Like many other disinfectants, hypochlorous acid solutions will destroypathogens, such asCOVID-19, adsorbed on surfaces.[8] In low concentrations, such solutions can serve to disinfectopen wounds.[9]
Hypochlorous acid was discovered in 1834 by the French chemistAntoine Jérôme Balard (1802–1876) by adding a dilute suspension ofmercury(II) oxide in water to a flask of chlorine gas.[10] He also named the acid and its compounds.[11]
Hypochlorous acid is relatively easy to make, but it is difficult to maintain a stable solution. It is not until recent years that scientists have been able to cost-effectively produce and maintain hypochlorous acid water for stable commercial use.
In medicine, hypochlorous acid water has been used as a disinfectant and sanitiser.[6][9][5]
Inwound care,[16][17][18] and as of early 2016 the U.S. Food and Drug Administration has approved products whose main active ingredient is hypochlorous acid for use in treating wounds and various infections in humans and pets. It is also FDA-approved as a preservative for saline solutions.
In disinfection, it has been used in the form of liquid spray, wet wipes and aerosolised application. Recent studies have shown hypochlorous acid water to be suitable for fog and aerosolised application for disinfection chambers and suitable for disinfecting indoor settings such as offices, hospitals and healthcare clinics.[19]
In food service and water distribution, specialized equipment to generate weak solutions of HClO from water and salt is sometimes used to generate adequate quantities of safe (unstable) disinfectant to treat food preparation surfaces and water supplies.[20][21] It is also commonly used in restaurants due to its non-flammable and nontoxic characteristics.
In water treatment, hypochlorous acid is the active sanitizer in hypochlorite-based products (e.g. used in swimming pools).[22]
Similarly, in ships and yachts, marine sanitation devices[23] use electricity to convert seawater into hypochlorous acid to disinfect macerated faecal waste before discharge into the sea.
In deodorization, hypochlorous acid has been tested to remove up to 99% of foul odours including garbage, rotten meat, toilet, stool, and urine odours.[citation needed]
When acids are added to aqueous salts of hypochlorous acid (such as sodium hypochlorite in commercial bleach solution), the resultant reaction is driven to the left, and chlorine gas is formed. Thus, the formation of stable hypochlorite bleaches is facilitated by dissolving chlorine gas into basic water solutions, such assodium hydroxide.
The acid can also be prepared by dissolvingdichlorine monoxide in water; under standard aqueous conditions, anhydrous hypochlorous acid is currently impossible to prepare due to the readily reversible equilibrium between it and its anhydride:[25]
The presence of light or transition metal oxides ofcopper,nickel, orcobalt accelerates the exothermic[dubious –discuss] decomposition into hydrochloric acid andoxygen:[25]
Knoxet al.[27] first noted that HClO is asulfhydryl inhibitor that, in sufficient quantity, could completely inactivate proteins containingsulfhydryl groups. This is because HClO oxidises sulfhydryl groups, leading to the formation ofdisulfide bonds[28] that can result in crosslinking ofproteins. The HClO mechanism of sulfhydryl oxidation is similar to that ofmonochloramine, and may only be bacteriostatic, because once the residual chlorine is dissipated, some sulfhydryl function can be restored.[29] One sulfhydryl-containing amino acid can scavenge up to four molecules of HClO.[30] Consistent with this, it has been proposed that sulfhydryl groups of sulfur-containingamino acids can be oxidized a total of three times by three HClO molecules, with the fourth reacting with the α-amino group. The first reaction yieldssulfenic acid (R−S−OH) thensulfinic acid (R−S(=O)−OH) and finallyR−S(=O)2−OH. Sulfenic acids form disulfides with another protein sulfhydryl group, causing cross-linking and aggregation of proteins. Sulfinic acid andR−S(=O)2−OH derivatives are produced only at high molar excesses of HClO, and disulfides are formed primarily at bacteriocidal levels.[31] Disulfide bonds can also be oxidized by HClO to sulfinic acid.[28] Because the oxidation of sulfhydryls anddisulfides evolveshydrochloric acid,[31] this process results in the depletion HClO.
