Temperature-Dependent Structural and Optical Analysis of Transition Metal-Amino Acid Complex Crystals Using XRD, Raman Spectroscopy, and Neutron Diffraction

Metal ions play a crucial role in both biological processes and optoelectronic applications. In biological systems, metal cations often interact with proteins, making the study of metal-protein interactions particularly relevant. Amino acids, due to their zwitterionic nature, with both proton-donating (-COO^-) and proton-accepting (-NH₂) groups, are promising organic materials for the synthesis of semi-organic nonlinear optical (NLO) crystals. When coordinated with transition metals, these crystals exhibit interesting physical properties, including nonlinear optical response, piezoelectricity, and ferroelectricity. Our research focuses on the structural and optical characterization of crystals formed by complexation of transition metals with amino acids, specifically Nickel L-Histidinate, Copper D-Alaninate, and Nickel L-Alaninate. Crystals were synthesized in-house and characterized using variable-temperature X-ray diffraction (XRD) and Raman spectroscopy. XRD measurements were carried out from room temperature down to -190 °C. Raman spectra were obtained at room temperature. For Nickel L-Alaninate, notable changes in diffraction patterns were observed at 0 °C, -80 °C, -170 °C, and -190 °C. For Nickel L-Histidinate, variations were detected at room temperature, -20 °C, -40 °C, -120 °C, and -190 °C. In addition to in-house studies, diffraction experiments were conducted at the SXD beamline at the ISIS Neutron and Muon Source, providing crystallographic information files (CIFs) at selected temperatures. These datasets will serve as a basis for future ab initio harmonic lattice dynamics and molecular dynamics simulations aimed at understanding temperature-induced structural transitions. This combined experimental and computational approach aims to elucidate the relationship between metal-ion coordination, thermal stability, and functional properties in these hybrid crystals. Therefore, we present the characterization of complexed crystals of Nickel L-Histidinate and Copper D-Alaninate, using X-ray  and neutron diffraction and Raman spectroscopy, focusing on their structural and optical properties, with the aim of expanding knowledge about their applications and potentialities.