Fig. 6.

Examples of plaque chemistry that change with pH. (A) Interfacial catechol bonding to metal oxide surfaces changes from H-bonds at acid pH to bidentate coordination at seawater pH. (B) Metal coordination by catechol (Dopa) increases in valency from none or one (no cross-linking) at acid pH to three at pH ∼8.0 (cross-linking). (C) Covalent cross-links (Yu et al., 2013a,b; Holten-Andersen et al., 2011). These are formed after the catechols are oxidized to quinones in a pH-dependent enzyme-catalyzed reaction (optimum pH 8). The blue, green, orange and purple squiggles denote different protein chains. The reactions shown are not the only pH-sensitive changes taking place.