UV SPECTROPHOTOMETRY OF PARAHYDROXYCINNAM ALDEHYDES AND PARAHYDROXYCINNAMIDES - COMPARISON OF THEIR COMPLEX-FORMING CAPACITY IN THE PRESENCE OF ZNBR2

The ability of zinc salts to be complexed by cinnamaldehydes and N-pyr idyl cinnamides has been investigated for modeling the active site of cinnamyl alcohol dehydrogenase (CAD), a zinc enzyme involved in the li gnification process. Parahydroxycinnamaldehydes 4a-6a are specific sub strates of the enzyme while cinnamides were synthesized as bidendate l igands towards zinc, and present inhibitory effects toward CAD. The in teraction 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) x 10(3) stronger with the corre sponding cinnamides, due to the aminopyridine moiety participation. Th e stoichiometry of the complexes was found to be ML (M: salt, L: ligan d) with monosubstituted aldehydes and M2L with 5a or 6a, which was att ributed to an additional complexation caused by the catechol effect. W ith the N-pyridylamides the complexation occurs according to a 1:1 (ML ) or 1:2 (ML2) stoichiometry. Strong bathochromic effects were also ob served; they are more important with the aldehydes than with the amide s (cross conjugation). Bathochromic effects due to the substitution of the aromatic ring are shown by a relationship between the ligands and their zinc complexes.