SRLS Analysis of 15N Spin Relaxation from E. coli Ribonuclease HI: The Tensorial Perspective

¹⁵N–H relaxation parameters from ribonuclease HI (RNase H), acquired in previous work at magnetic fields of 14.1 and 18.8 T, and at 300 K, are analyzed with the mode-coupling slowly relaxing local structure (SRLS) approach. In accordance with standard theoretical treatments of restricted motions, SRLS approaches N–H bond dynamics from a tensorial perspective. As shown previously, a physically adequate description of this phenomenon has to account for the asymmetry of the local spatial restrictions. So far, we used rhombic local ordering tensors; this is straightforward but computationally demanding. Here, we propose substantiating the asymmetry of the local spatial restrictions in terms of tilted axial local ordering (S) and local diffusion (D₂) tensors. Although less straightforward, this description provides physically sound structural and dynamic information and is efficient computationally. We find that the local order parameter, S₀², is on average 0.89 (0.84, and may be as small as 0.6) for the secondary structure elements (loops). The main local ordering axis deviates from the Cᵢ–₁ᵅ–Cᵢᵅ axis by less than 6°. At 300 K, D₂,⊥ is virtually the same as the global diffusion rate, D₁ = 1.8 × 10⁷ s–¹. The correlation time 1/6D₂,∥ ranges from 3–125 (208–344) ps for the secondary structure elements (loops) and is on average 125 ps for the C-terminal segment. The main local diffusion axis deviates from the N–H bond by less than 2° (10°) for the secondary structure elements (loops). An effective data-fitting protocol, which leads in most cases to unambiguous results with limited uncertainty, has been devised. A physically sound and computationally effective methodology for analyzing ¹⁵N relaxation in proteins, that provides a new picture of N–H bond structural dynamics in proteins, has been set forth.