From Biomimetic Ion Carriers to Helical Structures

Biomimetic chemistry aims at reproducing the functions of natural compounds with the simplest possible synthetic molecules. Our strategy in this endeavor involved: (i) first reproduction of elementary processes such as molecular recognition, mass- transport, electron-transport, and signaling, and (ii) subsequently integration of several of these properties into single molecules. We approached the problems of molecular recognition and mass- transport by concentrating on the design and synthesis of all-artificial iron(III)-carriers that mimic the properties of microbial siderophores (iron(III) carriers): (i) the capability to effectively bind iron(III), (ii) to interact with specific membrane receptors as their iron(III)-complexes, and (iii) to transport iron(III) into the cells’ interior. Conjugation of the synthetic carriers with fluorescent markers enabled us to couple molecular recognition with signaling and to thereby provide diagnostic tools for the identification of specific microorganisms. The knowledge gained in the course of this work was then applied to the synthesis of (i) triple-stranded binders that form helical, dinuclear complexes, and of (ii) helical structures where four elementary process are integrated into a single molecule to provide molecular »redox-switches«

Photoinduced Electron Transfer in Ruthenium Bipyridyl Complexes: Evidence for the Existence of a Cage with Molecular Oxygen

Ruthenium complexes with three bipyridyl ligands, one of which is modified by attaching one or two hydroxamic acids groups (Ru-1 and Ru-2, respectively), were synthesized. Using EPR spectroscopy, we have found that photoexcitation leads to formation of nitroxyl radicals. The nitroxyl radical concentration in Ru-2 increased dramatically in the presence of spin traps DMPO (5,5‘-dimethyl-1-pyrroline-N-oxide) and PBN (N-tert-butyl-α-phenylnitrone) characterized by strong affinity to superoxide radicals. We have attributed this behavior to the formation of a cage complex between Ru-2 and the superoxide radical. This paper concerns the study of cages formed between ruthenium complexes and molecular oxygen and the effect of functional groups attached to modified bipyridyl ligands on cage formation. The complex between Ru-2 and O_2 was formed in the ground state, probably with participation of the hydroxamic acid groups. The equilibrium constant of this complex was determined by EPR as K_(eq) ∼ 3 M^(-1). The formation of the Ru-2−O_2 complex is supported by the temperature-dependent rate of appearance of the EPR signal in the presence of PBN. Additional evidence comes from observation of paramagnetic shifts of the peaks in the 1H NMR spectrum of specific aromatic protons in the substituted bipyridyl ring upon exposure to O_2. Similar shifts were observed in the spectrum of Os-2, with osmium replacing ruthenium. Model compounds with functional groups that replace the hydroxamic acid or compounds without the metal center, but with the two hydroxamic acids, were synthesized. No shifts in the ^1H NMR spectra of these derivatives were observed in the presence of O_2. These results lead to the conclusion that both metal ions, Ru(II) or Os(II), and hydroxamic acid groups are essential components for the formation of the oxygen cage