Selective Labeling, Cross-Linking, and Cleavage of Proteins with Transition-Metal Complexes

Abstract

Because the substitution reactions of transition-metal complexes can be controlled precisely by the choice of the metal and the ligands, and because these complexes have various spectroscopic and electrochemical properties, they are well-suited for many applications in biochemical and biophysical laboratories. Complex [Pt(trpy)Cl)⁺ reacts, under mild conditions and noninvasively, with the exposed side chains of His, Cys, and even Arg residues in proteins. The derivatives can be separated chromatographically. The Pt(trpy)²⁺ tags are stable. They are easily detected and quantitated owing to the strong UV-vis bands, whose positions and relative intensities depend on the identity and the environment of the tagged residue. The selectivity of Pt(II) chloro complexes as labeling reagents can be controlled by purposeful changes of the other three ancillary ligands. Treatment of Cys residues with (η⁵-C₅H₅)Fe(η⁵-C₅H₄HgCl)results in attachment of the redox-active ferrocenyl tag to the protein. Proteins can be cross-linked covalently through Met residues with the monometallic trans-PtL₂Cl₂ reagents, and through His residues with the bimetallic Rh₂(RCOO)₄ reagents. Cross-linking with the trimetallic reagent [Ru₃(μ₃-O)(μ₂-OAc)₆(H₂O)₃] may yield covalent triprotein clusters. A PtCl₃- group attached to the Cys side chain in glutathione promotes selective hydrolysis of the Cys–Gly peptide bond. These findings point the way for the study of protein topography, of enzyme mechanism, of electron-transfer reactions in modified metalloproteins, of metal binding to Arg residues, and of protein cleavage by inorganic reagents.