The active sites of Cyt P450s contain a single heme b with a proximal, deprotonated Cys ligand (a thiolate ligand).1 In the catalytic mechanism of these enzymes, heterolytic cleavage of the O-O bond of the ferric hydroperoxo intermediate generates the catalytically active species, Compound I. One important role of the proximal thiolate is to facilitate rapid O-O bond cleavage via strong donation to the Fe center (the “push” effect, proposed by Dawson). To stabilize the proximal cysteinate ligand, Cyt P450s contain conserved amino acids which form hydrogen bonds with the cysteinate sulfur (the so-called “cys pocket”). These hydrogen bonds play important roles in fine tuning the reactivity of the heme, but also protect the thiolate ligand from undesired side reactions.
To interrogate the effect of the thiolate, we prepared a series of ferric heme-thiolate NO complexes where the donor strength of the thiolate is systematically varied.2,3 Here, [Fe(TPP)(SPh*)] type complexes were prepared with two series of thiophenolate ligands (SPh*-) that either carry electron-withdrawing substituents,2 or thiolates that are stabilized by a variable hydrogen bond.3 These precursors were then reacted with NO gas to obtain the corresponding ferric heme-NO complexes (Figure 1, left). We determined the Fe-NO and N-O stretching frequencies of these compounds and demonstrated experimentally, for the first time, that the thiolate donor strength directly controls the strength of the Fe-NO and N-O bonds in these complexes. Here, a direct correlation of the Fe-NO and N-O bond strengths is observed (Figure 1, right) as a function of the thiolate donor strength.2,3 Using computational studies, we were able to show that this s-trans effect of the thiolate provides an electronic-structural explanation for Dawson’s push effect. Vice versa, this means that NO can serve as a sensitive probe to quantify the thiolate donor strength in new Cyt P450 and related enzymes.