Observation of subtle dynamic transitions by a combination of neutron scattering, X-ray diffraction and DSC: A case study of the monoclinic l-cysteine

The paper illustrates the benefit of combining several experimental techniques (incoherent elastic and inelastic neutron scattering, DSC, and X-ray diffraction) to study subtle dynamic transitions in a biologically important system, probing a broad time (frequency) range of the molecular motions in a wide temperature interval of 2-300 K. As a case study the crystalline form (a monoclinic polymorph) of l-cysteine (+NH3-CH(CH2SH)-COO−) - an essential amino acid - has been selected. Crystals of amino acids are widely used to mimic important structural and dynamic features of peptides. The conformational lability of cysteine and the dynamics of the thiol-side chains are known to account for various phase transitions in the crystalline state and for the conformational transitions important for the biological function in the peptides. The effect of temperature on the monoclinic polymorph of l-cysteine, metastable at ambient conditions, has been studied for the first time. A dynamical transition at about 150 K, marking a crossover of the molecular fluctuations between harmonic and non-harmonic dynamical regimes, was evidenced by evaluating the evolution of the mean-square displacement obtained from the elastic fixed window approach using the backscattering spectrometer IN10 located at the ILL. Although this transition does not manifest itself in the DSC, it was clearly observed by incoherent inelastic neutron scattering. By analyzing the dynamical susceptibility contribution (χ″(ω)) obtained using IN6 also at ILL we were able to evidence another relaxation process at a different time scale. The disordered soft l-cysteine structure has an excess of inelastic scattering at about 3 meV, analogous to the “boson peak” observed in glass-like materials and proteins. High-precision X-ray diffraction has revealed an anomaly in the changes of selected unit cell parameters and volume at about 240 K.