Because bacterial growth and infection rely on adequate iron supply, the ability to scavenge iron in the iron-limited environment of the host is a virulence trait for pathogenic microorganisms. 2 To mediate their mandatory iron acquisition, the zoonotic causative agent of anthrax, Bacillus anthracis, and the human opportunistic pathogen Bacillus cereus both synthesize, by independent pathways, two low-molecular-weight ferric ion chelators, the hexadentate siderophores bacillibactin (BB) and petrobactin (PB) (Figure 1). 3 While BB is built on a central trilactone scaffold, linked to the common 2,3-dihydroxybenzoyl iron-chelating subunits, and exhibits a phenomenally high and selective affinity for iron (K f ) 10 48 ), 4 it is specifically bound by the mammalian protein siderocalin, a component of the antibacterial iron-depletion defense of the innate immune system. 5 Conversely, PB, first isolated from the marine bacterium Marinobacter hydrocarbonoclasticus, 6 is an unusual citrate-and 3,4-catecholate-based ligand, 7 the only natural hexadentate 3,4-catecholate siderophore observed in a pathogenic bacterium. Moreover, the full pathogenicity of B. anthracis in mice is observed only when PB is produced. 8 The protein siderocalin cannot accommodate 3,4-catecholate moieties within its binding pocket, precluding sequestration of PB. Hence, incorporation of such iron-binding moieties into a siderophore has been proposed as a bacterial strategy to evade the immune system. 5 However, little is known about the iron coordination chemistry of 3,4-catecholamide fragments. 9 Are the thermodynamic and kinetic properties of the ferric PB complex advantageous as compared to other more common hydroxamate and citrate siderophores?Another notable feature of PB is the photoreactivity of its ferric complex. 6 Photolysis of the complex into the citrate ligand-to-metal charge-transfer band is reported to result in decarboxylation and oxidation of the ligand, forming a 3-ketoglutarate residue at the former site of the citryl moiety ( Figure S1). 6 This process also involves reduction of the metal center; 10 in fact when 0.1 mM solutions of [Fe III (PB)] 3-and [Ga III (PB)] 3-contained in quartz cuvettes are exposed to sunlight, no spectral change is observed for [Ga III (PB)] 3-whereas the photolysis of [Fe III (PB)] 3-is characterized by significant changes ( Figure S2). 11 The incorporation of 3,4-catecholate and citrate units in PB therefore presents a challenge for a full description of the thermodynamic properties of the ligand. Indeed, while the 3,4-dihydroxybenzamide units are susceptible to oxidation and decomposition at high pH, compromising accurate pK a determination by potentiometric methods, 12 the photoreactivity of the ferric-citrate moiety requires rigorous dark conditions for spectrophotometric competition titrations. However, the affinities of PB and its photoproduct (PB ν ) for Fe(III) at physiological pH were measured via competition titration against EDTA. Titrations were performed on PB samples stored in the dark ov...