Modification of a Chemical Linker via the Finkelstein Reaction Mechanism

• Phillip Moquin, Biochemistry, University of Delaware

• Zhihao Zhuang, Chemistry and Biochemistry, University of Delaware

The Finkelstein reaction developed by Hans Finkelstein is a type of SN2 reaction wherein a halogen atom is exchanged for another. Such a mechanism can be used to affix a molecule with a halogen that can act as a more effective leaving group; a group that is more readily displaced in a nucleophilic substitution reaction. A non-hydrolyzable linker (NHL) molecule, one that is used in the construction of protein (particularly ubiquitin-based) probes, is commonly synthesized within our research group. Typically, this linker is synthesized with bromine, however, the treatment of an NHL intermediate with potassium iodide is able to exchange the bromine atom for an iodine by the aforementioned Finkelstein reaction mechanism. In the synthesis of the iodo-NHL, the brominated NHL intermediate is synthesized as per standard procedure and is subsequently dissolved in acetone and treated with an excess of potassium iodide for 24 hours. This reaction yields an iodinated NHL intermediate as well as solid potassium bromide that can be filtered out of the reaction solution. Similar to the synthesis of the typical bromo-NHL, this iodinated intermediate will undergo a diol protection (ketalization) and subsequent amine deprotection to yield the final product of the multistep synthesis, the iodo-NHL. The final product of the synthesis will be analyzed by proton nuclear magnetic resonance (NMR) spectroscopy to determine if the expected product has been successfully synthesized as well as the product’s purity. Following the successful completion of its synthesis, the iodinated linker can be used to produce polyubiquitin probes more effectively than the brominated linker; iodine is the most effective leaving group of the standard halogens. An increased protein ligation efficiency provided by this new linker molecule will contribute to the improved efficacy of future studies of ubiquitin pathways, thus advancing our understanding of the interactions between ubiquitin and other proteins.