## 3 Results and Discussion
### Photoconversion Mechanism of DHSAN into SAN  
Here are reported the outcomes of the investigation of the mechanism that allows the formation of SAN starting from its dihydro precursor DHSAN under light irradiation and, then, singlet oxygen \({}^{1}\)O\({}_{2}\) assistance. As first demonstrated by [PERSON] and coworkers using spectroscopic measurements, SAN exists almost exclusively as iminium and alkanolamine forms at physiological pH, being the equilibrium shifted toward alkanolamine formation [56]. Both forms of the alkaloid, the water-soluble cation and the less soluble neutral alkanolamine can be involved in interactions with biological systems. For that reason, the transformation of the iminium ion into the alkanolamine form was also examined. In the previous paragraph, it was demonstrated that the proposed mechanism [7] is reliable. Indeed, irradiation with light of proper wavelength causes the excitation of DHSAN from the ground state into a short-lived electronically excited state (S\({}_{1}\)). The excited singlet state undergoes, via intersystem crossing, spin inversion and populates the lower-energy first excited triplet state, and then from such state, a second spin-forbidden inversion occurs depopulating the excited triplet state T\({}_{1}\) by decaying to the ground state. The longer lifetime of this species is sufficient to allow the DHSAN T\({}_{1}\) state to interact with surrounding molecules including the ground triplet state of molecular oxygen, and being the released amount of energy more than enough, excited singlet oxygen is formed. Singlet oxygen, in turn, can react with DHSAN to produce SAN. The free energy profile describing the reaction between SAN and \({}^{1}\)O\({}_{2}\) is reported in Figure 2. Optimized geometrical structures of the stationary points located along the reaction pathway are reported in Figure S2 of the Supplementary Materials. After the formation of the first adduct between DHSAN and singlet oxygen, lying 5 kcal mol\({}^{-1}\) lower in energy than the separated reactants, the strongly oxidizing nature of \({}^{1}\)O\({}_{2}\) causes the transfer of a H\({}^{-}\) unit from the tertiary amine to molecular oxygen leading to the \begin{table}
\begin{tabular}{c c c c} \hline \hline \(\Delta\)E **(eV)** & \(\lambda\) **(nm)** & \(\langle\delta^{TPA}\rangle\)** **(a.u.)** & \(\sigma^{TPA}\) **(GM)** \\ \hline
3.47 & 715 & 1,490,000 & 6550 \\
3.80 & 653 & 3400 & 18 \\
3.94 & 629 & 2490 & 14 \\
4.19 & 592 & 2400 & 15 \\
4.29 & 578 & 11,400 & 77 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photon energy (\(\Delta\)E) and wavelength (\(\lambda\)), TPA strength \(\langle\delta^{TPA}\rangle\), and cross section \(\delta^{TPA}\) for five states TD-DFT calculated for DHSAN in implicit water solvent for linearly polarized light. formation of the iminium cation and the hydroperoxide anion HOO\({}^{-}\). The height of barrier that is necessary to overcome for this reaction to occur is only 1.1 kcal mol\({}^{-1}\), whereas the products are stabilized by 20.0 kcal mol\({}^{-1}\) with respect to the entrance channel. The reaction, then, can proceed towards the formation of the alkanolamine form of SAN. The favorable interaction with water further stabilizes the adduct formed with the iminium cation by 8.5 kcal mol\({}^{-1}\). The simultaneous abstraction of a proton from water by the HOO\({}^{-}\). species and the nucleophilic attack of the formed hydroxo species to the carbon atom in position 6 of the iminium cation affords the 6-hydroxylhydroderivative (SANOH). The corresponding transition state is higher by 3.4 kcal mol\({}^{-1}\) than the minimum leading to it. The formation of alkanolamine and hydrogen peroxide is calculated to be exergonic by 16.2 kcal mol\({}^{-1}\) with respect to the iminium cation and water.