Hypochlorous acid reacts readily with amino acids that haveamino group side-chains, with the chlorine from HClO displacing a hydrogen, resulting in an organic chloramine.[32] Chlorinatedamino acids rapidly decompose, butprotein chloramines are longer-lived and retain some oxidative capacity.[14][30] Thomaset al.[14] concluded from their results that most organic chloramines decayed by internal rearrangement and that fewer availableNH2 groups promoted attack on thepeptide bond, resulting in cleavage of theprotein. McKenna and Davies[33] found that 10 mM or greater HClO is necessary to fragment proteins in vivo. Consistent with these results, it was later proposed that the chloramine undergoes a molecular rearrangement, releasingHCl andammonia to form analdehyde.[34] Thealdehyde group can further react with anotheramino group to form aSchiff base, causing cross-linking and aggregation of proteins.[35]
Hypochlorous acid reacts slowly with DNA and RNA as well as allnucleotides in vitro.[36][37]GMP is the most reactive because HClO reacts with both the heterocyclic NH group and the amino group. In similar manner,TMP with only a heterocyclic NH group that is reactive with HClO is the second-most reactive.AMP andCMP, which have only a slowly reactive amino group, are less reactive with HClO.[37]UMP has been reported to be reactive only at a very slow rate.[15][36] The heterocyclic NH groups are more reactive than amino groups, and their secondary chloramines are able to donate the chlorine.[31] These reactions likely interfere with DNA base pairing, and, consistent with this, Prütz[37] has reported a decrease in viscosity of DNA exposed to HClO similar to that seen with heat denaturation. The sugar moieties are nonreactive and the DNA backbone is not broken.[37] NADH can react with chlorinated TMP and UMP as well as HClO. This reaction can regenerate UMP and TMP and results in the 5-hydroxy derivative of NADH. The reaction with TMP or UMP is slowly reversible to regenerate HClO. A second slower reaction that results in cleavage of the pyridine ring occurs when excess HClO is present.NAD+ is inert to HClO.[31][37]
Hypochlorous acid reacts withunsaturated bonds inlipids, but notsaturated bonds, and theClO− ion does not participate in this reaction. This reaction occurs byhydrolysis with addition ofchlorine to one of the carbons and ahydroxyl to the other. The resulting compound is a chlorohydrin.[38] The polar chlorine disruptslipid bilayers and could increase permeability.[39] When chlorohydrin formation occurs in lipid bilayers of red blood cells, increased permeability occurs. Disruption could occur if enough chlorohydrin is formed.[38][40] The addition of preformed chlorohydrin to red blood cells can affect permeability as well.[41]Cholesterol chlorohydrin have also been observed,[39][42] but do not greatly affect permeability, and it is believed thatCl2 is responsible for this reaction.[42] Hypochlorous acid also reacts with a subclass ofglycerophospholipids calledplasmalogens, yielding chlorinated fattyaldehydes which are capable of protein modification and may play a role in inflammatory processes such asplatelet aggregation and the formation ofneutrophil extracellular traps.[43][44][45]
E. coli exposed to hypochlorous acidlose viability in less than 0.1 seconds due to inactivation of many vital systems.[24][46][47][48][49] Hypochlorous acid has a reportedLD50 of 0.0104–0.156 ppm[50] and 2.6 ppm caused 100% growth inhibition in 5 minutes.[33] However, the concentration required for bactericidal activity is also highly dependent on bacterial concentration.[27]
In 1948, Knoxet al.[27] proposed the idea that inhibition ofglucose oxidation is a major factor in the bacteriocidal nature of chlorine solutions. They proposed that the active agent or agents diffuse across the cytoplasmic membrane to inactivate keysulfhydryl-containingenzymes in theglycolytic pathway. This group was also the first to note that chlorine solutions (HClO) inhibitsulfhydrylenzymes. Later studies have shown that, at bacteriocidal levels, thecytosol components do not react with HClO.[51] In agreement with this, McFeters and Camper[52] found thataldolase, anenzyme that Knoxet al.[27] proposes would be inactivated, was unaffected by HClOin vivo. It has been further shown that loss ofsulfhydryls does not correlate with inactivation.[29] That leaves the question concerning what causes inhibition ofglucose oxidation. The discovery that HClO blocks induction ofβ-galactosidase by addedlactose[53] led to a possible answer to this question. The uptake of radiolabeled substrates by both ATP hydrolysis and protonco-transport may be blocked by exposure to HClO preceding loss of viability.[51] From this observation, it proposed that HClO blocks uptake of nutrients by inactivating transport proteins.[54][51][52][55] The question of loss of glucose oxidation has been further explored in terms of loss of respiration. Venkobacharet al.[56] found that succinic dehydrogenase was inhibited in vitro by HClO, which led to the investigation of the possibility that disruption ofelectron transport could be the cause of bacterial inactivation. Albrichet al.[15] subsequently found that HClO destroyscytochromes andiron-sulfur clusters and observed that oxygen uptake is abolished by HClO and adenine nucleotides are lost. It was also observed that irreversible oxidation ofcytochromes paralleled the loss of respiratory activity. One way of addressing the loss of oxygen uptake was by studying the effects of HClO on succinate-dependentelectron transport.[57] Rosenet al.[49] found that levels of reductablecytochromes in HClO-treated cells were normal, and these cells were unable to reduce them. Succinate dehydrogenase was also inhibited by HClO, stopping the flow of electrons to oxygen. Later studies[47] revealed that Ubiquinol oxidase activity ceases first, and the still-activecytochromes reduce the remaining quinone. Thecytochromes then pass theelectrons tooxygen, which explains why thecytochromes cannot be reoxidized, as observed by Rosenet al.[49] However, this line of inquiry was ended when Albrichet al.[58] found that cellular inactivation precedes loss of respiration by using a flow mixing system that allowed evaluation of viability on much smaller time scales. This group found that cells capable of respiring could not divide after exposure to HClO.
