Calcium sulfide (CaS) is a promising candidate to limit the growth and proliferation of cancer cells. It was demonstrated that traces of CaS nanostructures limit the growth of breast cancer cells with no significant effect on the population or life cycle of the corresponding benign cells. Our hypothesis is that the CaS nanostructures are selectively dissociated in the characteristic acidic extracellular environment of cancer cells to Ca2+ and protonated sulfides. To challenge this hypothesis, theoretical calculations at the DFT/B3LYP/DGVZVP level of theory of bare and protonated (CaS)n clusters (n = 1 to 4) were performed to establish cluster stability and reaction pathways in neutral and acidic media. The total energy decreases with CaS cluster size, and the total energy of the protonated cluster decreases with the number of protons up to saturation of the sulfur with a proton. Further addition of protons results in rupture of the cluster into H2S and several protonated CaS clusters. The results are consistent with the formation of H2S from CaS clusters in strongly acidic environment, although partially protonated clusters likely coexist with the H2S.

Reactions of calcium sulfide nanoclusters with free protons: Quantum simulation of the chemical process in the cancer and benign cell environment

ACS Spring 2022

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