Dynamic properties of artificial protein hydrogels assembled through aggregation of leucine zipper peptide domains

Network relaxation dynamics of hydrogels formed from a genetically engineered multidomain protein (AC₁₀A, where A is an associative leucine zipper domain and C₁₀ 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 (τ_r) of these gels varies strongly with pH, increasing from τ_r ≈ 80 s at pH 8.0 to τ_r ≈ 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 (τ_e,) of the A domain, As pH decreases from 8.0 to 7.0, τ_e increases from ca. 200 s to ca. 4500 s. Thus, τ_r of AC₁₀A hydrogels and τ_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.