## 1 Introduction  
Sanguinarine (SAN) belongs to the quaternary benzo[c]phenanthridine alkaloids family having methylene dioxy substituents at the 2, 3 and 7, 8 positions of the chromophore as shown in Scheme 1. Sanguinarine is, very likely, the most important member of such family possessing remarkable biological significance. Indeed, this alkaloid has been shown to possess antimicrobial, antioxidant, and antitumor properties and it is widely used for the prevention and treatment of many inflammatory conditions [1, 2, 3, 4, 5, 6]. Figure 1: Molecular structure of sanguinarine (SAN) and its dihydro precursor dihydrosanguinarine (DHSAN). The interest in SAN anticancer properties has recently been renewed [7] by the proposal to use this alkaloid in the framework of the strategy named photoactivated chemotherapy (PACT) [8]. SAN, indeed, as well as all the chemotherapeutics, is plagued by lack of selectivity and off-target toxicity [9; 10]. Dihydrosanguinarine (DHSAN), instead, being a SAN dihydro precursor (see Scheme 1), is not cytotoxic [11], and can be used to form the SAN active species by irradiation with light of proper wavelength in presence of singlet molecular oxygen \({}^{1}\)O\({}_{2}\) to undergo photooxidative dehydrogenation in situ allowing selective killing of cancer cells. Nevertheless, in spite of the displayed multiple anticancer effects, the underlying mechanisms by which SAN exerts its antitumor activity have not been yet fully elucidated. As demonstrated, first, by the biophysical studies of [PERSON] et al. [12], in comparison with the classical DNA ethidium bromide intercalator, one of the most important properties of SAN is its strong DNA binding ability that, surely, is relevant for its therapeutic anticancer action [13]. Intercalation is the way of binding and several studies of the base sequence specificity for the sanguinarine DNA binding [14; 15; 16] have evidenced the strong preference for alternating purine-pyrimidine sequences. In particular, it has been revealed that sanguinarine is a guanine-cytosine (GC) specific DNA intercalator. The alternating GC sequence binding affinity is followed by that for alternating adenine-thymine (AT) sequences. SAN, together with other alkaloids, has been also examined for its ability to produce singlet oxygen under light irradiation [17; 18; 19; 20]. The exploration of the capability of such compounds to generate singlet oxygen is strictly connected to their use as photosensizers in Photodynamic Therapy (PDT) [21; 22]. PDT is a well-known less aggressive treatment used for combatting cancer, whose indispensable components are a photosensitizer, molecular oxygen, and irradiation with light of proper wavelength. Under light irradiation of the photosensitizer, reactive oxygen species (ROS) are produced, including excited singlet oxygen, as cytotoxic agents able to selectively destroy diseased cells. However, the studies of the photosensitizing properties of SAN are scarce and not up-to-date [21; 22]. Moreover, on the capability of DHSAN to work as a photosensitizer generating singlet oxygen \({}^{1}\)O\({}_{2}\) relies the possibility that its photooxidative dehydrogenation occurs for the controlled generation of the active species SAN. On the basis of such premises, in the present paper we have focused our attention on the investigation of the photophysical properties of SAN to evaluate whether it can act as a photosensitizer in PDT, the mode of interaction of SAN with DNA occurring through intercalation and how the spectroscopic properties are influenced by DNA intercalation. Photophysical properties of DHSAN have also been examined to prove that it is phoactivatable and, then, able to generate singlet oxygen, which causes the formation of SAN from its dihydro precursor DHSAN. To this aim, DFT and TDDFT were used for examining the spectroscopic properties of both DHSAN and SAN alone and in its DNA intercalated forms. The photosensitizer nature of SAN for its use in PDT and binding effects on sensitization activity were compared.