DETERMINATION OF THE 3-DIMENSIONAL STRUCTURE OF A NEW CRYSTALLINE FORM OF N-ACETYL-PRO-GLY-PHE AS REVEALED BY C-13 REDOR, X-RAY-DIFFRACTION, AND MOLECULAR-DYNAMICS CALCULATION

The interatomic distances in the crystalline specimen of C-13, N-15 do ubly 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]P he (III) evaluated from rotational echo double resonance (REDOR) data were compared with those from X-ray diffraction studies and justify ou r novel approach. The minimization of B-1 inhomogeneity was critical t o obtain accurate distances, which were achieved by confinement of the samples in the central portion (50% of the total filling volume of th e 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 rot or cycle. The C-13 ...N-15 distances obtained from C-13 REDOR were thu s 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 (ort horhombic crystal). The maximum deviation of the distances determined by NMR and X-ray diffraction is 0.08 Angstrom despite the complete neg lect of the dipolar interactions with the labeled nuclei of neighborin g molecules and natural abundance nuclei. The precision and accuracy g iven by C-13 REDOR experiments are on the order of +/- 0.05 Angstrom. Distinction between the two types of beta-turn forms including the bet a-turn type II found in the monoclinic crystal of this peptide whose i nteratomic distances are different by about 0.57 Angstrom is made poss ible only by the very accurate REDOR measurement. Finally, we found th at the three-dimensional structure of this peptide was well reproduced by a molecular dynamics simulation by taking into account all the int ermolecular interactions in the crystals.