Photochemical reactivity of the iron(III) complex of a mixed-donor, α-hydroxy acid-containing chelate and its biological relevance to photoactive marine siderophores.

The trimeric clusters Fe(III)3(X-Sal-AHA)3(μ3-OCH3), where X-Sal-AHA is a tetradentate chelate incorporating an α-hydroxy acid moiety (AHA) and a salicylidene moiety (X-Sal with X being 5-NO2, 3,5-diCl, all-H, 3-OCH3, or 3,5-di-t-Bu substituents on the phenolate ring), undergo a photochemical reaction resulting in reduction of two Fe(III) to Fe(II) for each AHA group that is oxidatively cleaved. However, photolysis of structurally analogous mixed Fe/Ga clusters demonstrate that a similar photolysis reaction will occur with only a single Fe(III) in the cluster. Quantum yields of iron reduction for the series of Fe(III)3(X-Sal-AHA)3(μ3-OCH3) complexes measured by monitoring Fe(II) production are twice those for ligand oxidation, measured by loss of the CD signal for the complex due to cleavage of the chiral AHA group.The quantum yields, 2-13% in the UVA and UVB ranges, are higher for complexes with electron-withdrawing X groups than for those with electron-donating X groups [corrected]. The observed final photolysis product of the chelate is different if irradiation is done in the air than if it is done under Ar. The first observed photochemical product is the aldehyde resulting from decarboxylation of the AHA. This is the final product under anaerobic conditions. In air, this is followed by an Fe- and O2-dependent reaction oxidizing the aldehyde to the corresponding carboxylate, then a second Fe- and light-dependent decarboxylation reaction giving a product that is two carbons smaller than the initial ligand. These reactivity studies have important biological implications for the photoactive marine siderophores. They suggest that different types of photochemical products for different siderophore structure types do not result from different initial photochemical steps, but rather from different susceptibility of the initial photochemical product to air oxidation.