Hydroxyl radicals are much too unstable in aqueous solutions (e. g., cytoplasm) for being easily quenchable. When they are formed they immediately react with any molecules in their vicinity. So in my opinion hydrogen peroxide is not a scavenger of hydroxyl radicals.

With best regards, Christian

Hydrogen peroxide generates hydroxyl radicals (especially in presence of metals which exists in two or more valence states, in particular iron). To the best of my knowledge it can not scavenge hydroxyl radicals, therefore, I agree with Christian.

Hello. H2O2 gives OH in the Fenton reaction. OH is so reactive, that it can react with anything prior to the molecule it came from, having such a short lifetime compared to H2O2. There are really a lot of compounds that can react with OH "easier" than H2O2 would. (See also "ROS-mediated redox signaling during cell differentiation in plants" by Romy Schmidt and Jos H.M. Schippers). Best regards.

The most important reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and hydroxyl radicals are common by-products of normal aerobic metabolism. Hydrogen peroxide is the simplest peroxide and finds use as a strong oxidizer, bleaching agent and disinfectant. Its chemistry is dominated by the nature of its unstable peroxide bond. Hydrogen peroxide is thermodynamically unstable and decomposes to form water and oxygen. Free hydrogen peroxide will damage any tissue it encounters via oxidative stress. Not a free radical itself, but easily converts to free radicals like OH, which then do the damage. Hydrogen peroxide is neutralized by peroxidase (an enzymatic antioxidant). ROS can also stimulate lipid peroxidation (LPO). Although ROS are necessary for defence of the host, they also expose the host tissue to oxidative damage. These mechanisms include a disruption of the extracellular matrix, induction of lipid peroxidation and proinflammatory cytokines that cause DNA damage and oxidation of enzymes and increased apoptosis.

The hydroxyl radicals are a highly reactive and non-selective intermediates (E0=2.8 V), which reacts very rapidly with most organic and inorganic compounds. Advanced Oxidation Processes (AOPs) are based on the formation and reactions of .OH radicals that are much more reactive than all other oxidizing species that are used in oxidative pollution abatement in drinking water and in wastewater. One of the oldest methods for generation hydroxyl radicals invvolves photolysis of aqueous hydrogen peroxide solutions:

H2O2 + UV = 2 .OH

The hydroxyl radicals then react with hydrogen peroxide to yield perhydroxyl radicals (HO2.): 

.OH + H2O2  = HO2. + H2O, where k = 2.7 × 107 M-1 * s-1.

Therefore, the excess radicals generated may undergo recombination or may become involved in the side reactions (for example, with hydrogen peroxide).

I am also in agree with the answer that Hydrogen peroxide cannot be a scavenger of hydroxy radicals.

Whether a compound scavenges a radical depends on the product of rate constant and concentration.  Let us compare a typical organic molecule with hydrogen peroxide. The hydroxyl radical reacts with the organic molecule with a rate constant of typically  3 x 109 M-1s-1, with hydrogen peroxide with 3 x 107 M-1s-1.  If the concentration of H2O2 is 100 times larger that that of the organic molecule, H2O2 and the organic molecule react with equal numbers of HO..  Thus, hydrogen peroxide can be a scavenger, but not under physiological conditions, where high H2O2 concentrations are not found/cannot be used. I noticed that many of the answers given above do not pertain to the question asked. 

Corrections of the answers provided above:

The hydroxyl radical is not unstable.  It is reactive.

Whether the Fenton reaction yields the hydroxyl radical or a higher oxidation state of iron is still being debated.

The name of HO2· is not perhydroxyl radical, but hydrogen dioxide, hydridodioxygen(dot), or dioxidanyl radical, see  W. H. Koppenol, Names for inorganic radicals (IUPAC recommendations 2000), Pure Appl.Chem. 72:437-446, 2000.

The standard electrode potential of the couple HO.,H+/H2O is +2.73 V (vs NHE), see

W. H. Koppenol, D. M. Stanbury, and P. L. Bounds. Electrode potentials of partially reduced oxygen species, from dioxygen to water. Free Radical Biol.Med. 49:317-322, 2010.

Superoxide, hydrogen peroxide and hydroxyl have very different reactivities, see http://kinetics.nist.gov/solution/.  Of these, superoxide and hydrogen peroxide are not very reactive at all - otherwise the enzymes SOD and catalase could  not do their job.  One should therefore not use the abbreviation ROS; instead use the name of the species you are discussing.

