¹³C Nuclear Magnetic Resonance Relaxation-Derived Ψ, Φ Bond Rotational Energy Barriers and Rotational Restrictions for Glycine ¹³Cα-Methylenes in a GXX-Repeat Hexadecapeptide

Spin-lattice relaxation of ¹³C multiplet spectra and {¹H}-¹³C nuclear Overhauser enhancement (NOE) coefficients of selectively ¹³C-enriched glycines in a collagen GXX-repeat motif hexadecapeptide, G₁VKG₄DKG₇NPG₁₀WPG₁₃APY, has been investigated. Data have been collected at two ¹³C Larmor frequencies (90 and 150 MHz) over the temperature range from 5 to 70°C. Relaxation data indicate that the most restricted internal rotations are at G7 and G10. Mobility of other glycine residues can be arranged in the order G4, G13, and G1. G1 glycine shows the least change in motional anisotropy with temperature. Several motional models have been used to explain the experimental data. While any one model is not completely satisfactory in describing all experimental parameters, only the model of restricted internal diffusion yields the observed positive sign for the cross-correlated spectral densities. Energetic and angular limits of Ψ, Φ bond rotational motions derived from relaxation data and the restricted diffusion model are in good agreement with those calculated as Ramachandran potentional energy profiles. G1 rotational energy barriers for overall tumbling and internal rotation are approximately equal, suggesting strong interaction between the N-terminus and water. Internal rotational parameters for GV and GG dipeptides confirm this view. Nonterminal glycine internal motions are apparently less dependent on water-peptide interactions.