Alex Zozula, 2010, Chemistry
The hydroxylation of unactivated hydrocarbons is one of the most difficult chemical reactions known. Yet despite its complexity, many organisms are able to use hydrocarbons as their sole energy source, and an wide array of enzymes have evolved to facilitate these reactions. Naphthalene 1,2-dioxygenase (NDO), a Rieske protein homologue that catalyzes the stereoselective oxidation of naphthalene to (1R,2S)-1,2-dihydronaphthalene-1,2-diol is one such enzyme. The cis-1,2-dihydroxylation of olefins and arenes carried out by NDO is of immense synthetic interest; NDO has the ability to catalyze the creation of two chiral centers in a reaction that proceeds almost to completion.
Despite all the work on NDO in recent years, the mechanisms for itsdi- and mono-oxygenations have not been elucidated. In vitro work had suggested that mono-oxygenation proceeds through a radical pathway, indicative of the formation of a high-valent Fe(V)-oxo-hydroxo species before substrate attack. I have demonstrated that the same mechanism is at work in vivo using an even wider range of diagnostic substrates than used in vitro. As further evidence of the formation of a Fe(V)- oxo-hydroxo species, I performed studies using 18O labeled water and showed that there is non-trivial exchange into the iron center based on product analysis; oxo-hydroxo tautomerization, expected for a high- valent Fe(V) species, would result in this solvent exchange. I have also synthesized a range of selectively deuterated materials to serve as intramolecular probes of NDO’s kinetic isotope effect. This represents one of the first instances of a KIE being determined solely by product analysis for an in vivo system.