Dynamics of crystalline acetanilide: Analysis using neutron scattering and computer simulation

Abstract

The unusual temperature dependence of several optical spectroscopic vibrational bands in crystalline acetanilide has been interpreted as providing evidence for dynamic localization. Here we examine the vibrational dynamics of crystalline acetanilide over a spectral range of ϳ20-4000 cm Ϫ1 using incoherent neutron scattering experiments, phonon normal mode calculations and molecular dynamics simulations. A molecular mechanics energy function is parametrized and used to perform the normal mode analyses in the full configurational space of the crystal i.e., including the intramolecular and intermolecular degrees of freedom. One-and multiphonon incoherent inelastic neutron scattering intensities are calculated from harmonic analyses in the first Brillouin zone and compared with the experimental data presented here. Phonon dispersion relations and mean-square atomic displacements are derived from the harmonic model and compared with data derived from coherent inelastic neutron scattering and neutron and x-ray diffraction. To examine the temperature effects on the vibrations the full, anharmonic potential function is used in molecular dynamics simulations of the crystal at 80, 140, and 300 K. Several, but not all, of the spectral features calculated from the molecular dynamics simulations exhibit temperature-dependent behavior in agreement with experiment. The significance of the results for the interpretation of the optical spectroscopic results and possible improvements to the model are discussed.

High-Resolution Vibrational Inelastic Neutron Scattering: A New Spectroscopic Tool for Globular Proteins

“…It is of interest to compare the present experimental value for the methyl torsional frequency with that previously documented in ACN. 17 In ACN the torsion is found at 145 cm -1 both experimentally and in a vibrational analysis of the crystal using the same form of potential function as in the present work. The rotational barrier associated with the ACN sp 2 -sp 3 methyl torsion in the above theoretical analysis is ∼1 kcal/mol, significantly lower than that expected (∼3 kcal/mol) for the aliphatic sp 3 -sp 3 methyl rotations in protein side chains.…”

“…However, the intensity should in principle be nonzero in this region due to an approximately monotonic multiphonon scattering contribution, as illustrated in recent calculations up to the three-phonon level on a molecular crystal, acetanilide (CH 3 NHCOC 6 H 5 , ACN). 17 Differences between the theoretical and experimental spectrum in the region below 100 cm -1 are not significant as the experimental data suffer from poor counting statistics and low energy resolution. An analysis of neutron scattering by a small globular protein in this frequency range is given in ref 7.…”

“…Improvements similar to the above were undertaken in the recent analysis of the TFXA spectrum of crystalline acetanilide. 17 The inclusion of bound water molecules in the SNase normal mode analysis might also produce improved agreement, particularly in the O-D‚‚‚O hydrogen-bond bend region. The water content of the present sample is equivalent to about 40 water molecules per protein.…”

Painting biological low-frequency vibrational modes from small peptides to proteins

“…Neutron scattering investigations on NMA have also shown a band at 4800 GHz that involves the motion of both methyl groups. 62 More recently, Smith and coworkers have analyzed the VDOS of methyl groups from small molecules and proteins, [29][30][31][32][33] assigning the vibrational band observed between 1500-3750 GHz to a librational-type motion of the methyl hydrogens and the band around 6900-9300 GHz to hindered torsions of methyl groups. 29 This suggests a preliminary assignment of NALMA vibrational modes in the 1-5 THz frequency range to methyl groups' librations; those between 5 and 10 THz to CH 3 hindered torsions.…”

“…29 They are also thought to be sensitive to their local microenvironment. [29][30][31][32][33] The other region of interest in the low-frequency spectra of proteins using neutron scattering (NS) and depolarized light scattering (DLS) is a broad band occurring in an energy range of B350 GHz and B1 THz, which is sensitive to temperature and hydration conditions. 34 This is generally interpreted as the Boson peak (BP).…”