Étude par spectrophotométrie UV d'aldéhydes et d'amides parahydroxycinnamiques. Comparaison de leurs capacités complexantes vis à vis de ZnBr₂

Abstract

The ability of zinc salts to be complexed by cinnamaldehydes and N-pyridyl cinnamides has been investigated for modeling the active site of cinnamyl alcohol dehydrogenase (CAD), a zinc enzyme involved in the lignification process. Parahydroxycinnamaldehydes 4a–6a are specific substrates of the enzyme while cinnamides were synthesized as bidendate ligands towards zinc, and present inhibitory effects toward CAD. The interaction C=O … Zn was studied by UV methods and was observed in each example; the complexation constants are rather low with aldehydes (K = 3–16 L mol−1) and of the order (2–12) × 103 stronger with the corresponding cinnamides, due to the aminopyridine moiety participation. The stoichiometry of the complexes was found to be ML (M: salt, L: ligand) with mono substituted aldehydes and M2L with 5a or 6a, which was attributed to an additional complexation caused by the catechol effect. With the N-pyridylamides the complexation occurs according to a 1:1 (ML) or 1:2 (ML2) stoichiometry. Strong bathochromic effects were also observed; they are more important with the aldehydes than with the amides (cross conjugation). Bathochromic effects due to the substitution of the aromatic ring are shown by a relationship between the ligands and their zinc complexes.