## 3 Results
### Interaction of CaS with Relevant Biological Functional Groups  
We performed theoretical calculations at the DFT/B3 LYP/6-311G level of theory to learn about favorable interactions of CaS with functional groups relevant to the cell environment. We sought to identify those interactions that could result in the dissociation of CaS. The results of the optimized structures are summarized in Table 2. None of the optimized structures exhibited negative frequencies. The bond length of CaS was found to be 2.58 A. This distance is short enough to consider localized interactions of CaS with relevant functional groups that may be found in macromolecules useful in the interpretation of results. The optimization process was performed by placing CaS at about 2.7 A from a given functional group. Two calculations were performed for each functional group interacting with CaS. These calculations differed only in the initial orientation of CaS with respect to the functional group. In one calculation, CaS was oriented with the Ca end pointing toward the functional group of interest. The sulfur end pointed toward the functional group in the second calculation. We found that the optimized structures of these complexes were independent of the initial orientation of CaS with respect to the functional group. The resulting distances between the closest atom in the functional group and CaS in the optimized complex are also indicated in Table 2. These distances ranged from about 2 to 3 A. All interactions were found to be exergonic. The interaction energies reported in Table 2 are taken as the difference in energy between the optimized complex and the initial CaS and organic molecule. The interaction of CaS was found to interact with the thiol (R-SH), alcoholic (R-OH), and amino (R-NH\({}_{2}\)) functional groups by the calcium end. The  
Figure 7: The squares and circles represent the dependence of normal fibroblast cell density fed with normal media and media mixed with CaS clusters, respectively. The time \(t=0\) h is taken to correspond to the time of the initial dose. The solid line is to guide the eyes through the discussion and does not represent a model fit. The spectrum of the CaS nanocluster dispersion employed is indicated in the insert. The total number of mol of Ca\({}^{2+}\) in the dispersion is \(4\times 10^{-8}\). oxygen atoms in the carboxylic acid functional group also exhibited a strong interaction with the calcium end of CaS. The complete proton transfer from the carboxylic acid group to the sulfur end was found only in phenylalanine. The relevant physiological pH is significantly higher than the pK\({}_{\text{a}}\) of most organic carboxylic acids [35]. Functional groups containing NH\({}_{4}\)\({}^{+}\) and COO\({}^{-}\) were present at pH values between 6 and 7.45, which were typical values found in cancer and normal cells, respectively [36]. The ammonium (R-NH\({}_{3}\)\({}^{+}\)) and carboxylate (R-COO\({}^{-}\)) functional groups were found to interact with the sulfur and the calcium end of CaS, respectively. The interaction between the calcium end of CaS and the carboxylate anion resulted in lower energy and shorter distances than the interaction of the sulfur end with the ammonium functional group. None of the interactions studied resulted in the dissociation of Ca-S into calcium and sulfide ions, although we found that the aromatic group in phenylalanine interacted with the CaS monomer well enough to facilitate the proton transfer from the carboxylic group to form CaSH\({}^{+}\). This proton transfer was not observed with other carboxylic acids, including acetic acid and glycine. Figure 8: The upper and lower panel summarizes the effect of CaS nanoclusters in the progression of the cell cycle of breast adenocarcinoma and normal fibroblasts, respectively. The vehicle control is 2% DMSO. The total amount of Ca\({}^{2+}\) in the dispersion is 4 \(\times\) 10\({}^{-8}\) moles. The * indicates statistically significant values.