Persulfides (RSSH/RSS-) participate in sulfur trafficking and metabolic processes, and are proposed to mediate the signaling effects of H2S. Despite their growing relevance, their chemical properties are poorly understood. Herein, we studied experimentally and computationally the formation, acidity, and nucleophilicity of glutathione persulfide (GSSH/GSS-), the derivative of the abundant cellular thiol, glutathione (GSH). We characterized the kinetics and equilibrium of GSSH formation from glutathione disulfide and H2S. A pKa of 5.45 for GSSH was determined, which is 3.49 units below that of GSH. The reactions of GSSH with the physiologically-relevant electrophiles peroxynitrite and hydrogen peroxide, as well as with the probe monobromobimane, were studied and compared with those of thiols. These reactions occurred through SN2 mechanisms. At neutral pH, GSSH reacted faster than GSH due to increased availability of the anion and, depending on the electrophile, increased reactivity. In addition, GSS- presented higher nucleophilicity with respect to a thiolate with similar basicity. This can be interpreted in terms of the so-called alpha effect, i.e. the increased reactivity of a nucleophile when the atom adjacent to the nucleophilic atom has high electron density. The magnitude of the alpha effect correlated with the Brønsted nucleophilic factor, βnuc, for the reactions with thiolates and with the ability of the leaving group. Our study constitutes the first determination of the pKa of a biological persulfide and the first examination of the alpha effect in sulfur nucleophiles, and sheds light on the chemical basis of the biological properties of persulfides.

In this work we present the current advances in the development and the applications of LIO, a lab-made code designed for density functional theory calculations in graphical processing units (GPU), that can be coupled with different classical molecular dynamics engines. This code has been thoroughly optimized to perform efficient molecular dynamics simulations at the QM/MM DFT level, allowing for an exhaustive sampling of the configurational space. Selected examples are presented for the description of chemical reactivity in terms of free energy profiles, and also for the computation of optical properties, such as vibrational and electronic spectra in solvent and protein environments.