¹³C relaxation experiments for aromatic side chains employing longitudinal- and transverse-relaxation optimized NMR spectroscopy.

Aromatic side chains are prevalent in protein binding sites, perform functional roles in enzymatic catalysis, and form an integral part of the hydrophobic core of proteins. Thus, it is of great interest to probe the conformational dynamics of aromatic side chains and its response to biologically relevant events. Indeed, measurements of (13)C relaxation rates in aromatic moieties have a long history in biomolecular NMR, primarily in the context of samples without isotope enrichment that avoid complications due to the strong coupling between neighboring (13)C spins present in uniformly enriched proteins. Recently established protocols for specific (13)C labeling of aromatic side chains enable measurement of (13)C relaxation that can be analyzed in a straightforward manner. Here we present longitudinal- and transverse-relaxation optimized pulse sequences for measuring R (1), R (2), and {(1)H}-(13)C NOE in specifically (13)C-labeled aromatic side chains. The optimized R (1) and R (2) experiments offer an increase in sensitivity of up to 35 % for medium-sized proteins, and increasingly greater gains are expected with increasing molecular weight and higher static magnetic field strengths. Our results highlight the importance of controlling the magnetizations of water and aliphatic protons during the relaxation period in order to obtain accurate relaxation rate measurements and achieve full sensitivity enhancement. We further demonstrate that potential complications due to residual two-bond (13)C-(13)C scalar couplings or dipolar interactions with neighboring (1)H spins do not significantly affect the experiments. The approach presented here should serve as a valuable complement to methods developed for other types of protein side chains.