Singlet oxygen generation in PUVA therapy studied using electronic structure calculations Juan José Serrano-Pérez, Gloria Olaso-González, Manuela Merchán, Luis Serrano-Andrés * Instituto de Ciencia Molecular, Universitat de València, Apartado 22085, ES-46071 Valencia, Spain article info Article history: Received 23 January 2009 Accepted 22 April 2009 Available online 3 May 2009 Keywords: Phototherapy Furocoumarins Energy transfer Singlet oxygen generation PDT abstract The ability of furocoumarins to participate in the PUVA (Psoralen + UV-A) therapy against skin disorders and some types of cancer, is analyzed on quantum chemical grounds. The efficiency of the process relies on its capability to populate its lowest triplet excited state, and then either form adducts with thymine which interfere DNA replication or transfer its energy, generating singlet molecular oxygen damaging the cell membrane in photoactivated tissues. By determining the spin–orbit couplings, shown to be the key property, in the intersystem crossing yielding the triplet state of the furocoumarin, the electronic cou- plings in the triplet–triplet energy transfer process producing the singlet oxygen, and the reaction rates and lifetimes, the efficiency in the phototherapeutic action of the furocoumarin family is predicted as: khellin < 5-methoxypsoralen (5-MOP) < 8-methoxypsoralen (8-MOP) < psoralen < 4,5 0 ,8-trimethylpsora- len (TMP) < 3-carbethoxypsoralen (3-CPS), the latter being the most efficient photosensitizer and singlet oxygen generator. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Photomedicine applies the principles of photobiology, photo- chemistry, and photophysics to the diagnosis and therapy of dis- eases. One of the most active research areas in this field is photodynamic therapy (PDT), in which the affected living tissue is treated with a combination of a photosensitizer, activated by UV light, and molecular oxygen [1–3]. Utility of light in the medical field as a therapeutic tool traces its origin back to several thou- sands years, but the development of Phototherapy did not reach its heyday until the 20th century [4–6]. In particular for PDT, it was in 1976 that Weishaupt et al. [7] made a breakthrough postu- lating that singlet oxygen is the cytotoxic agent responsible for the photoinactivation of tumor cells. In PDT, unlike in radiation ther- apy, DNA is not the major target because photosensitizers tend to localize at the cell membranes. Future advances in the applica- tion of PDT rely on understanding the nature of the interaction of light, photosensitizer, and tissue [8–11]. Among the several groups of molecules with photosensitizing ability for PDT [4], we will focus the present research on fur- ocoumarins (psoralens), a family of heterocyclic compounds pro- tagonists of the PUVA (Psoralen + UV-A) therapy [12–15]. This technique, which is nowadays providing successful treatments against many diseases such as skin disorders (psoriasis, vitiligo) as well as some forms of cancer, is known to have a two-fold type of therapeutic mechanism. Furocoumarins are known to exert their photosensitizing ability by reacting with thymine and forming ad- ducts, interfering DNA replication in the affected cell [16]. Addi- tionally, these compounds also interact with molecular oxygen in its triplet ground state ( 3 O 2 ) to produce singlet excited state ( 1 O 2 ) oxygen, a extremely reactive and cytotoxic species [17–23], together with the simultaneous generation of other radicals such us O Å 2 or HO Å 2 [17]. These systems induce photooxidation of lipids and are considered responsible for cell membrane damaging ef- fects, causing also the appearance of erythema and pigmentation activity in the human skin [18]. The relative contribution of one (adduct formation) or other (singlet oxygen generation) process to the effectiveness of the PUVA therapy is still unclear. It has been observed that molecular oxygen quenches the photochemical reac- tion between psoralens and thymine [20,24], whereas there are evidences indicating that the binding of 8-methoxypsoralen (8- MOP) to double stranded poly-(dA-dT) inhibits the furocoumarin ability to sensitize via singlet oxygen generation [20]. In some cases, however, furocoumarins are described to produce 1 O 2 even when complexed with or covalently bound to DNA [25]. In princi- ple both mechanisms can be expected to be competitive, not syn- ergic, although higher values of singlet oxygen production have been reported for complexed furocoumarins than for the free com- pounds for psoralen, 8-MOP, and 5-methoxypsoralen (5-MOP) [26,27]. The yield of formation and activity of singlet oxygen from the different furocoumarins has been estimated by several research groups, but no agreement has been reached due to the problems 0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2009.04.014 * Corresponding author. Tel.: +34 963544427; fax: +34 963543274. E-mail address: Luis.Serrano@uv.es (L. Serrano-Andrés). Chemical Physics 360 (2009) 85–96 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys