The lanthanoids are peculiar elements with a very unusual electronic structure. It makes the lanthanoid cations Ln3+ chemically almost indistinguishable, leads to almost vanishing ligand field effects, and is the origin of their outstanding physical properties. These make the lanthanoids extremely valuable for diverse high-tech applications, for example as contrast agents in magnetic resonance imaging (MRI) or as luminescent probes for bioassays. At the same time the very weak ligand field effects make it very difficult to construct stable lanthanoid coordination compounds. An even more challenging task is the preparation of heteropolymetallic lanthanoid complexes. These promise an improvement of already established applications of the lanthanoids and, even more importantly, also genuinely new applications. The one we are most fascinated by is the preparation of molecular luminescent nanocodes. Inspired by Nature’s extremely successful data storage device, the DNA, such materials could allow for the storage of any type of information on a molecular scale. The information is encoded in the sequence and/or ratio of reliably connected monomers, the resulting nanocode can for example be used for the reliable labelling of biomolecular samples or for anti-counterfeiting purposes. Compared to the nucleotides which are used in the DNA, the use of luminescent monomers would lead to a drastic advantage for such applications: While the artificial read-out of DNA requires rather tedious and time consuming biochemical methods, a luminescent nanocode could be read out completely non-invasively with the simple and well-established technique of luminescence spectroscopy.
Due to their fingerprint-like emission spectra consisting of very sharp bands, lanthanoid coordination compounds are the perfect luminophores for such a purpose. As indicated before, the biggest struggle to realise this potential was to find a way to connect the individual lanthanoid luminophores in a highly controlled and reliable fashion. Coordination chemistry cannot provide bonds which fulfill these requirements, so from a conceptual point of view it was clear that the aid of the toolbox of organic chemistry would be needed to realise the desired heteropolymetallic lanthanoid compounds. The most suitable technique of synthetic organic chemistry is solid-phase peptide synthesis (SPPS), which is well established, very reliable, and can even be automated. Yet the harsh reaction conditions applied during SPPS are a challenge for the notoriously labile lanthanoid complexes. In our new study we describe the strategy we recently developed to circumvent that problem: The lanthanoids were encapsulated in a cryptate ligand, which in our previous studies had proven to provide lanthanoid complexes of outstanding stability and reliability.
The ligand scaffold was decorated with a lysine derivative, yielding new amino acid-functionalised lanthanoid cryptates. In a proof-of-concept study we showed how these new building blocks can be covalently connected with the standard methods of SPPS and prepared a first nanocode with the sequence Sm-Tb-Eu. The luminescence spectrum of this nanocode exhibits the characteristic emission bands of all three lanthanoids, just as desired for the application as a molecular nanocode.
Importantly the applicability of this new strategy is not limited to the preparation of luminescent nanocodes. It provides a universally applicable strategy for the preparation of heteropolymetallic lanthanoid coordination compounds of any desired sequence. The amino acid-functionalised complexes should also allow for e.g. the simple introduction of lanthanoid-tags into any polypeptide. In this sense we hope that our findings will facilitate and stimulate the development of new applications of and new fundamental research on the fascinating properties of this special group of elements. You can find our paper here: https://www.nature.com/articles/s41467-020-15199-8.pdf