A Study of the Interaction Between trans-Dehydrocrotonin, a Bioactive Natural 19-nor-Clerodane, and Serum Albumin

The interaction between 19-nor-clerodane trans-dehydrocrotonin (from Croton cajucara Benth.) and bovine serum albumin was studied, applying spectroscopic techniques (fluorescence and circular dichroism), combined with molecular modeling. Fluorescence quenching of albumin by the nor-clerodane (k(q) ca. 10(11) mol L-1 s(-1) and Stern-Volmer, K-SV, increase with temperature) indicates a combination of static and dynamic quenching mechanism. The binding constant (K-b ca. 10(3) mol L-1) and circular dichroism data suggest that this association is weak and causes only a moderate change in the alpha-helix content of the protein. Thermodynamic parameters indicate a spontaneous (Gibbs free energy, Delta G degrees, ca. -21.28 kJ mol(-1) at 310 K) and probably entropy-driven (Delta S degrees = 0.072 kJ mol(-1) K-1) association, typical of hydrophobic interactions. The number of binding sites (n ca. 1) indicates one main binding site and molecular modeling suggests subdomain IIIA (Sudlow's site II) as the main binding site to the nor-clerodane, which is able to make hydrophobic interactions with leucine (Leu)-24, phenylalanine (Phe)-36, valine (Val)-40 and tryptophan (Trp)-134 residues.