7. Project Summary/Abstract The adaptation of abiotic transition metal catalysts for applications in biological systems has undergone remarkable growth in recent years with examples ranging from artificial metalloenzymes to molecular probes for detection of challenging analytes such as carbon monoxide. The PI has a standing interest in discovery, development, and understanding of strategies for detecting challenging biologically relevant molecules via the unique reactivity of transition metals. Most recently, the Michel group reported BODIPY Ethylene Probes (BEPs) as the first profluorescent Activity Based Sensors (ABS) for the detection of ethylene in biological systems. These ABS adapted well-known ruthenium olefin metathesis catalyst, which are known to readily react with ethylene. While ethylene has long been known as an important plant hormone it has also been demonstrated to be produced in mammals as a result of oxidative stress that is hallmark to numerous diseases. In particular ethylene arises from the radical fragmentation of lipid peroxides and/or intermediates in their formation. The formation of lipid peroxides is a result of reactive oxygen species, which are implicated as playing stress or signaling roles in numerous diseases including cancer, cardiovascular disease, and neurodegenerative diseases amongst others. While there are some sophisticated spectroscopic methods for sensitively measuring the biomarker ethylene in exhaled breath, these approaches are necessarily limited in spatial resolution and complexity of sample. Since reporting our initial ABS approach, our group has conducted systematic ligand modulation studies to improve probe response time and sensitivity. Through this work the limit-of-detection was improved nearly two orders of magnitude. Despite these advancements, important questions remain for broad applications in the detection of endogenous ethylene related to modifications that further improve sensitivity while retaining robust stability in biological systems. The next stage of developing this technology will build on mechanistic insight and recent advancements in catalytic olefin metathesis. This will be accomplished through an interdisciplinary approach of synthesis, analytical and photophysical characterization, and subcellular localization studies. It is anticipated that the proposed research will result in highly sensitive ethylene ABS localized to subcellular locations where ethylene is expected to be found in the highest concentrations. Further we expect to explore novel strategies for ethylene detection beyond the dosimetric responses generally employed for ethylene and other small molecule analytes. As has been the case over the past 10-15 years, we envision that there will be continued growth of transition metal catalysts operating in cellular environments to perform critical functions that would otherwise not be possible. The research and concepts of the proposed program will continue to signifi...