Biology-inspired chemistry


 

Research in my group is directed towards the synthesis and characterization of novel supramolecular structures containing transition metal ions, which offer the opportunity to employ the rich aspects of electronic structure and physico-chemical properties of transition metal ions to obtain special electric, magnetic, and chemical properties for the supramolecules. The design of our molecules is inspired by biological structures and their potential applications are in molecular electronics and nanotechnology. The two main directions in which we focus our efforts are (1); the use of peptide nucleic acids as scaffold for transition metal ions, and (2); the spectroscopic characterization of Fe-containing polynuclear complexes that exhibit spin transitions and/or intramolecular electron transfer.

1); The structural and spectroscopic characterization of metallo-proteins has revealed that the metal ion organization and the presence of unpaired electrons in transition metal ions are features essential for the efficient function of these systems as catalysts or electron-transfer mediators. We have developed a synthetic system that makes possible the rational organization of a variety of metal ions in supramolecular assemblies and have demonstrated that peptide nucleic acid (PNA);, an analogue of DNA that has a pseudo-peptide backbone and forms helical duplexes, represents a platform especially amenable as scaffold for metal ions. The rational modification of PNA oligomers by substitution of the natural nucleobases with ligands that have high affinity for metal ions affords modified PNA duplexes that have high binding specificity for metal ions at predefined positions (Figure 1);. To demonstrate the broad utility of this approach and to provide a rational way for the design of new metal-containing systems, we have also studied the thermodynamics and kinetics of the duplex assembly process and showed that the properties of the metal-containing duplexes depend in a synergetic way on the hybridization properties of the PNA and on the coordination properties of the metal ions. We have also shown that strong metal-ligand interactions lead to high mismatch tolerance and, consequently, to the formation of duplexes from partly complementary PNA oligomers, a property that makes possible the construction of higher hierarchy metal-containing PNA structures. Our present studies focus on the possibility of building functional PNA-based, molecular electronics devices. As a first step, we investigate in collaboration with David Waldeck at University of Pittsburgh the effect of spin and electronic structure of the metal ions on the magnetic and electron-transfer properties of the PNA-metal based assemblies (Figure 2);. In another research direction we pursue the use of the recognition properties of positively-charged metal-PNA duplexes for creating molecular imaging reagents for biological applications.

2); Complexes with spin transitions can be used for information storage if they are bistable as a result of cooperativity. Studies of polynuclear complexes with spin transitions are few and very recent in contrast to those of mononuclear complexes, which have been extensively investigated. Polynuclear complexes offer the opportunity to study how the spin transition is affected by intramolecular interactions between the metal ions, which underscore the cooperativity observed in 3-D structures. We use variable-temperature, variable-field Mössbauer and EPR spectroscopy to characterize several classes of polynuclear complexes that exhibit spin transitions.

In collaboration with Kim Dunbar at Texas A&M, we discovered that [Fe2Co3(tmphen);3(CN);12] 1, a cluster with a trigonal-bipyramid core of Fe and Co ions (Figure 3);, is the first molecular-based material that exhibits a charge-transfer induced spin transition. The cluster exhibits valence isomerism and can exist as {CoIICoIII2FeII2}, {CoII2CoIIIFeIIFeIII}, or {CoII3FeIII2}, depending on temperature and solvent content. In single crystals, temperature-induced intramolecular electron transfer FeIICoIIIàFeIIICoII interconverts the valence isomers and leads to drastic changes in the cluster spin states. We focus on the detailed spectroscopic and computational investigation of extrinsic factors that determine the special properties of 1 and 1+. Furthermore, the rich electronic properties of 1 together with the existence in the cluster of terminal cyanide ligands offer the entry into a new class of magnetic materials in which 1 is a building block. We investigate these novel materials synthesized in the Dunbar lab, as well as new analogs of 1, which contain iron and other transition metal ions.

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Carnegie Mellon University
Department of Chemistry
4400 Fifth Ave.
Pittsburgh, PA 15213

Phone: (412) 268-9588
Fax: (412) 268-1061

E-mail: achim@cmu.edu
Website: http://www.chem.cmu.edu/groups/achim/