Catalysis of Helix Inversion of Zinc Bilindiones by Amines and Amino Acid Esters

Abstract

Control of the rate of conformational changes of a molecule by intermolecular forces is one of challenging goals in host-guest chemistry. Dynamic ¹H NMR studies demonstrated that the conformational change of zinc bilindione complexes was found to be catalyzed by amines. Isomerization of [3,8,12,17-tetraethyl-1, 21-dihydro-19-methoxy-2,7,13,18-tetra-methyl-23H-bilin-1-onatolzinc(II), the zinc chelate of aetiobiliverdin-IV y, from P-to M-helical conformation was slow (K < 1 s⁻¹) at 223 K in CD₂Cl₂. It was accelerated up to k = 13s⁻¹ by adding (R-1-(1-naph-thyl)ethylamine (NEA) until all the zinc bilindione was complexed. The rate was further accelerated by the addition of more NEA with smaller dependence on the NEA concentration. The rate of helix inversion was thus catalyzed by one amine molecule, and further accelerated by the second amine molecule. Amino acid esters also showed catalytic activity, although the activity was lower than that of amines. The helix inversion rates were decelerated, as the peripheral alkyl groups introduced at the terminal pyrroles of bilindione became bulkier from methyl, ethyl to propyl groups. Thermal fluctuation, namely, an entropic effect, of these peripheral alkyl groups was responsible for the retardation.

Key Words:

• Bilindion
• conformational change
• kinetics
• activation entropy