Protein Protected Silver Nanoclusters (PP-AgNCs) stand out by their attractive optical properties as well as their facile, low cost, and environmentally friendly synthesis methods. This work focuses on understanding tyrosine amino acid's role in the structural and optical properties of one type of PP-AgNCS: egg albumin templated silver nanoclusters. For this purpose, we performed DFT calculations to elucidate the Global Minima geometries of Agn clusters and Agn-Tyr (n = 3 - 12) complexes. We analyzed the structural stability of the Ag clusters through the Vertical Potential Energy (VIP) and Vertical Electronic Affinity (VEA) descriptors. For complexes, besides VIP and VEA, we considered the Adsorption Energy parameter, and Non-Covalent Interaction (NCI) calculations were useful to analyze the interaction between the Ag cluster and tyrosine molecule in each one. For plotting UV-Vis spectra, we make excited-state calculations for clusters and complexes using TDDFT. The resultant lowest-lying energy structures of the bare Agn (n = 3 - 12) clusters show that the adopted geometry is planar for n = 3 - 6, whereas for n = 7 - 12, acquire three-dimensional shapes. Resultant Agn-Tyr complexes geometries show that Agn (n = 3 - 12) clusters coordinate with tyrosine by interacting with the aromatic ring, and even in some cases, tyrosine bends to maximize the interaction with the silver cluster, what is confirmed by NCI analysis. The structural stability analysis from VIP and VEA values yields that clusters with an even number of Ag atoms are more stable than their odd counterparts. Also, we observed that the coordination of Agn with tyrosine molecule reduces complexes' stability and makes them more prone to be ionized in comparison to naked clusters. Calculated silver naked clusters UV-Vis spectra are in good agreement with measures done on synthesized in an argon matrix at 28 K. On the other hand, UV-Vis complexes spectra show that coordination with tyrosine induces a redshift in the highest intensity signal for naked clusters for most of the cases.

Geometry elucidation and UV-Vis absorption spectra for Agn clusters and Agn-Tyr complexes (n = 3 - 12)

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