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

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 perform a comparative analysis, we also studied other bromo-β-carbolines and the corresponding non-substituted-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 (ϕf) in acetonitrile and acetonitrile + 1% perchloric acid solutions at 298 K were measured. The HOMO and LUMO energy, the positions (λmax) and oscillator strength (f) of the 1S1 ← 1S0 band for all the neutral and protonated β-carbolines studied was 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 spectrophotometrically measured ( p K a ( H 2 O ) ) . The change of the acid–base character of these compounds on going from the ground state (pKa) to the first electronic excited singlet state ( p K a ∗ ) as Δ p K a = p K a ∗ - p K a = 0.625 Δ ν ¯ / T , in ethanol solution at 298 K were calculated (ΔpKa(EtOH)). Proton affinities (PA) for all the compounds studied defined as minus the enthalpy change of the reaction M + H+ → MH+ (gas state) were calculated. Basicity relative to pyridine (ΔHrPy) defined as the enthalpy change of the isodesmic reaction MH+ + Py → M + PyH+ (gas state) was also calculated. The effect of bromine as substituent on the properties of the β-carboline moiety in nor-harmane, harmane and harmine is discussed.