Metal sites in proteins often cannot be studied nonperturbatively in all redox states because of the reliance of many methods on bulky, non-site-specific probes or unpaired valence electrons. Infrared (IR) spectroscopy of vibrational probes inserted site-specifically as ligands to the metal center circumvent these challenges. Deuterated amino acids provide a nonperturbative vibrational oscillator which absorbs in a region of the spectrum free from intrinsic vibrations of the protein. These probes have been used to investigate changes in the bonding of the plastocyanin (Pc) copper center to its ligands upon binding to its redox partner, cytochrome <i>f</i>. The impact of complexation on the observed frequency of the carbon-deuterium bonds is greater than that observed when directly varying the redox state of the metal ion, indicating that redox partner binding has a dramatic impact on the bonding strength of copper with its cysteine ligand. This in turn facilitates electron transfer from cytochrome <i>f</i>, linking binding to the role of Pc as a shuttle in the photosynthetic electron transport chain. IR spectroscopy of deuterated amino acid probes in this system has also been used to investigate changes to the metal center brought on by mutation of the secondary coordination sphere. These studies demonstrate the unique utility of carbon-deuterium bonds as site-specific probes of metal site coordination in proteins.

Deuterated amino acids as sensitive vibrational probes of metal site coordination in plastocyanin

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