Having eliminated loss of respiration, Albrichet al.[58] proposes that the cause of death may be due to metabolic dysfunction caused by depletion of adenine nucleotides. Barretteet al.[53] studied the loss of adenine nucleotides by studying the energy charge of HClO-exposed cells and found that cells exposed to HClO were unable to step up their energy charge after addition of nutrients. The conclusion was that exposed cells have lost the ability to regulate their adenylate pool, based on the fact that metabolite uptake was only 45% deficient after exposure to HClO and the observation that HClO causes intracellular ATP hydrolysis. It was also confirmed that, at bacteriocidal levels of HClO, cytosolic components are unaffected. So it was proposed that modification of some membrane-bound protein results in extensive ATP hydrolysis, and this, coupled with the cells inability to remove AMP from the cytosol, depresses metabolic function. One protein involved in loss of ability to regenerate ATP has been found to beATP synthetase.[54] Much of this research on respiration reconfirms the observation that relevant bacteriocidal reactions take place at the cell membrane.[54][53][59]
Recently it has been proposed that bacterial inactivation by HClO is the result of inhibition ofDNA replication. When bacteria are exposed to HClO, there is a precipitous decline inDNA synthesis that precedes inhibition ofprotein synthesis, and closely parallels loss of viability.[33][60] During bacterial genome replication, theorigin of replication (oriC inE. coli) binds to proteins that are associated with the cell membrane, and it was observed that HClO treatment decreases the affinity of extracted membranes for oriC, and this decreased affinity also parallels loss of viability. A study by Rosenet al.[61] compared the rate of HClO inhibition of DNA replication of plasmids with different replication origins and found that certain plasmids exhibited a delay in the inhibition of replication when compared to plasmids containing oriC. Rosen's group proposed that inactivation of membrane proteins involved in DNA replication are the mechanism of action of HClO.
HClO is known to cause post-translational modifications toproteins, the notable ones beingcysteine andmethionine oxidation. A recent examination of HClO's bactericidal role revealed it to be a potent inducer of protein aggregation.[62] Hsp33, a chaperone known to be activated by oxidative heat stress, protects bacteria from the effects of HClO by acting as aholdase, effectively preventing protein aggregation. Strains ofEscherichia coli andVibrio cholerae lacking Hsp33 were rendered especially sensitive to HClO. Hsp33 protected many essential proteins from aggregation and inactivation due to HClO, which is a probable mediator of HClO's bactericidal effects.
Solutions of hypochlorites can be produced in-situ by electrolysis of an aqueous sodium chloride solution in both batch and flow processes.[63] The composition of the resulting solution depends on the pH at the anode. In acid conditions the solution produced will have a high hypochlorous acid concentration, but will also contain dissolved gaseous chlorine, which can be corrosive; at a neutral pH the solution will be around 75% hypochlorous acid and 25% hypochlorite. Some of the chlorine gas produced will dissolve forming hypochlorite ions. Hypochlorites are also produced by thedisproportionation of chlorine gas in alkaline solutions.
HClO is classified as non-hazardous by theEnvironmental Protection Agency in the US. As an oxidising agent, it can be corrosive or irritant depending on its concentration and pH.
In a clinical test, hypochlorous acid water was tested for eye irritation, skin irritation, and toxicity. The test concluded that it was non-toxic and non-irritating to the eye and skin.[64]
In a 2017 study, a saline hygiene solution preserved with pure hypochlorous acid was shown to reduce the bacterial load significantly without altering the diversity of bacterial species on the eyelids. After 20 minutes of treatment, there was more than 99% reduction of theStaphylococci bacteria.[65]
Commercial disinfection applications remained elusive for a long time after the discovery of hypochlorous acid because the stability of its solution in water is difficult to maintain. The active compounds quickly deteriorate back into salt water, losing the solution its disinfecting capability, which makes it difficult to transport for wide use. It is less commonly used as a disinfectant compared to bleach and alcohol due to cost, despite its stronger disinfecting capabilities.
Technological developments have reduced manufacturing costs and allow for manufacturing and bottling of hypochlorous acid water for home and commercial use. However, most hypochlorous acid water has a short shelf life. Storing away from heat and direct sunlight can help slow the deterioration. The further development of continuous flow electrochemical cells has been implemented in new products, allowing the commercialisation of domestic and industrial continuous flow devices for the in-situ generation of hypochlorous acid for disinfection purposes.[66]
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