The potential of15N NMR relaxation data for conformational studies of small linear peptides is discussed, using15N enriched enkephalin derivatives as follows: Tyr-*Gly-*Gly-*Phe, I; Boc-Tyr-*Gly-*Gly-*Phe-OCH3, II; and Tyr-*Gly-*Gly-*Phe-*Leu, III.15N relaxation data (T1/NOE) are interpreted in terms of molecular motion using the simplified two-spin system assumption for the NH bonds. In that way, two different approaches are developed: (i) the comparison of the temperature variation of T1for15N and for13Cαwhich shows the occurrence of concerted motion for linked15N and13C, nuclei along the peptide backbone; (ii) the correlation of15N relaxation data pairs (T1/NOE) by means of different molecular motion models to check which best fits with the experimental results. The isotropic overall motion model with internal libration appears most appropriate. The15N "antisymmetric" T1obtained with an INEPT-derived pulse sequence provide useful parameters to study the conformational preferences in free linear peptides, because of the sensitivity of these quantities to intramolecular exchange processes which are distance dependent. All the results support folded conformations for the free peptides I and III and a random one for the diprotected peptide II. © 1983.
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