Tyrosine–silver interactions: Comparing [Tyr,H]+ and [Tyr,Ag]+ using mass spectrometry, UVPD, and IRMPD action spectroscopy

Metal complexes of peptides and proteins are central to biological systems and to the synthesis of functional subnanometer biomaterial templates, such as silver nanoclusters (NCs). Although tyrosine (Tyr) is widely recognised as an effective reducing and stabilising agent in silver nanoparticle production, its fundamental interaction with an isolated silver cation remains poorly understood. Here, we present a joint experimental and theoretical investigation of the structural and optical properties of the mass-selected [Tyr,Ag]+ complex, using [Tyr,H]+ as a structural benchmark. We introduce a new experimental platform that, for the first time, combines a benchtop continuouswave (cw) mid-infrared (MIR) laser and a tunable ultraviolet (UV) optical parametric oscillator (OPO) in a single iontrap setup. Action spectra were recorded in both the mid-IR (1134–1938 cm−1) and UV (234–390 nm) spectral regions. Because [Tyr,Ag]+ exhibits a low propensity for fragmentation, mid-IR photodissociation was achieved by attaching a weakly bound H2O tag. The experimental infrared multiphoton dissociation (IRMPD) and ultraviolet photodissociation (UVPD) spectra were combined with quantum-chemical simulations to identify low-energy conformers and assign their vibrational and electronic transitions. This complementary approach fingerprints the structural and electronic motifs associated with Ag+ binding. The UVPD spectra provide clear evidence of chargetransfer excitations between the aromatic ring of tyrosine and the silver cation. These results provide molecularlevel insight into the early stages of silver–biomolecule coordination.