Photochemistry of the alkaloids eudistomin N (6-bromo-nor-harmane) and eudistomin O (8-bromo-nor-harmane) and other bromo-b-carbolines

I. TARZI, OLGA; ERRA-BALSELLS, ROSA; ERRA BALSELLS, ROSA

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY

Elsevier

Lugar: Amsterdam; Año: 2005 vol. 80 p. 29 - 45

1011-1344

Abstract The UV-absorption, fluorescence excitation and emission spectra of the alkaloids eudistomin N (6-bromo-nor-harmane) and eudistomin O (8-bromo-nor-harmane), were described. In order to performe a comparative analysis we also studied other bromo-b?carbolines and the corresponding non-substituted b?carboline. Thus 6-bromo-, 8-bromo-, 6,8-dibromo-, 3,6-dibromo- and 3,6,8-tribromo-derivatives of nor- harmane, harmane and harmine were studied. These studies were performed in EtOH and in EtOH  +  1% perchloric acid solutions (pa). Furthermore, fluorescence quantum yields (ff) in acetonitrile and acetonitrile + 1% perchloric acid solutions at 298 K were measured. The HOMO and LUMO energy, the positions (lmax) and oscillator strength (f) of the 1S1<---1S0  band for all the neutral and protonated b-carbolines studied were calculated and compared with the experimental data. The pKa values in aqueous solution for eudistomin N and O (6-bromo-and 8-bromo-nor-harmane), for 6-bromo-, 8-bromo- and 6,8-dibromo-harmane, and for 6-bromo- and 8-bromo-harmine were spectrophotometrycally measured (pKa(H2O)). The change of the acid-base character of these compounds on going from the ground state (pKa) to the first electronic excited singlet state (pKa*) as DpKa = pKa* - pKa = 0.625 D`n / T, in ethanol solution at 298 K were calculated (DpKa(EtOH)). Proton affinities (PA) for all the compounds studied defined as minus the enthalpy change of the reaction M + H+ ® MH+ (gas state) were also calculated. The effect of bromine as substituent on the properties of the b?carboline moity in nor-harmane, harmane and harmine is discussed. Keywords: eudistomin N; eudistomin O; bromo-b-carbolines; bromo-nor-harmane; bromo-harmane; bromo-harmine