The transition-metal complex (Pt(trpy)Cl) [superscript]+ selectively labels the amino acids Cys and His, but is unreactive toward Met. The (Pt(trpy)Cl) [superscript]+ complex also forms complexes with nonbiological ligands to form novel compounds with metal-metal or [pi]-[pi] interactions;The homologous gold reagent, (Au(trpy)Cl) [superscript]2+ reacts with Trp, Cys, His, and Met. The oxidative properties of the gold complex makes it an invasive reagent toward proteins, however;In reactions with proteins, the complex (Pt(trpy)Cl) [superscript]+ exhibits unexpected selectivity toward amino acid side chains in cytochromes c. His residues are labeled in greater yield than the Cys residues. The Pr(trpy)[superscript]2+ tags are stable and are easily detected and quantitated. The new reagent does not alter the structural and redox properties of the cytochromes c;Subsequent research involves modification of the active sites of serine and sulfhydryl proteases with (Pt(trpy)Cl) [superscript]+. Although the tagging modifies the catalytic triad and disrupts the charge relay, the platinated enzymes retain significant activity for substrates. The (Pt(trpy)Cl) [superscript]+ complex labels both Cys and His residues in papain. Selective removal of these tags with judiciously chosen nucleophiles provides a new approach for site-selective modification of proteins;Cyt c was also modified with the negatively charged complex (Pt(sbpaphy)Cl) [superscript]-. The yield of protein derivatives is low compared with (Pt(trpy)Cl) [superscript]+ derivatives;Electron-transfer between covalently tethered cyt c and pc is investigated. The tethers prohibit surface diffusion of the proteins and therefore prohibit a productive electron-transfer complex from forming. Rates of electron transfer between the heterodimeric proteins and a third, free, monomeric protein provide an empirical approach for understanding how the 2 proteins are cross-linked and why they cannot participate in intracomplex electron transfer;Methylene blue has long been known to be a photoreductant, but its reactions with biomolecules have not been quantitated. Rates of reaction of methylene blue and derivatives of methylene blue with cyt c are measured.