Catalyst-free "click" reactions for the direct conjugation of metal complexes to bio(macro)molecules
Inorganic Molecular Chemistry - Synthesis and Characterisation
Final Report Abstract
The modification of bio(macro)molecules with metal-based functionalities is an active area of research in bioinorganic and bioorganometallic chemistry. However, the regiospecific and bioorthogonal introduction of emissive as well as electrochemical markers, stable and radioisotope labelling for speciation studies, and metal sites with tunable ligand exchange kinetics for the generation of novel catalytic activity is a particular challenge. In bioorganic chemistry, numerous conjugation methods have been developed in the last decade, which can also be applied to the ligand periphery of metal complexes. On the other hand, the great potential of metal-centered conjugation reactions involving small ligands directly coordinated to a metal center has not been explored to a significant extend so far, although variation of the metal-coligand combination would allow for a very flexible tuning of the reaction kinetics. In the context of the present project, the inner sphere "iClick" (inorganic Click) cycloaddition reaction of metal-coordinated azide groups with alkynes and "masked" alkynes was established for a wide range of 3d–5d transition metals. The choice of proper alkyne coupling partners was delineated and the kinetics of the reaction studied with a range of complementary analytical methods. Metal azide complexes of Mo, W, Ma, Re, Ru, Rh, Pd, and Pt were easily accessible from halide precursors and smoothly underwent the iClick reaction with electron-poor alkynes such as dimethyl acetylenedicarboxylate (DMAD) or F3C-C≡C-COOCH3. Two electronwithdrawing groups were required for the reaction to take place, which usually proceeded at room-temperature in the case of octahedral compounds while square-planer complexes required heating. Kinetic studies based on HPLC, solution IR spectroscopy, or variable-temperature 19F NMR spectroscopy in the case of F3C-C≡C-COOCH3 gave reaction rate constants comparable to the well-established Staudinger ligation, and in some case approaching those of the strainpromoted azide-alkyne cycloaddition (SPAAC). Electron-donating groups on the coligands also accelerated the reaction, and in the case of the Mo vs. W case, an unprecedented effect of the metal was discovered, with the molybdenum azide complex reacting significantly faster than the tungsten analogue. In particular in the case of some ruthenium triazolate "iClick" products, an exceptional robustness of these organometallic compounds in aqueous medium was observed, which were stable in the pH range of 1–8 for at least 24 hrs and could be kept in neutral solution for 7 1/2 months without noticeable decomposition. Preliminary biological assessment on a small panel of human cancer cell lines showed, for another series of isostructural palladium and platinum triazolate "iClick" compounds, submicromolar IC50 values and an unexpected reversal of the biological potential, with the palladium compounds more active than the platinum analogues, which could be correlated with an inhibition of the thioredoxin reductase (TrxR) enzyme currently in the focus of much research as an alternative, non-DNA target for anticancer chemotherapy. In summary, in this project, we have shown that the "iClick" reaction of metal-azide complexes with electron-poor alkynes leads to highly stable triazolate-linked conjugates under kinetics comparable to bioorthogonal coupling reactions well-established in bioorganic chemistry. The next challenge will now be to incorporate the alkyne coupling partner into functional biomolecules for applications in Inorganic Chemical Biology and the elucidation of the cellular mechanisms behind the surprising biological activity discovered along the way.
Publications
- „Amino acid bioconjugation via iClick reaction of an oxanorbornadiene-masked alkyne with a MnI(bpy)(CO)3-coordinated azide”, Chem. Commun. 2014, 50, 15692–15695
L. Henry, C. Schneider, B. Mützel, P.V. Simpson, C. Nagel, K. Fucke, U. Schatzschneider
(See online at https://doi.org/10.1039/c4cc07892f) - „Sonogashira, CuAAC, and oxime ligations for the synthesis of Mn(I) tricarbonyl PhotoCORM peptide conjugates”, Eur. J. Inorg. Chem. 2014, 2886–2895
S. Pai, K. Radacki, U. Schatzschneider
(See online at https://doi.org/10.1002/ejic.201402123) - „Catalyst-free room-temperature iClick reaction of molybdenum(II) and tungsten(II) azide complexes with electron-poor alkynes: Structural preferences and kinetic studies”, Dalton Trans. 2017
P. Schmid, M. Maier, H. Pfeiffer, L. Henry, A. Belz, A. Friedrich, F. Schönfeld, K. Edkins, U. Schatzschneider
(See online at https://doi.org/10.1039/c7dt03096g) - „Electronic influences on the stability and kinetics of Cp* rhodium(III) azide complexes in the iClick reaction with electron-poor alkynes”, Eur. J. Inorg. Chem. 2017, 3024–3029
L. Hiersch, J. Mößeler, U. Schatzschneider
(See online at https://doi.org/10.1002/ejic.201700199)
