Hypochlorous acid

Chloric(I) acid is a weak disinfectant.

Formation

Addition of hydrochloric acid^[2] (HCl):

Cl₂ + H₂O → HOCl + HCl

Uses

In alkenes to chlorohydrins.^[3]

In biology, hypochlorous acid is generated in activated neutrophils by myeloperoxidase mediated peroxidation of chloride ions, and contributes to the destruction of bacteria.^[4][5][6]

Hypochlorous acid is the active sanitizer in hypochlorite based swimming pool products.

Chemical reactions

In aqueous solution, hypochlorous acid partially dissociates into the anion hypochlorite ClO^-:

HOCl OCl^- + H⁺

NaClO, the active ingredient in bleach.

In the presence of sunlight, hypochlorous acid decomposes into oxygen, so this reaction is sometimes seen as:

2Cl₂ + 2H₂O → 4HCl + O₂

HOCl is considered to be a stronger oxidant than chlorine.

Reactivity of HOCl with biomolecules

Hypochlorous acid reacts with a wide variety of biomolecules including DNA, RNA,^[6][7][8][9] fatty acid groups, cholesterol^{[10][11][12][13][14][15][16][17]} and proteins.^{[2][18][13][19][20][21][22]}

Reaction with protein sulfhydral groups

Knox et al.^[20] first noted that HOCl was a hydrochloric acid,^[9] this process results in the depletion HOCl.

Reaction with protein amino groups

Hypochlorous acid reacts readily with amino acids that have Schiff base causing cross linking and aggregation of proteins.^[13]

Reaction with DNA and Nucleotides

Hypochlourous acid reacts slowly with DNA and RNA as well as all nucleotides in vitro.^[7][27] GMP is the most reactive because HOCl reacts with both the heterocyclic NH group and the amino group. Similarly TMP with only a heterocyclic NH group that is reactive with HOCl is the second most reactive. AMP and CMP which only have a slowly reactive amino group are less reactive with HOCl.^[27] UMP has been reported to be reactive only at a very slow rate.^[6][7] The heterocyclic NH groups are more reactive than amino groups and their secondary chloramines are able to donate the chlorine.^[9] These reactions likely interfere with DNA base pairing and consistent with this, Prütz^[27] has reported a decrease in viscosity of DNA exposed to HOCl similar to that seen with heat denaturation. The sugar moieties are unreactive and the DNA backbone is not broken.^[27] NADH can react with chlorinated TMP and UMP as well as HOCl. 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 HOCl. A second slower reaction that results in cleavage of the pyridine ring occurs when excess HOCl is present. NAD+ is inert to HOCl.^[27][9]

Reaction with lipids

Hypochlorous acid reacts with unsaturated bonds in Cl₂ is responsible for this reaction.^[14]

Mode of disinfectant action

Escherichia coli exposed to hypochlorous acid lose viability in less than 100 ms due to inactivation of many vital systems.^{[2][28][29][30][31]} Hypochlorous acid has a reported LD₅₀ of 0.0104 ppm - 0.156 ppm^[32] and 2.6 ppm caused 100% growth inhibition in 5 minutes.^[25] However it should be noted that the concentration required for bactericidal activity is also highly dependent on bacterial concentration.^[20]

Inhibition of glucose oxidation

In 1948, Knox et al.^[20] proposed the idea that inhibition of cytochromes cannot be reoxidized as observed by Rosen et al.^[31] However, this line of inquiry was ended when Albrich et al.^[2] 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 HOCl.

Depletion of adenine nucleotides

Having eliminated loss of respiration Albrich et al.^[2] proposes that the cause of death may be due to metabolic dysfunction caused by depletion of adenine nucleotides. Barrette et al.^[34] studied the loss of adenine nucleotides by studying the energy charge of HOCl exposed cells and found that cells exposed to HOCl 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 HOCl and the observation that HOCl causes intracellular ATP hydrolysis. Also confirmed was; that at bacteriocidal levels of HOCl, 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 be ATP synthetase.^[18] Much of this research on respiration reconfirms the observation that relevant bacteriocidal reactions take place at the cell membrane.^[34][18][38]

Inhibition of DNA replication

Recently it has been proposed that bacterial inactivation by HOCl is the result of inhibition of protein synthesis, and closely parallels loss of viability.^[25][39] During bacterial genome replication, the origin of replication (oriC in E. coli) binds to proteins that are associated with the cell membrane, and it was observed that HOCl treatment decreases the affinity of extracted membranes for oriC, and this decreased affinity also parallels loss of viability. A study by Rosen et al^[40] compared the rate of HOCl 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 HOCl.

Safety

HOCl is a strong oxidizer and can form explosive mixtures.