Unraveling the pH-Dependent Coordination Chemistry of Copper with Aspartic Acid: From Molecular Binding to Supramolecular Morphology.

The coordination chemistry of copper(II) with aspartic acid (Asp) is of fundamental interest in both biological systems and materials science, yet the pH-dependent binding preferences of Cu(II) toward the individual functional groups of Asp remain incompletely understood. Here, we systematically investigate the pH-modulated coordination and self-assembly of Cu-Asp complexes, establishing the vibrational fingerprints of the four protonation states of free Asp. IR spectral changes indicate that Cu(II) coordination strongly promotes amino deprotonation but barely affects the carboxyl groups. Chelation proceeds via the α-carboxylate and amino groups at pH 2, with the β-carboxylate participating sequentially at pH 3. Polarization-dependent 2D IR spectroscopy reveals a perpendicular alignment of the two carboxylate transition dipoles within an eight-membered chelate ring, a structural motif confirmed by crystallography. Above neutral pH, weakening of the Cu-amino bond disrupts this rigidity, driving a morphological transition from well-defined fiber-like assemblies to gel-like structures. These results provide a molecular-level framework for understanding pH-responsive changes in both the coordination environment and supramolecular morphology, offering a design principle for stimuli-responsive metal-organic materials.