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.