AbstractNMR measurements of cross-correlated nuclear spin relaxation provide powerful probes of polypeptide dynamics and rotational diffusion, free from contributions due to chemical exchange or interactions with external spins. Here, we report on the development of a sensitivity-optimized pulse sequence for the measurement of cross-correlated relaxation in methyl spin systems by analysis of the differential relaxation of transitions within the13C multiplet. We describe the application of optimal design theory to implement a real-time ‘on-the-fly’ adaptive sampling scheme that maximizes the accuracy of the measured rate constants. The increase in sensitivity obtained using this approach enables, for the first time, quantitative measurements of rotational diffusion within folded states of translationally-arrested ribosome–nascent chain complexes of the FLN5 filamin domain, and can be used to place strong limits on interactions between the domain and the ribosome surface.