Abstract
Dynamic nuclear polarization (DNP) has shown great promise as a tool to enhance the nuclear magnetic resonance signals of proteins in the cellular environment. As sensitivity increases, the ability to select and efficiently polarize a specific macromolecule over the cellular background has become desirable. Herein, we address this need and present a tetrazine-based DNP agent that can be targeted selectively to proteins containing the unnatural amino acid (UAA) norbornene-lysine. This UAA can be introduced efficiently into the cellular milieu by genetic means. Our approach is bio-orthogonal and easily adaptable to any protein of interest. We illustrate the scope of our methodology and investigate the DNP transfer mechanisms in several biological systems. Our results shed light on the complex polarization-transfer pathways in targeted DNP and ultimately pave the way to selective DNP-enhanced NMR spectroscopy in both bacterial and mammalian cells.