Département de chimie moléculaire

Electrospray ionization mass spectrometry was used to investigate the intrinsic chem. properties of $\beta$-carbolines: Eudistomin U (as EU) and 5-Br-Eudistomin U (as BrEU) yielding self-assocn. in neg. ion mode. It was obsd. that the deprotonated homodimer of EU was more stable in the gas phase than BrEU. Indeed, BrEU presents a higher polarizability that could influence the dimerization energy. On the contrary, the stability energies calcd. at B3LYP/6-31+G*//AM1 level showed a small difference for these two dimers. To understand the exptl. stability variation, the exptl. and theor. gas phase acidities of these two $\beta$-carbolines have been estd. It has been shown that EU is less acidic than BrEU in the gas phase, which could explain the lower stability of the [2BrEU-H]- dimer. In addn., the investigation of non-covalent interaction of $\beta$-carbolines and nucleobases has been carried out. It was exptl. demonstrated that the deprotonated heterodimer's relative stability scale was [Gua + M-H]- » [Thy + M-H]- {\textgreater} [Ade + M-H]-, but the Cyt/$\beta$-carboline complexes were not obsd. However, the absence of Cyt/$\beta$-carboline complexes could be explained by the great acidity difference between Cyt and $\beta$-carbolines, yielding unstable non-covalent complexes. The studied $\beta$-carbolines showed a particular affinity with guanine which suggested a strong interaction, from a structural and reactivity point of view. Calcd. reaction pathways rationalize the set of exptl. results concerning the heterodimer stabilization reflected by the V1/2 value of breakdown curves of deprotonated heterodimers. [on SciFinder(R)]