Figure 1.

Mussel byssus, model for pH-dependent Fe³⁺-catechol chemistry, and molecules studied. (a) Picture of mussel with labeled byssal structures. (b) Model for pH-dependent interactions between Fe³⁺ and catechol. At acidic pH, catechols and Fe³⁺ react to produce o-quinones, leading to catechol-catechol oligomerization. At basic pH, catechols form coordination bonds with Fe³⁺ that prevent oxidation of the catechol. In the context of the mussel byssus, Fe³⁺ could hypothetically induce covalent cross-linking during acidic processing of byssal protein secretions and enhance the mechanical properties of the byssal thread and adhesive by formation of mechanically active Fe³⁺-catechol coordination interactions upon equilibration to basic marine pH. Note that Fe³⁺-catechol coordination bonds would be expected to be mechanically reversible, whereas covalent catechol-catechol bonds would not.²³ (c) Catechol-containing molecules studied: (1) DHPA and (2) Ac-Ser-DOPA-NH₂ (dipeptide).