Monitoring intracellular metal ion complexation with an acetylene-tagged ligand by Raman spectroscopy

Abstract

We propose to monitor molecular vibrations to identify metal ion–ligand complexation by means of Raman spectroscopy, which has been applied to track vibrational modes of molecules and to obtain a structural fingerprint. We prepared ligand molecules for Zn²⁺ ion complexation with a dipycolylaminoethyl aniline (DPEA) skeleton and phenylacetylene unit as the Raman tag which showed a typical band around 2200 cm⁻¹. Among the labeled ligands synthesized in this study, A-DPEA showed a strong band attributed to the acetylene unit at 2212 cm⁻¹, while the addition of Zn²⁺ ion resulted in a band shift to 2220 cm⁻¹ due to complex formation. The addition of other metal ions and titration experiments showed that A-DPEA bound with Zn²⁺ selectively with a dissociation constant (K_d) that was estimated to be 0.22 μM. We also conducted cellular experiments and found that complexation between A-DPEA and Zn²⁺ also occurred in cells, with a shift in the Raman signal of the ligand from 2212 to 2215 cm⁻¹. Thus, complex formation of the metal ion was identified by monitoring the Raman band shift.