The interatomic distances in the crystalline specimen of C-13, N-15 doubly labeled peptides [1-C-13]N-acetyl-Pro-[N-15]Gly-Phe (I), N-acetyl-[1-C-13]Pro-Gly-[N-15]Phe (II), and [1-C-13]-N-acetyl-Pro-Gly-[N-15]Phe (III) evaluated from rotational echo double resonance (REDOR) data were compared with those from X-ray diffraction studies and justify our novel approach. The minimization of B-1 inhomogeneity was critical to obtain accurate distances, which were achieved by confinement of the samples in the central portion (50% of the total filling volume of the rotor). The effect of the finite length of the pi pulse was found to be negligible as long as the pulse length is less than 10% of the rotor cycle. The C-13 ...N-15 distances obtained from C-13 REDOR were thus 3.24 +/- 0.05, 3.43 +/- 0.05, and 4.07 +/- 0.05 Angstrom for I, II, and III, respectively. The REDOR-derived conformation of this peptide was beta-turn type I, consistent with our X-ray diffraction study (orthorhombic crystal). The maximum deviation of the distances determined by NMR and X-ray diffraction is 0.08 Angstrom despite the complete neglect of the dipolar interactions with the labeled nuclei of neighboring molecules and natural abundance nuclei. The precision and accuracy given by C-13 REDOR experiments are on the order of +/- 0.05 Angstrom. Distinction between the two types of beta-turn forms including the beta-turn type II found in the monoclinic crystal of this peptide whose interatomic distances are different by about 0.57 Angstrom is made possible only by the very accurate REDOR measurement. Finally, we found that the three-dimensional structure of this peptide was well reproduced by a molecular dynamics simulation by taking into account all the intermolecular interactions in the crystals.