Chemical control of CSA geometry enables relaxation-optimized 19F-13C NMR probes

Fluorine NMR is a powerful tool for probing biomolecular structure and dynamics, yet the performance of 19F probes is fundamentally constrained by rapid transverse relaxation driven by chemical shift anisotropy (CSA). Despite its central role, CSA has largely been treated as an immutable nuclear property rather than a chemically addressable design parameter. Here we demonstrate that the geometry of the CSA tensor - specifically its magnitude, symmetry, and orientation relative to the internuclear dipolar interaction - constitutes a decisive and engineerable determinant of relaxation behavior in coupled 19F-13C spin systems. Guided by electronic-structure calculations and Bloch-Redfield-Wangsness relaxation theory, we establish quantitative design rules that predict when CSA-dipolar interference can be exploited to suppress transverse relaxation. Implementation of these principles in a cysteine-reactive fluoropyrimidine scaffold yields a reporter that supports simultaneous 19F and 13C TROSY optimization, validated by solid-state MAS NMR and protein-based experiments. When incorporated into the 42 kDa maltose binding protein, the probe exhibits exceptionally slow 13C transverse relaxation (R2 {approx} 2-3 s-1) corresponding to linewidths of [~]2 Hz that persist even at apparent molecular weights exceeding 200 kDa. These results recast relaxation optimization as a chemically programmable problem and provide a general framework for the rational design of next-generation NMR probes tailored to large, dynamic, and heterogeneous biomolecular systems.