HIGH-RESOLUTION SOLID-STATE C-13 AND N-15 NMR-STUDY ON CRYSTALLINE LEU-ENKEPHALIN AND MET-ENKEPHALIN - DISTINCTION OF POLYMORPHS - BACKBONEDYNAMICS, AND LOCAL CONFORMATIONAL REARRANGEMENTS INDUCED BY DEHYDRATION OR FREEZING OF MOTIONS OF BOUND SOLVENT MOLECULES
M. Kamihira et al., HIGH-RESOLUTION SOLID-STATE C-13 AND N-15 NMR-STUDY ON CRYSTALLINE LEU-ENKEPHALIN AND MET-ENKEPHALIN - DISTINCTION OF POLYMORPHS - BACKBONEDYNAMICS, AND LOCAL CONFORMATIONAL REARRANGEMENTS INDUCED BY DEHYDRATION OR FREEZING OF MOTIONS OF BOUND SOLVENT MOLECULES, JOURNAL OF PHYSICAL CHEMISTRY B, 102(15), 1998, pp. 2826-2834
We have recorded (13) and N-15 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(u
nspecified solvent) (3, crystallized from aqueous N,N-dimethylformamid
e (DMF)), and Metenkephalin . 5.3H(2)O (4. crystallized from aqueous e
thanol), at temperatures between 20 and -120 degrees C, to gain insigh
t into the distinction of these crystalline polymorphs, backbone dynam
ics, and local conformational rearrangements induced by dehydration or
freezing of motions of bound solvent molecules in the crystalline sta
te. Four kinds of crystal forms revealed by X-ray diffraction studies
were readily distinguished by C-13 NMR spectral patterns, and the numb
er of nonequivalent chains in a unit cell was counted by either N-15 o
r C-13 split peaks. NR IR signals of two types of independent chains f
or 1 were resolved at 0 degrees C but were not distinguishable at ambi
ent temperature owing to either two-sits exchange or phase transition.
Up to four chains. however, were distinguished for 2-4 from the C-13
and (15) NMR splittings of the backbone C-alpha, C=O, N-H. COO-, and/o
r side-chain Tyr C-zeta signals. Noticeable spectral changes were indu
ced at the C-13 NMR signals of the carboxyl group of the terminal Leu
residue and Gly C-alpha by dehydration. It turned out that a distinct
conformational change was accompanied with dehydration as manifested f
rom the shortened interatomic distance between the C-13=O and N-15-H g
roup. which is four bonds apart. In addition, pronounced spectral chan
ges 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 o
f the C-13 and proton spin-lattice relaxation times in the laboratory
(T-1(C)) and rotating frames (T-1p(H)), respectively. In particular, t
he beta-bend structure of crystalline Leu-enkephalin trihydrate turned
out to be very flexible in view of T-1(C) and T-1p(H) values as compa
red with the other crystalline forms taking the beta-sheet form. In co
ntrast, there appears no such flexibility in Leu-enkephalin dihydrate
despite a claimed similar beta-bend structure as determined by X-ray d
iffraction. It was also demonstrated that the presence or absence of s
ide-chain motions was conveniently monitored by relative peak intensit
ies of individual residues which were strongly influenced by the manne
r of local motions interfered with proton decoupling frequency. It was
further shown that these molecular motions were also strongly affecte
d by the bound solvent molecules.