INTERNAL MOBILITY OF CYCLIC RGD HEXAPEPTIDES STUDIED BY C-13 NMR RELAXATION AND THE MODEL-FREE APPROACH

The internal mobility of three isomeric cyclic RGD hexapeptides design ed to contain two p-turns in defined positions, cyclo(Arg-Gly-Asp-Gly- D-Pro-Pro) (I), cyclo(Arg-Gly-Asp-D-Pro-Gly-Pro) (II) and cyclo(Arg-Gl y-Asp-D-Pro-Pro-Gly) (III), have been studied by C-13 NMR longitudinal and transverse relaxation experiments and measurements of steady-stat e heteronuclear (H-1)-C-13 NOE enhancement with C-13 at natural abunda nce. The data were interpreted according to the model-free formalism o f Lipari and Szabo, which is usually applied to data from macromolecul es or larger sized peptides with overall rotational correlation times exceeding 1 ns, to yield information about internal motions on the 10- 100 ps time scale. The applicability of the model-free analysis with a cceptable uncertainties to these small peptides, with overall rotation al correlation times slightly below 0.3 ns, was demonstrated for this specific instance. Chemical exchange contributions to T-2 from slower motions were also identified in the process. According to the order pa rameters obtained for its backbone a-carbon atoms, II has the most rig id backbone conformation on the 10-100 ps time scale: and I the most f lexible. This result coincides with the results of earlier NMR-constra ined conformational searches, which indicated greatest uncertainty in the structure of I and least in II.