Potential roles for the involvement of metals in mussel protein cross-linking have also been considered. Both a mussel adhesive precursor protein and hydrolyzed peptides thereof have been shown to chelate iron through DOPA. Copper(II) can aggregate DOPA proteins and decrease viscoelasticity via interprotein chelation . When examined by atomic force microscopy, the adhesive energy of a mussel protein was increased upon addition of Fe3+. The affinity of iron for DOPA and catechol (Figure 2) is established . Oxidation of catechol and reduction of Fe3+ occur at similar potentials (0.75 V). Thus Fe interactions with DOPA proteins could be via chelation, redox chemistry, or a combination of both. Total DOPA content of solid plaques is difficult to quantify but we can make a rough estimate of DOPA being present at levels approximately equimolar to that of iron.