An Iron Grip

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Science  17 Oct 2003:
Vol. 302, Issue 5644, pp. 359-361
DOI: 10.1126/science.302.5644.359e

The utility of enzymes such as horseradish peroxidase (HRP), which can be used to activate hydrogen peroxide for subsequent organic reactions such as bleaching, is often limited by the loss of activity at high or low pH. Recently, small molecules that exhibit the activity of these metalloporphyrin-containing enzymes have been developed. They carry a porphyrin mimic, a tetraamido macrocyclic ligand (TAML) for binding FeIII, but do not need the surrounding protein shell for activity. Unfortunately, these catalysts are perhaps too biomimetic, in that they also lose activity at pH < 4.

Ghosh et al. explored ways of stabilizing these TAML catalysts at low pH. They found that by placing electron-withdrawing substituents, such as fluorine, at the carbon that bridges the two carbonyl groups, they could increase the resistance against acid-induced hydrolysis, which demetalates the compound, by up to 11 orders of magnitude at low pH, presumably by inhibiting the protonation of the amide oxygens. These results not only expand the range of these catalysts, but may also provide a model system for studying demetalation of enzymes. — PDS

J. Am. Chem. Soc. 10.1021/ja0367344 (2003).

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