Structural and Spectroscopic Characterization of a Histidine-Containing Tetrapeptide Crystallized with Copper Chloride.

Metal coordination has emerged as a promising strategy to modulate peptide self-assembly and enhance crystal stability. In this study, we investigated the crystallization behavior of a histidine-containing tetrapeptide in the presence and absence of copper ions to elucidate the role of metal coordination in peptide assembly. Crystals were obtained under both conditions and characterized using single-crystal X-ray diffraction (sc-XRD) and angle-resolved polarized Raman spectroscopy. The copper-free crystals exhibited a rod-like morphology stabilized predominantly by hydrogen bonding, with molecular alignment along the a-axis. In contrast, the copper-containing crystals adopted a needle-like form with a distinct molecular orientation. Raman spectroscopy revealed coordination between histidine side chains and copper ions, suggesting that metal binding modulates the intermolecular packing and alters the crystal architecture. These findings demonstrate how metal-peptide interactions influence peptide crystallization and highlight the utility of vibrational spectroscopy in probing such interactions at the molecular level.