Comparison of ¹³C_αH and ¹⁵NH backbone dynamics in protein GB1

This study presents a site-resolved experimental view of backbone C_αH and NH internal motions in the 56-residue immunoglobulin-binding domain of streptococcal protein G, GB1. Using ¹³C_αH and ¹⁵NH NMR relaxation data [T₁, T₂, and NOE] acquired at three resonance frequencies (¹H frequencies of 500, 600, and 800 MHz), spectral density functions were calculated as F(ω) = 2ωJ(ω) to provide a model-independent way to visualize and analyze internal motional correlation time distributions for backbone groups in GB1. Line broadening in F(ω) curves indicates the presence of nanosecond time scale internal motions (0.8 to 5 nsec) for all C_αH and NH groups. Deconvolution of F(ω) curves effectively separates overall tumbling and internal motional correlation time distributions to yield more accurate order parameters than determined by using standard model free approaches. Compared to NH groups, C_αH internal motions are more broadly distributed on the nanosecond time scale, and larger C_αH order parameters are related to correlated bond rotations for C_αH fluctuations. Motional parameters for NH groups are more structurally correlated, with NH order parameters, for example, being larger for residues in more structured regions of β-sheet and helix and generally smaller for residues in the loop and turns. This is most likely related to the observation that NH order parameters are correlated to hydrogen bonding. This study contributes to the general understanding of protein dynamics and exemplifies an alternative and easier way to analyze NMR relaxation data.