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

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 Zn 2+ 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 Zn 2+ 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 Zn 2+ 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 Zn 2+ 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.