Influence of Head-to-Tail Cyclization of Lanmodulin-Inspired Peptides on Lanthanide Affinity and Structure

Over the recent years, lanthanide (Ln)-binding proteins such as lanmodulin, a naturally Ln-binding EF-hand protein from Methylorubrum extorquens AM1, as well as protein-inspired short Ln-binding peptides, have become of increasing interest for the development of separation and recycling methods for rare earth elements. Here, we comprehensively examined a set of head-to-tail cyclized lanmodulin-EF-hand sequence-inspired peptides and their complexation behavior toward Lns using time-resolved laser-induced fluorescence spectroscopy (TRLFS), isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR), and circular dichroism (CD) spectroscopies, complemented by molecular dynamic (MD) simulations and advanced mass spectrometry methods. Results obtained from this multi-method approach evidence that the head-to-tail cyclization can indeed increase the Ln affinity in some cases and positively impact the pre-organization of the metal-free peptide while predominantly forming 1:1 complexes in comparison to linear analogues, which can form both 1:1 and 1:2 species. Furthermore, the impact of a sequence alteration by inserting glycine at the head-to-tail junction is shown to be useful in NMR experiments for probing alterations in the local environment of neighboring amino acids (AA) potentially involved in coordination. Lastly, K_D values for the peptides with the whole Ln series (except Pm) and the actinide curium are presented.