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The cross-linking behavior of mussel adhesive protein Mefp-1 was studied by measuring the rate of aggregation of the protein by photon correlation spectroscopy. To be able to calculate the aggregation numbers, the hydrodynamic radius of monomer Mefp-1 (10 nm) was determined under reducing conditions. The aggregation is controlled by the redox potential of the solution, and the aggregation number varied, independent of pH, over a factor 2 within the experimentally accessible redox potential window. A kinetic model for cross-linking, based on the intricate interplay of the oxidation and auto-oxidation of the hydroquinones of Mefp-1, is proposed. The oxidation rate strongly depends on redox potential. The cross-linking rate is taken to be proportional to the rate of auto-oxidation. The model correctly predicts the experimentally observed phenomena. When the oxidation rate is slower than the auto-oxidation rate, cross-linking is efficient and controlled by the oxidation rate. When the rate of auto-oxidation rate is slower than the oxidation rate, the cross-linking is inefficient due to the quick exhaustion of the hydroquinones. The experimentally determined rate constant for cross-linking is found to be much smaller than those found for auto-oxidation of hydroquinones because of the excluded volume interactions imposed by the protein backbone. Tuning the interplay between oxidation and auto-oxidation presents the potential of controlling cross-linking density independent of the density of reactive groups.

The coagulation and the repeptization of electrocratic colloids can be treated in one theory provided that the appropriate bounda.ry conditions are chosen. From this version of the DLVO theory it follows that for each sol there exists a critical value Z® c of the double layer parameter Z~ , Z~ = ze~,~/kT. A sol is stable, and floes can repeptize if Z~ > Z® e. For Z~< Z~ ° the traditional DLVO theory is obtained. Some examples are given to illustrate this concept. In the appendix the interaction of double layers with a constant charge is related to that at a constant potential.