NMR spin relaxation experiments provide a powerful tool for the measurement of global and local biomolecular rotational dynamics at subnanosecond time scales. Technical limitations restrict most spin relaxation studies to biomolecules weighing less than 10 kDa, considerably smaller than the average protein molecular weight of 30 kDa. In particular, experiments measuring ηz, the longitudinal 1HN–15N dipole-dipole (DD)∕15N chemical shift anisotropy (CSA) cross-correlated relaxation rate, are among those least suitable for use with larger biosystems. This is unfortunate because these experiments yield valuable insight into the variability of the 15N CSA tensor over the polypeptide backbone, and this knowledge is critical to the correct interpretation of most 15N-NMR backbone relaxation experiments, including R2 and R1. In order to remedy this situation, we present a new 1HN–15N transverse relaxation optimized spectroscopy experiment measuring ηz suitable for applications with larger proteins (up to at least 30 kDa). The presented experiment also yields κ, the site-specific rate of longitudinal 1HN–1H′ DD cross relaxation. We describe the ηz∕κ experiment’s performance in protonated human ubiquitin at 30.0 °C and in protonated calcium-saturated calmodulin∕peptide complex at 20.0 °C, and demonstrate preliminary experimental results for a deuterated E. coli DnaK ATPase domain construct at 34 °C
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