The salts and esters of the unstable thiocyanic acid ( hydrofluoric acid ) HSCN are called thiocyanates (outdated also rhodanides ) . The name "Rhodanid" is derived from the Greek rhodos for "red", as iron (III) thiocyanate has a deep red color. The easiest way to produce the salts is to melt the corresponding cyanides with sulfur .
The thiocyanate ion (SCN - ) can coordinate to the central atom as a ligand in complexes both via the nitrogen and the sulfur atom. In its chemical behavior it is similar to the halogens and has therefore belonged to the group of pseudohalogens since 1925 . Decisive for the biological activity of thiocyanate ions is the variety of possible arrangements and distribution of its 16 electrons, which, in addition to ionic interactions, enter into coordinative bonds via NS ligator atoms in the form of mono- to pentagonal bonds as well as covalent or coordinative fixation to receptors and binding partners can.
The esters R − S − C≡N (R = organic radical, such as alkyl radical, aryl radical, etc.) of thiocyanic acid are called thiocyanic acid esters and are constitutional isomers of isothiocyanic acid esters RN = C = S. They smell like garlic and are not very persistent.
Thiocyanic acid esters can be synthesized from Bunte salts and sodium cyanide in one step.
Thiocyanate is ubiquitous in living nature with the peculiarity that it was already present in the prebiotic-chemical evolution and numerous SCN atom groups were detected even in interstellar space. The reason for this is that ammonium thiocyanate is formed from carbon disulfide and ammonia under pressure and at an elevated temperature (110 ° C). For the importance of thiocyanate in cell metabolism (see below), the exogenous and endogenous presence during the evolution of life may have been a prerequisite. In 1798 the chemist Buchholz discovered the reaction of sulfur with cyanides to form thiocyanate. Porett (1809) described the cooking of potassium sulphide and Prussian blue compound formed when sulfur hydrogen cyanide. In 1814 Gottfried Reinhold Treviranus described the red color of saliva when a saturated iron (III) salt solution in nitric or sulfuric acid was added . Tiedemann and Gmelin (1826) attributed this reaction to KSCN; thus the natural occurrence of SCN- in saliva was discovered. In 1829, Friedrich Wöhler first synthesized free hydrofluoric acid. The investigations by Hofmeister in 1888 were the beginning of the targeted investigation of the influence of SCN and other anions occurring in the organism on physiological processes. The human liver cells in particular produce thiocyanate by detoxifying the cyanides produced in the cell metabolism by the enzyme rhodanase (thiosulfate: cyanide sulfur transferase), which is located in the mitochondria and discovered by Lang . In the mitochondria as well as in the cytoplasm of the liver, kidneys, brain and myocardium there is another enzyme responsible for the breakdown of cyanide, the 3-mercaptopyruvate cyanide sulfur transferase . From some foods, such as cabbage, thiocyanates can be enzymatically released from the mustard oil glycosides contained therein , such as glucobrassicin , while other foods contain the thiocyanate directly. Abundant amounts of thiocyanate are absorbed, especially with lacto-vegetable diets. The view that thiocyanate inhibits iodine uptake in the thyroid could not be confirmed either experimentally or epidemiologically; essentially only the iodine loss is increased, and in the range of physiological concentrations the thyroid function is even stimulated. With dietary intake in physiological ranges of 4–8 mg SCN- / d), no thyroid inhibition is to be expected. Cyanide added with smoking is also detoxified to thiocyanate, so that tobacco consumption was detected earlier in this way.
Thiocyanate formation takes place in the so-called thiocyanate-cyanide cycle , in which there is an equilibrium with cyanide that is shifted to thiocyanate. When the thiocyanate serum level falls, activation occurs; The two transferases and thiosulfate reductase are involved in the formation . On the other hand, thiocyanate and hydrogen peroxide react by means of the enzymes lactoperoxidase , myeloperoxidase and eosinophilic peroxidase to form hypothiocyanite and higher oxidation products.
For a long time, thiocyanate was only seen as a detoxification product from cyanide without any physiological or biochemical significance of its own. Due to the ubiquitous presence of thiocyanate in all cells and body fluids of humans and mammals, the observed changes in concentration in the human organism z. B. in immunization, infection, stress, toxic exposure, UV radiation and certain diseases and the first evidence of stimulation of the humoral immune response carried out in 1968, intensive research into further effects of this bioactive anion began. As a result, the following effects could be confirmed at physiological doses within the physiological range: stimulation of wound healing, phagocytosis , spermogenesis , hair formation and interferon production as well as chemofusion in protoplasts . The stimulation is particularly pronounced with SCN deficiency or when there is an increased need. Thiocyanate also has an anti- inflammatory and protective effect in the event of infectious, allergic, toxic, irritative and mutagenic loads. The anti-infectious protective effect is based both on the promotion of colonization resistance and indirectly through the formation of hypothiocyanite. In the plant, vegetative development, yield and resistance to microorganisms are promoted and a protective effect is achieved in the event of toxic stress. The hypothiocyanites produced by oxidation are highly antimicrobial and essential for the microbial defense in the oral cavity, the respiratory tract, the tear fluid, the milk, in the vaginal secretion and other compartments .
Mechanism of action
Thiocyanate changes the conformation of so-called conformationally labile proteins depending on the type of ligand of the iron porphyrins . The activity-increasing effect on a number of drug-metabolizing enzymes and other enzymes such as thiocyanate in physiological concentrations is apparently also based on a change in conformation. B. collagenase, lysozyme, Na + -, K + -, Mg 2+ - and anion-sensitive ATPase (myelo- and lactoperoxidase), phosphodiesterase ; Via the latter, thiocyanate can influence growth and division processes via the “second messenger” cAMP . Further effects at the molecular level are the shifting of thermodynamic equilibria, the protection of SH groups, the loosening of H-bonds with an increase in entropy , the influence on the hydration and affinity of biomacromolecules (e.g. in antibodies and hormone receptors ), of cations - and anion transport processes, the increase in the transmembrane potential with the associated stabilization of the cell membrane and the modulation of transport processes, the inhibition of the formation of free radicals, the stabilization of DNA and the inhibition of oxidative metabolism. Through interactions with the hydrogen peroxide peroxidase systems, thiocyanate is involved in physiological cycle processes with different effects depending on the concentration, e.g. B. Influence of glycolysis and glucose transport , immune regulation, cytolytic lymphocyte activity with inhibition of inflammatory reactions and reduction of DR antigens on the cell surface. In vitro, thiocyanate ions have significant effects on glucocorticoid receptors. Apparently, the biological activity of thiocyanate ions is not based on a uniform mechanism of action, but is to be understood as the sum of various partial effects.
As sodium thiocyanate , thiocyanate is used in cosmetic or pharmaceutical applications as a hair restorer and for skin care in atopic dermatitis.
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- Patent DE4100975 : Cosmetic or pharmaceutical preparations for improving hair quality and promoting hair growth. Published on July 16, 1992 , inventors: Stefan Koch, Axel Kramer, Hans Meffert, Wolfgang Weuffen.