Photodecarboxylation of the Siderophore Aerobactin with the Lewis Acidic Metal Ions Fe(III), Ga(III), and Ti(IV)

TL;DR

This paper explores how different metal ions affect the light-driven breakdown of a natural compound called aerobactin, revealing new insights into its chemical reactions.

Contribution

The study demonstrates that photodecarboxylation of aerobactin occurs with Ga(III) and Ti(IV) ions, extending beyond previously known Fe(III) reactivity.

Findings

Photoirradiation of [Ga(AB)]3– and [Ti(AB)]2– causes decarboxylation at two distinct sites.

TD-DFT calculations show hydroxamate groups, not α-hydroxy carboxylates, drive LMCT excitation and radical formation.

Shorter wavelength irradiation of [Ga(AB)]3– and [Ti(AB)]2– induces secondary decarboxylation of lysine carboxylate.

Abstract

The class of α-hydroxy carboxylic acid-containing
ferric
siderophore natural products undergo photochemical modification by
decarboxylation. To date, there is only limited mechanistic understanding
of the metal-ion-mediated photodegradation of photoactive siderophores.
This study investigates the photoreactivity of the α-hydroxy
carboxylic acid-containing siderophore aerobactin (AB) and the corresponding
Ga3+ and Ti4+ metal complexes in direct comparison
with their Fe3+-bound counterpart. Using UV–vis
and nuclear magnetic resonance (NMR) spectroscopy, complemented by
time-dependent density functional theory (TD-DFT) calculations, we
demonstrate that ligand-to-metal charge transfer (LMCT)-driven photocleavage
of aerobactin–metal complexes is feasible beyond Fe3
+. We show that photoirradiation at shorter wavelengths
of [Ga­(AB)]3– and [Ti­(AB)]2– results
in decarboxylative photocleavage at two distinct sites. While [Fe­(AB)]3– exhibits distinct reactivity upon photoexcitation
from 254 to 575 nm, producing C–C bond cleavage to release
CO2 and form the corresponding tautomer, the analogous
[Ti­(AB)]2– complex can be selectively exited. Lower
energy excitation of [Ti­(AB)]2– within the LMCT
band centered at 295 nm induces decarboxylation in direct homology
with the parent ferric complex, whereas secondary decarboxylation
of the lysine carboxylate is observed using short wavelength irradiation
of [Ga­(AB)]3– and [Ti­(AB)]2–.
These experimental results, supported by TD-DFT findings, reveal that
the coordinating hydroxamate groups, rather than the α-hydroxy
carboxylate, are the source of efficient LMCT excitation and radical
formation, challenging previous assumptions about aerobactin’s
photochemical decarboxylation mechanism. We provide a mechanistic
framework for siderophore-mediated photochemistry and highlight its
applicability to xenometal ions.

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