*Article* **Intermediate Detection in the Casiopeina–Cysteine Interaction Ending in the Disulfide Bond Formation and Copper Reduction**

**Lillian G. Ramírez-Palma 1,2,†, Adrián Espinoza-Guillén 3,† , Fabiola Nieto-Camacho <sup>3</sup> , Alexis E. López-Guerra <sup>3</sup> , Virginia Gómez-Vidales <sup>1</sup> , Fernando Cortés-Guzmán 1,2,\* and Lena Ruiz-Azuara 3,\***


**Abstract:** A strategy to improve the cancer therapies involves agents that cause the depletion of the endogenous antioxidant glutathione (GSH), increasing its efflux out of cells and inducing apoptosis in tumoral cells due to the presence of reactive oxygen species. It has been shown that Casiopeina copper complexes caused a dramatic intracellular GSH drop, forming disulfide bonds and reducing CuII to Cu<sup>I</sup> . Herein, through the determination of the [CuII]–SH bond before reduction, we present evidence of the adduct between cysteine and one Casiopeina as an intermediate in the cystine formation and as a model to understand the anticancer activity of copper complexes. Evidence of such an intermediate has never been presented before.

**Keywords:** copper complexes; glutathione depletion; cysteine; Casiopeina; intermediate; copper reduction
