Controlling the stable structures of metallic nanoparticles on mesoscopic and macroscopic length scales is of great interest in nanotechnology. Here, this task is accomplished using a synthetic biopolymer that is responsive to external stimuli and undergoes changes in secondary structure. Reversible aggregation of gold nanoparticles (GNP) is induced by pH dependent changes in a self-assembled monolayer of disulfide modified poly(L-glutamic acid) (SSPLGA) with M(w) approximately 27000. The disulfide anchoring group drives chemisorption onto the gold nanoparticles and leads to the formation of a self-assembled monolayer. Characterization of the modified GNP and its aggregation behavior is performed using dynamic light scattering (DLS), UV-vis and IR spectroscopy, and transmission electron microscopy (TEM). Experimental results show that decrease in pH near 5.5 leads to aggregation of the modified GNP. The change in aggregation behavior with pH occurs within minutes, is reversible, and happens within a narrow range of pH from about 4.5 to 5.5. Comparison with literature data on molar enthalpy of hydrogen bonding, specific optical rotation, and ionization for the helix-coil transition of PLGA indicates that the aggregation of the SSPLGA modified GNP corresponds to the transition in the secondary structure of the polyacid.