Network relaxation dynamics of hydrogels formed from a genetically engineered multidomain protein (AC(10)A, where A is an associative leucine zipper domain and C-10 is a random-coil polyelectrolyte domain) were investigated by shear rheometry. Physical gels form by tetrameric association of the leucine zipper end-blocks (A). The longest stress relaxation time (tau(r)) of these gels varies strongly with pH, increasing from tau(r) approximate to 80 s at pH 8.0 to tau(r) approximate to 1000 s at pH 7.0. The rate of strand exchange of the end-blocks was studied by using fluorescence quenching of the labeled form of the A domain. Fluorescence is quenched in solutions of fluorescein-labeled A; dequenching occurs when labeled A is mixed with a 60-fold excess of the unlabeled peptide. The dequenching transient after mixing reveals the characteristic strand exchange time (tau(e)) of the A domain. As pH decreases from 8.0 to 7.0, tau(e) increases from ca. 200 s to ca. 4500 s. Thus, tau(r) of AC(10)A hydrogels and tau(e) of the A domain vary in parallel with pH. The strong correlation between macroscopic and molecular properties indicates that network relaxation is regulated by the lifetime of associations in the transient network. Because the rate of leucine zipper strand exchange is sensitive to interstrand electrostatic interactions, the relaxation behavior of artificial protein hydrogels can be engineered systematically by genetic programming of the amino acid sequence.