Direct Vibrational Structure of Protein Metal-binding Sites from Near-infrared Yb3+ Vibronic Side Band Spectroscopy

Near-infrared Yb3+ vibronic side band (VSB) spectroscopy is used to obtain structural information of metal binding sites in metalloproteins. This technique provides a selective "IR-like" vibrational spectrum of those ligands chelated to the Yb3+ ion. VSB spectra of various model complexes of Yb3+ representing different ligand types were studied to provide references for the VSB spectra of Yb(3+)-reconstituted metalloproteins. Ca2+ in the calcium-binding protein parvalbumin and Fe3+ in the iron-transporting protein transferrin were replaced with Yb3+. The fluorescence of Yb3+ reconstituted into these two proteins exhibits weak VSBs whose energy shifts, with respect to the main 2F5/2-->2F7/2 Yb3+ electronic transition, represent the vibrational frequencies of the Yb3+ ligands. The chemical nature of the ligands of the Yb3+ in these proteins, as deduced by the observed VSB frequencies, is entirely in agreement with their known crystal structures. For transferrin, replacement of the 12CO3(2-) metal counterion with 13CO3(2-) yielded the expected isotopic shift for the VSBs corresponding to the carbonate vibrational modes. This technique demonstrates enormous potential in elucidating the localized structure of metal binding sites in proteins.