Relaxation-optimized correlation spectroscopy ROCSY for assigning ¹H or ¹³C spin systems in large proteins

Solution NMR studies of large systems are hampered by rapid signal decay. We hereby introduce ROCSY (relaxation-optimized total correlation spectroscopy), which maximizes transfer efficiency across J-coupling-connected spin networks by minimizing the amount of time magnetization spends in the transverse plane. Hard pulses are substituted into the Clean-CITY TOCSY pulse element first developed by Ernst and co-workers, allowing for longer delays in which magnetization is aligned along the z-axis. This has the following consequences: 1) transverse relaxation is minimized, 2) resonance offset effects are minimized, 3) and through-space nuclear Overhauser enhancement (NOE) adds to J-coupling-mediated magnetization transfer. The major drawbacks of the technique are additional heat generation produced by the hard pulses and complication of analysis by the additional through-space NOE magnetization transfer. We demonstrate H_alpha-to-HN correlations not possible using conventional ¹H-TOCSY in a ¹⁵N-enriched sample of PagP (a 161-residue integral membrane protein) in dodecylphosphocholine detergent micelles. We also demonstrate enhanced signal-to-noise compared to ¹³C-TOCSY in a ¹⁵N,¹³C-enriched sample of cardiac troponin C N-terminal domain. We thus propose that ROCSY can be used to boost signal in any protein NMR experiment that utilizes TOCSY, with greater enhancements seen in higher molecular weight systems.