Background

L-Cysteine hydrochloride monohydrate (Figure 1) is a compound identified with the Chemical Abstracts Service (CAS) No 7048-04-6. It has a molecular mass of 175.6 g/mol. The molecular formula is C₃H₇NO₂S HCl H2O. The additive is an off-white crystalline powder, with a bulk density of 600-800 kg/m³, a pH (10% water solution) ranging from 0.8 to 1.2, and a solubility in water (at 25℃) of 50 g/L.[1]

L-cysteine has gathered a great attention on account of its high reactivity and numerous biological functions. It is known that L-cysteine is a pivotal amino acid in the binding site of the cysticfibrosis transmembrane regulator ion channel which is associated with the devastating effects of cystic fibrosis . L-cysteine is capable of forming chemical bonds with metal surfaces through its thiol side chain and provides the molecular hook between several biomolecules and metal surfaces. In addition, L-cysteine also participates in the biosynthesis of coenzyme A and may be essential for infants, the elderly and individuals with certain metabolic diseases. As the hydrochloride salt of L-cysteine, L-cysteine hydrochloride monohydrate is a residue within chloride ion channel proteins associated with disease. L-Cysteine hydrochloride monohydrate is implicated in the mutations of the CLCNKB (Chloride channel Kb) chloride channel which leads to Bartter's syndrome type III. L-Cysteine hydrochloride monohydrate is also one of the most efficient known radiation protection agents.[2]

Application as a flavouring additive for all animal species

L-Cysteine hydrochloride monohydrate has been assessed by the EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP) as a flavouring substance in pets only. The FEEDAP Panel also evaluated L-cysteine hydrochloride monohydrate in an opinion on the safety and efficacy of the use of amino acids (chemical group 34) when used as flavourings for all animal species. In both opinions, the FEEDAP Panel did not conclude on it produced by fermentation because no information on the production strains was submitted. It has not been assessed for use as food flavour. L-Cysteine hydrochloride monohydrate is listed in the European Union Register of Feed Additives as feed flavourings. L-Cysteine hydrochloride monohydrate produced by chemical synthesis or protein hydrolysis instead of using production strains is currently authorised as a sensory additive for use in all animal species except cats and dogs in accordance with Regulation (EU) 2018/2493 and it produced by hydrolysis of keratin from avian feathers is authorised in dogs and cats in accordance with Regulation (EU) 2015/2306. Since L-cysteine hydrochloride monohydrate is used in food as flavouring, it is to be expected that it can provide a similar function in feed and no further demonstration of efficacy is necessary when used at concentrations up to 25 mg/kg complete feed and the corresponding concentration in water.[1]

Application as photonic or opto-electronic material

Recently, Chapman et al. [3] reported that L-cysteine hydrochloride monohydrate belongs to the orthorhombic crystal system with space group P2₁2₁2₁ and unit-cell parameters a = 5.4588 (9), b = 7.1570 (11), c = 19.389 (3) A? . [Note that there is another organic compound with a very similar name, l-cystine hydrochloride, but its molecular formula is C₆H₁₂N₂O₄S₂·HCl and it crystallizes in a monoclinic structure.] The reported space group of L-Cys HCl·H2O indicates that the crystal is noncentrosymmetric and may possess Nonlinear optical properties.

A bulk size (~60 mm length and 20 mm diameter) single crystal of L-cysteine hydrochloride monohydrate has been successfully grown for the first time by the unidirectional Sankaranarayanan–Ramasamy method and found to be a nonlinear optical material by observing the second harmonic generation (SHG). The crystal structure has been confirmed by single-crystal as well as powder X-ray diffraction analysis. The Fourier transform–Raman technique was used to study the vibrational modes and functional groups. The grown crystal was found to be highly transparent. The crystalline perfection has been evaluated by high-resolution X-ray diffractometry and the grown crystal was found to be free from macroscopic defects such as structural grain boundaries. The relative SHG efficiency and laser damage threshold values were found to be 1.2 and 5.5 times that of a potassium dihydrogen phosphate crystal, respectively. The dielectric constant, dielectric loss and alternating current conductivity were measured by an impedance analyser over a wide range of frequency between 100 Hz and 5 MHz. The dielectric constant was found to be stable for the entire range of frequency with very small dielectric loss, making this material suitable for device applications. The present investigation reveals that the title compound, since it can be grown with desirable size along a predetermined direction and it is a good nonlinear optical material with the prerequisite optical and dielectric properties, is a good candidate for photonic or opto-electronic applications.[3]

References

[1] EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), Bampidis V, Azimonti G, et al. Safety and efficacy of l-cysteine hydrochloride monohydrate produced by fermentation using Escherichia coli KCCM 80180 and Escherichia coli KCCM 80181 as a flavouring additive for all animal species. EFSA J. 2020;18(2):e06003. Published 2020 Feb 10. doi:10.2903/j.efsa.2020.6003

[2] Ren G, Zong S, Zhu Z, et al. Far-infrared terahertz properties of L-cysteine and its hydrochloride monohydrate. Spectrochim Acta A Mol Biomol Spectrosc. 2020;225:117476. doi:10.1016/j.saa.2019.117476

[3] Bhagavannarayana G , Kumar S , Shakir M ,et al. Unidirectional growth of L-cysteine hydrochloride monohydrate: first time observation as nonlinear optical material and its characterization[J].Journal of Applied Crystallography, 2010, 43(4):710-715.DOI:10.1107/S0021889810016870.