Photochemistry of b-carbolines in aqueous solution upon one and two photon excitation.

M. MICAELA GONZALEZ; JACOB ARNBJERG; M. PAULA DENOFRIO; ROSA ERRA-BALSELLS; FRANCO M. CABRERIZO; PETER R. OGILBY

Cubato / Santos

Congreso; (IX ELAFOT) IX Encuentro Latino-Americano de Fotoquimica y Fotobiologia; 2008

Sociedad Latinoamericana de Fotoquimica y Fotobiologia

b-carboline alkaloids are a group of heterocyclic compounds derived from 9Hpyrido[3,4-b]indole or norharmane. In aqueous solution, in the pH-range 4-11, these alkaloids show an acid-base equilibrium characterized by a pKa value of 7.2. The other functional group of the b-carboline moiety, the nitrogen of the indolic ring, has a pKa value higher than 12. In this contribution, we present a comparative study of the photochemistry and photophysics of five b-carboline derivatives (norharmane (nHo), harmane (Ho), harmine (Ha), N-methyl-norharmane (N-Me-nHo) and N-methyl-harmane (N-Me-Ho)), upon one- and two-photon excitation. The experiments were performed in aqueous solution, at room temperature; and the influences of pH and atmospheric conditions were analyzed.The quantum yields of b-carbolines disappearance, upon one-photon excitation (UV-A light), were determined. These results show that b-carbolines are quite photostable. Additionally, fluorescence quantum yields (FF) have been measured under different experimental conditions. These results are analyzed and compared with the FF values already published in the literature, and the quenching of the fluorescence emission by O2 is also reported. In addition, we conducted a quantitative study of the capability of hydrogen peroxide (H2O2) and singlet oxygen (1O2) production by these five b-carbolines. Furthermore, the physical quenching of 1O2 by b-carbolines has been studied and the corresponding rate constants of 1O2  total quenching (kt) were calculated; the results obtained show a strong pH-dependence. To the best of our knowledge, we also present the first study of the photophysical behavior of b-carbolines upon two-photon excitation. Using a femtosecond laser, the two-photon excitation spectra of both acidic and basic forms of nHo, Ho and Ha have been recorded, and the two-photon absorption cross sections values (d) were determined at seven different wavelengths ranging from 540 to 780 nm.The capability of 1O2 production, upon two-photon excitation of b-carbolines, has been also demonstrated.