Determination of the Three-Dimensional Structure of a New Crystalline Form of N-Acetyl-Pro-Gly-Phe As Revealed by 13C REDOR, X-Ray Diffraction, and Molecular Dynamics Calculation

Abstract

The interatomic distances in the crystalline specimen of 13C,15N doubly labeled peptides [1-13C]N-acetyl-Pro-[15N]Gly-Phe (I), N-acetyl-[1-13C]Pro-Gly-[15N]Phe (II), and [1-13C]N-acetyl-Pro-Gly-[15N]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 B1 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 π pulse was found to be negligible as long as the pulse length is less than 10% of the rotor cycle. The 13C···15N distances obtained from 13C REDOR were thus 3.24 ± 0.05, 3.43 ± 0.05, and 4.07 ± 0.05 A for I, II, and III, respectively. The REDOR-derived conformation of this peptide was β-turn type I, consistent with our X-ray diffraction study (orthorhombic crystal). The maximum deviation of the distances determined by...