High-Resolution Solid-State 13C and 15N NMR Study on Crystalline Leu- and Met-enkephalins: Distinction of Polymorphs, Backbone Dynamics, and Local Conformational Rearrangements Induced by Dehydration or Freezing of Motions of Bound Solvent Molecules

We have recorded 13 C and 15 N high-resolution solid-state NMR spectra of Leu-enkephalin trihydrate ( 1, crystallized from aqueous solution), dihydrate ( 2, crystallized from aqueous methanol), −2H 2 O, 2DMF, X(unspecified solvent) ( 3, crystallized from aqueous N, N -dimethylformamide (DMF)), and Met-enkephalin·5.3H 2 O ( 4, crystallized from aqueous ethanol), at temperatures between 20 and −120 °C, to gain insight into the distinction of these crystalline polymorphs, backbone dynamics, and local conformational rearrangements induced by dehydration or freezing of motions of bound solvent molecules in the crystalline state. Four kinds of crystal forms revealed by X-ray diffraction studies were readily distinguished by 13 C NMR spectral patterns, and the number of nonequivalent chains in a unit cell was counted by either 15 N or 13 C split peaks. NMR signals of two types of independent chains for 1 were resolved at 0 °C but were not distinguishable at ambient temperature owing to either two-site exchange or phase transition. Up to four chains, however, were distinguished for 2 − 4 from the 13 C and 15 N NMR splittings of the backbone C α, C O, N−H, COO -, and/or side-chain Tyr C ζ signals. Noticeable spectral changes were induced at the 13 C NMR signals of the carboxyl group of the terminal Leu residue and Gly C α by dehydration. It turned out that a distinct conformational change was accompanied with dehydration as manifested from the shortened interatomic distance between the 13 C O and 15 N−H group, which is four bonds apart. In addition, pronounced spectral changes were noted when motions of bound solvent molecules in the crystals were frozen at lower temperatures. They were interpreted in terms of induced local conformational rearrangement, especially at the N- or C-terminus to which these solvent molecules were coordinated. We further examined the manner of the backbone and side-chain motions by means of the 13 C and proton spin−lattice relaxation times in the laboratory ( T 1 C ) and rotating frames ( T 1 ρ H ), respectively. In particular, the β-bend structure of crystalline Leu-enkephalin trihydrate turned out to be very flexible in view of T 1 C and T 1 ρ H values as compared with the other crystalline forms taking the β-sheet form. In contrast, there appears no such flexibility in Leu-enkephalin dihydrate despite a claimed similar β-bend structure as determined by X-ray diffraction. It was also demonstrated that the presence or absence of side-chain motions was conveniently monitored by relative peak intensities of individual residues which were strongly influenced by the manner of local motions interfered with proton decoupling frequency. It was further shown that these molecular motions were also strongly affected by the bound solvent molecules.