The backbone dynamics of specific residues in two collagen-like triple-helical peptides with (X-Y-Gly)n sequences have been investigated using two-dimensional inverse-detected 15N NMR relaxation measurements and hydrogen-exchange experiments. One peptide, (POG)10, has the highest possible imino acid content and is considered to be a very stable prototype of a triple helix. The second peptide, (POG)3ITGARGLAGPOG(POG)3 (denoted T3-785), models an imino acid poor region of type III collagen and contains 12 residues from near the unique collagenase cleavage site. 15N relaxation parameters and hydrogen-exchange data were obtained for a glycine residue in the center of (POG)10 and for the tripeptide unit Gly-Leu-Ala in the middle of T3-785. Analysis of the relaxation data of the rodlike triple-helical peptides required the assumption of anisotropic overall motion, and the model-free approach of Lipari and Szabo (1982) was used to derive overall motional parameters and the order parameter, S2, that describes the amplitudes of the internal motion. First the mobilities of the Gly, Leu, and Ala residues in peptide T3-785 were compared. Both hydrogen-exchange methods and relaxation measurements indicated that the residue in the Y position (Ala) is more mobile than residues in the Gly and X positions (Leu). The slower exchange rates of Gly and Leu compared to that of Ala are consistent with the two-hydrogen-bonded model for the triple helix. Then the backbone mobilities of the central Gly residue were compared for the two peptides (POG)10 and T3-785. In this case, 15N relaxation measurements give different results from hydrogen exchange. The glycine residues in the trimer form of both T3-785 and (POG)10 have high values for the order parameter (near 0.85), suggesting similar small-amplitude internal motions and rigid backbones in both peptides. In contrast to the similar values of the order parameters, hydrogen-exchange data indicate that the central Gly exchanges at a faster rate in the trimer form of T3-785 than in (POG)10. These results suggest that a Gly in the imino acid rich environment of (POG)10 is dynamically different from a Gly in the imino acid poor environment of T3-785 and that the difference lies in the slower motion related to stability, rather than the faster motion on the picosecond time scale. This sequence-dependent difference in dynamical properties may have important consequences for recognition processes in collagen.