Étude ab initio des effets des solvants aprotiques polaire et non polaire sur des dérives hydroxychromones et hydroxyquinolones

Abstract

Hydroxychromones and hydroxyquinolones, dual-emission, environmentally sensitive spectroscopic probes, have been developed for DNA labeling to track interactions between proteins,and DNA or membrane. In this work we have determined, using ab initio methods, the properties of a hydroxychromone probe, BTHC, and two hydroxyquinolone probes, FHQ and MFHQ. The calculation method used is the fundamental state DFT and the excited states TDDFT, methods associated with the TZVP base and the PBE0 functional. The two solvents, acetonitrile and n-hexane, were taken into account by the PCM method. Thus, we have Örst determined for each probe the most stable conformation in the fundamental state. This conformation informs us in particular about the áatness of the molecules studied. The results predicted that, unlike the distorted MFHQ probe, BTHC and FHQ probes with a planar structure could be inserted into DNA sequences. The calculated lengths of the inter-atomic and intramolecular links have been analyzed and interpreted. The value of the pka in the fondamental state makes it possible to establish the non-anionic nature of the probes in the two solvents, a necessary condition for the e§ectiveness of the ESIPT reaction which leads to the dual emission. This non-anionic character is further developed for the hydroxyquinolone probes in n-hexane, a nonpolar aprotic solvent. Increasing the Stokes shift from acetonitrile to n-hexane indicates easier detection in the latter solvent. The introduction of the N-methyl group at the 1-position of the quinolone part of the FHQ probe also leads to an increase in Stokes displacement. The absorption spectra show a very high absorption e¢ ciency for the BTHC probe in the two solvents. For all the probes studied, the emission bands move towards short wavelengths when the polarity of the solvent is increased. Oscillator forces at áuorescent relaxation states predict a very high quantum yield of the BTHC probe in both solvents. This is a necessary but not su¢ cient asset for good radiative deactivation and therefore for good detection. The excited states of the three probes are characterized by the lack of activation barrier of the ESIPT reaction because of a large energy gap between the áuorescent relaxation state of the normal form and the áuorescent relaxation state of the tautomeric form. Therefore, for all three probes, the ESIPT reaction is irreversible. The addition of the N-methyl group to the FHQ probe almost doubles the gap of vibrational relaxation energy in the excited state on the one hand and makes negligible the energy gap between the fundamental state of áuorescent relaxation and the initial ground state. This favors the return of the probe to the initial ground state after áuorescence emission of the normal form. Taking into account the áatness of our probes and their quantum yield, the BTHC and FHQ probes are much more favorable for an insertion in a DNA sequence for the spectroscopic detection of the latter.