Photophysical studies of 1,4-diazabicyclo(2,2,2)octane as a bifunctional ligand to fix the conformation of a flexibly-linked phorbinato-zinc dimer

The binding of a bifunctional ligand 1,4-diazabicyclo(2,2,2)octane (DABCO) to a phorbin-dimer (PP) in which a phytochlorin is covalently linked with a flexible spacer to a modified phytyl residue of pyropheophytin was studied in order to establish a rigid face-to-face intramolecular dimer. Steady-state and time-resolved fluorescence and absorption spectroscopies were employed to characterize the binding equilibrium in CH2Cl2 and to verify the formation of a 1:1 complex where DABCO is bound between the Zn-phorbin rings of the dimer. The proportion of this complex is highest, about 88%, at the molar ratio of ligand:substrate = 1.5:1 while at higher molar ratios, a second DABCO molecule binds to the dimer breaking down the fixed complex. (C) 2000 Elsevier Science S.A. All rights reserved.