Reevaluating Gold(I) Thiophilicity in Reduced Dielectric Environments: IRMPD Spectroscopy and Theoretical Investigation of Auranofin-Glutathione Complexes.

The coordination of gold(I) to glutathione (GSH) is a fundamental process in the medicinal chemistry of auranofin and related therapeutics. Utilizing infrared multiple-photon dissociation (IRMPD) spectroscopy, electron-activated dissociation (EAD), and quantum-chemical calculations, we characterize the gas-phase structure of the complex [(Et3P)AuGSH]+. Our results reveal that in the gas phase, the NH2 group of the N-terminal glutamate residue of GSH serves as a competitive coordination site, challenging coordination by the canonical cysteine thiol, with both isomers coexisting in comparable abundances. This finding suggests that in microenvironments characterized by a reduced dielectric constant, the GSH-Au(I) complex might also exist as an N-bound isomer with a free thiol, which may significantly affect its biological reactivity and functions. Moreover, considering that the dielectric constant within buried protein active sites is substantially lower than that of bulk water, this N-binding motif identifies the unprotonated N-terminus as a plausible target for metalation, even in the presence of free thiols. These insights offer a broader perspective on the competitive binding landscape of gold(I) drugs. As a future direction, we hypothesize potential implications for the inhibition of enzymes with N-terminal catalytic groups, which are crucial for tumor progression, such as nucleophile (Ntn) hydrolases.