Different Dynamics of Chiral and Racemic (l- and dl-) Serine Crystals: Evidenced by Incoherent Inelastic Neutron and Raman Scattering

Abstract

Interest in chiral versus racemic amino acid forms stems from their different solid-state properties associated to the packing arrangement that leads to starkly different physical behavior. Here we focus on the different dynamic properties of the chiral (l-form) and racemic (dl-form) crystals of serine, (C3H7NO3). Measuring incoherent inelastic neutron and Raman scattering over a wide temperature range we could assess a subtle dynamical transition in l-serine near 150 K. The difference in the dynamic properties of the crystals of l- and dl-serine evidenced from our spectroscopic studies is in a good agreement with the different CP(T) dependences and different structural strain of the two forms on cooling. Despite a higher bulk density, the molecular fragments in the crystals of l-serine are more dynamic than in dl-serine, and this manifests itself in the incoherent inelastic neutron and Raman spectra, as well as in a higher heat capacity, a higher thermal expansion coefficient, or in the NMR-spectra.