Peptide dynamics in triglycine: Coupling of internal bond rotations and overall molecular tumbling

As a model system, triglycine (G1-G2-G3) permits a thorough investigation of peptide backbone motional dynamics by using ¹³C- and ¹⁵N-NMR relaxation. Previously, rotational model analyses of the nonterminal glycine, G2, could not adequately explain ¹³C-NMR relaxation data (Daragan, V. A.; Mayo, K. H. Biochemistry 1993, 32, 11488). In this study, ¹⁵N-NMR relaxation measurements on ¹⁵N-enriched triglycine provide additional motional vectors for more complete rotational model analyses. The inadequancy in describing G2 internal motions with models of anisotropic or restricted rotational diffusion is overcome by using a rotational jump model which has been parameterized with a semiempirical coefficient for backbone recoil rotation. Effectively, this recoil action couples internal bond rotations and overall molecular tumbling. Stochastic dynamics computer simulations using this recoil coefficient allow calculation of triglycine ¹³CH and ¹⁵NH autocorrelation times and ¹³CH₂ cross-correlation times. Good agreement between experiment and theory is found only when strong recoil coupling is taken into account.