiles with fast reaction kinetics and stable fluorescence emission to further applications in advanced fluorescence microscopy.