To elucidate the effects of uniformity of molecular architecture on gel properties, a protein polymer based on the elastin-mimetic repeat sequence [(Val-Pro-Gly-Val-Gly)(4)(Val-Pro-Gly-Lys-Gly)], 1 (Lys-25), was synthesized using genetic engineering and microbial protein expression. The regularly placed lysine residues in poly(Lys-25) underwent selective reaction with electrophilic cross-linkers under mild conditions in either dimethyl sulfoxide or aqueous phosphate buffer to afford solvent-swollen networks. Chemical derivatization and spectroscopic investigations of the cross-linking reaction indicated that approximately 85% of the lysine residues reacted with the cross-linker. The protein hydrogel exhibited reversible, temperature-dependent expansion and contraction with an estimated midpoint temperature for the phase transition at 35 degrees C. Scanning electron microscopy (SEM) investigations indicated profound differences in morphology between protein gels prepared in organic vs aqueous solution.