Novel Crown Ether Amino Acids as Fluorescent Reporters for Metal Ions

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28083326/s1, 1H and 13C NMR spectra of compounds 3a–e.
Unnatural amino acids with enhanced properties, such as increased complexing ability and luminescence, are considered to be highly attractive building blocks for bioinspired frameworks, such as probes for biomolecule dynamics, sensitive fluorescent chemosensors, and peptides for molecular imaging, among others. Therefore, a novel series of highly emissive heterocyclic alanines bearing a benzo[d]oxazolyl unit functionalized with different heterocyclic π-spacers and (aza)crown ether moieties was synthesized. The new compounds were completely characterized using the usual spectroscopic techniques and evaluated as fluorimetric chemosensors in acetonitrile and aqueous mixtures in the presence of various alkaline, alkaline-earth, and transition metal ions. The different crown ether binding moieties as well as the electronic nature of the π-bridge allowed for fine tuning of the sensory properties of these unnatural amino acids towards Pd2+ and Fe3+, as seen by spectrofluorimetric titrations.
This research was funded by Fundação para a Ciência e Tecnologia—FCT (Portugal) through CQUM (UIDB/00686/2020) and a PhD grant to C.D.F. Martins (2020.05277.BD). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network (PTNMR) and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT.