Designer Proteins from Fluorinated Amino Acids

     This project was funded by FRAC in 1999. As proposed, funds were used to hire an undergraduate researcher for the summer of 1999, to purchase a pH-meter, and to purchase chemicals necessary to complete the research. A new, highly unusual rearrangement of a macrocyclic copper complex discovered in our laboratory was investigated in detail. FRAC funds helped us to understand the scope and mechanism of this reaction, and to develop convenient synthetic methodology for the preparation of a series of macrocyclic complexes for applications in biomimetics and catalysis.
     It was found that an isomerization is indeed controlled by the metal ion. This control is even more dramatic than we originally expected. In case of copper, one double bond in the ring shifts, in order to accommodate the metal ion better. Both the starting material and the product have the same five nitrogen atoms bound to the copper ion. This coordination environment is favorable for copper. In case of nickel, however, no rearrangement occurs under mild conditions. Upon heating, the complex completely changes the structure due to addition of water to one of its double bonds. In the resulting compound, there are six donor atoms (nitrogens and oxygens) bound to the nickel ion in an octahedral fashion. This geometry is ideally suited to bind nickel, and the new acyclic ligand is now flexible, so that all metal-ligand bonds can adopt their typical values. The coordination requirements of a particular metal ion dictate the outcome of the reaction. These findings answer the question: "What happens in the reaction?"
     The next question to be addresses was: "How does this reaction occur?" Mechanistic investigations allow us to answer this question as well. One striking observation was made by the undergraduate researcher: alcohols are required for the reaction to proceed cleanly. We proved that alcohol molecule does not interact with the metal center, but can be added to the double bond in the organic ligand. In the absence of an alcohol, a competing process occurs: a water molecule adds to the double bond. Alcohol addition yields stable compound, while water addition leads to an eventual opening of the cycle and full decomposition of the complex. It was also found that the rearrangement of the copper complex involves a proton transfer. A series of isotopic labeling experiments and investigations of reaction rates in the presence of a base support this pathway. The same pathway for C=N double bond migration is known in biochemistry for vitamin B6 catalyzed reactions.
     Finally, systematic studies on macrocycle formation allowed us to modify our synthetic methodology and develop convenient methods for the synthesis of a series of related complexes. In particular, we no longer have to run the condensations in very dilute solutions. Proper choice of metal "templates" allows us to obtain desirable cyclic products and to suppress "head-to tail" side reactions. We can easily obtain substantial quantities of cyclic copper and nickel complexes. These materials can be reduced, demetallated, and another metal ion can be incorporated. Our preliminary studies show that iron complexes can be synthesized, and they are promising catalysts in oxidation reactions.
     One manuscript on the scope and mechanism of metal ion directed rearrangements of the macrocycles is now in preparation. Another manuscript will be prepared on synthetic and catalytic aspects of the project. FRAC support will be acknowledged in both publications.
A talented undergraduate researcher supported by FRAC funds, Ann Kalayda, is now a graduate student in chemistry at the University of Leiden (Netherlands).