Optimal-Control ¹⁹F PROJECT Improves the Reliability of CPMG-Derived Relaxation Rates

In fluorine-based fragment screening, protein-ligand interactions are monitored through the impact of binding on ¹⁹F NMR relaxation properties. Most often, filters based on transverse relaxation are applied, whereas some protocols foresee the measurement of relaxation constants to rank different hits. The largest contribution to relaxation, even at the most common fields used in biomolecular NMR, is chemical shielding anisotropy, proportional to the reorientation correlation time and to the square of the magnetic field. R₂ values are thus usually measured using CPMG-type echo trains to mitigate exchange effects. When the observed ¹⁹F spin is scalarly coupled to another ¹⁹F spin, the CPMG refocusing train imposes a coherent J-modulation on the echo train, biasing the extracted R₂. We benchmarked the J-compensated variant proposed by Aguilar et al. ("PROJECT"), implemented with seedless optimal-control (OC) shaped pulses specifically designed for covering more than 30 ppm at 659 MHz ¹⁹F Larmor frequency (16.4 T), directly against CPMG performed with the same OC shaped pulses. The comparison is performed on two model proteins across multiple sample conditions and two spin echo times. In this application note, we show that seedless-PROJECT recovers R₂ estimates that are systematically more consistent with a J-free reference than a naive CPMG analysis-indistinguishable from CPMG when the coupling is negligible, and significantly better fit when it is resolved-at the price of a modest reduction in SNR. We outline a reusable protocol for other laboratories and provide a practical signal-to-noise ratio (SNR) threshold below which neither method returns reliable rates.