Vibrational and Structural Behavior of l‑Cysteine Ethyl Ester Hydrochloride in the Solid State and in Aqueous Solution

The aim of this work is to evaluate
the vibrational and structural properties of l-cysteine ethyl
ester hydrochloride (CE), and its electronic behavior mainly in relation
to the action of the thiol and amine groups at different degrees of
solvation. The crystal structure of CE was determined at room temperature
by X-ray diffraction methods. Infrared and Raman spectra were collected
to compare the behavior of different functional groups in the molecule,
both in the solid phase and in aqueous solution. Its UV and circular
dichroism spectra were also measured in aqueous solution. The influence
of an aqueous environment on the CE spectra was simulated by means
of implicit (polarizable continuum model) and explicit (molecular
dynamics, solute–solvent clusters) methods. Calculations in
explicit and continuous solvent are of interest to explain the behavior
of bioavailable sites in this medium. The study was completed by natural
bond orbital analysis to determine the presence of hyperconjugative
interactions.