The phenyl ring dynamics of [H-2(5)]Phe(4)-labeled Leu(5)- and Met(5)-enkephalin molecules in crystals grown from four solvents were examined using solid state H-2 NMR spectroscopy. H-2 NMR powder patterns clearly indicated the presence of 180 degrees flip motions about the C-beta-C-gamma, bond axis of the phenyl rings. Frequencies of the 180 degrees flip motions were estimated to be 5.0 x 10(3), 3.0 x 10(4), and 2.4 x 10(6) Hz for Leu(5)-enkephalin crystallized from H2O, methanol/H2O, and N,N-dimethylformamide (DMF)/H2O, respectively and 1.0 x 10(4) Hz for Met(5)-enkephalin crystallized from ethanol/H2O at ambient temperature. The difference of the frequencies for the motion was attributed to the manner of their moleculer packing in the crystals as determined by X-ray diffraction. Because the correlation times determined from the H-2 spin-lattice relaxation times (T-1(D) values) were much shorter than those of the 180 degrees flip motions, it was shown that the phenyl rings of these four crystals have small amplitude librations. Therefore, we concluded that the T-1(D) values were dominated by the librations, even for the ring deuterium. These motions became slower at lowered temperatures and caused the change of the peak intensities and increased quadrupole splittings which were observed in each H-2 NMR spectra. Isotropic sharp signals due to naturally abundant solvent molecules were observed at the center of the H-2 NMR spectra. The stepwise loss of the signal intensity was interpteted in terms of differential temperatures of freezing of motions of both bound water or organic solvent and the mixed solvent as the temperature was lowered, consistent with the buildup of solvent peaks in the C-13 CP-MAS NMR spectra. It is suggested that there are a number of bound mobile solvent molecules in the crystals and the freezing of the solvents causes considerable changes in the conformations and dynamics of enkephalin molecules.