Lattice vibrations in crystalline L-alanine

Area of Science:

• Solid-state physics
• Biophysics
• Materials science

Background:

• Zwitterionic L-alanine crystals possess intricate hydrogen-bonding and methyl-methyl interactions.
• Low-frequency lattice vibrations in these crystals involve correlated intermolecular motions on the picosecond timescale.
• Characterizing these vibrations offers insights into nonbonded interactions in peptides and proteins.

Purpose of the Study:

• To characterize low-frequency lattice vibrations in zwitterionic L-alanine crystals.
• To investigate the relationship between these vibrations and intermolecular interactions.
• To provide a foundation for understanding peptide and protein dynamics.

Main Methods:

• Coherent inelastic neutron scattering experiments were performed.
• Computer simulations using the CHARMM program were employed for normal mode analysis.
• Phonon dispersion relations for acoustic and low-frequency optic modes were determined.

Main Results:

• Evidence of interaction between the two lowest frequency optical phonons and the longitudinal acoustic mode was observed.
• Anisotropic sound velocity was found and correlated with the crystal's hydrogen-bonding arrangement.
• Calculated phonon dispersion relations showed good agreement with experimental data, despite some frequency overestimation.

Conclusions:

• Lattice vibrations in L-alanine crystals are strongly influenced by hydrogen bonding.
• Neutron scattering and simulation techniques effectively probe intermolecular dynamics.
• The study enhances understanding of vibrational properties and nonbonded interactions in amino acid crystals.