Dear Prof. Koppenol,

I am fully aware that you are authority in the field of free radical research. I have few doubts and I request you to clarify the same:

(i) in one of the statements you stated that hydroxyl radical is not unstable, but it is reactive. In general it is believed that free radicals tend react with various biological molecules/entities to regain stability (it is generally accepted that reactivity often depends on stability and the molecules/radicals tend to regain stability by reacting with some molecule/entity).

(ii) yes, it is true that hydrogen peroxide is less reactive (we do have catalase and peroxidases to detoxify this molecule). But, superoxide anion radical (although we have SOD in cells) tends to promote generation of hydroxyl radicals. It is also generally believed that amongst the three hydroxyl radical is most toxic followed by superoxide anion radical and then hydrogen peroxide (accordingly life time of hydroxyl radical is least followed by superoxide anion radical and hydrogen peroxide possesses relatively longer life time.

Please educate me on above points.

 (i) The hydroxyl radical by itself is not unstable, that is, it does not decay to another species. You appear to be mixing thermodynamics with kinetics. An example is hydrogen peroxide:  It is thermodynamically unstable with respect to disproportionation to dioxygen and water, but kinetically inert. 

(ii) That superoxide may generate hydroxyl radicals depends on its disproportionation to hydrogen peroxide.  That it could be a precursor to the hydroxyl radical does not make superoxide itself reactive . Superoxide only reacts fast with hydrogen dioxide (providing hydrogen peroxide and dioxygen), with Fe-, Mn- and Cu/Zn-SOD, with nitrogen monoxide, and with other radicals.  For instance, it does not react with amino acids, and, thus, not with proteins, unless there is a metal that has the proper electrode potential to be reduced, as in cytochrome c.  It also does not react with lipids.

Thank you very much for addressing to my queries. Your response has opened up further queries in my mind as certain biomolecules such as ascorbate, proline, mannitol  are known to scavenge free radicals. For instance proline is known to scavenge hydroxyl radicals and singlet oxygen (which is not a free radical), but not superoxide anion radicals. I believe we are going too much away from the original question and hence, I shall try to bother you through email.

Dear Dr. Saradhi,

I tried to degrade DNA molecule using high concentration of hydrogen peroxide alone in a test tube.  It did not degrade DNA.  So is hydrogen peroxide kinetically inert as mentioned by Dr. Koppenol.  In the presence of transition metal it does through Fenton reaction.  

Dear Malcolm,

I am not expert in the field, but based on our experiences let me address your query.

Firstly, it is not clear how exactly you treated DNA with hydrogen peroxide. Did you mix DNA with hydrogen peroxide alone? Did you incubate this mixture in light or dark? Even intensity of light matters. Did you add any salt (of Fe, Cu...) to this mixture? Yes, transition metal ions promote Fenton reaction.  We have recorded enhanced inactivation of functional units such as photosystems (in particular PS II) and certain enzymes in presence of hydrogen peroxide in presence of light. We have recorded acceleration in inactivation in presence of iron and copper salts in presence of light.

Hydrogen peroxide is an oxidant/oxidative agent that may symbolize a ticking bomb always readily produced in the body and liable to give off oxidative radicals when there is low level of either or both glutathione peroxidase and/or catalase enzymes supposedly expected to serve as antioxidants against this reactive peroxide. Hydrogen peroxide does not scavenge but rather produce hydroxyl radicals, especially lipid hydoxyl when the parent peroxide reacts with membrane lipid forming lipid hydoxyl.

Dear all,

I would like to know the reference of these sentence: " If the concentration of H2O2 is 100 times larger that that of the organic molecule, H2O2 and the organic molecule react with equal numbers of HO*", commented by Willem H Koppenol.

Can you give me the article where we can find this data?

Thank you!

Dear Dr. Clusellas,

There is no reference. It goes back to comparing the product of rate constant and concentration. Let us assume that we have 0.10 mM organic material that reacts with HO with a rate constant of 3.0 x 109 M-1s-1, a typical rate constant for the reaction of HO with an organic molecule. The product of these two numbers is 3.0 x 105 s-1. Let us also say that we want to prevent that reaction, using H2O2 as a scavenger for HO. Given that HO reacts with H2O2 a hunderd times slower - the rate constant is 3 x 107 M-1s-1, we need a hundred times more H2O2, or 10 mM H2O2 and the product is again 3.0 x 105 s-1. At this point half of HO reacts with the organic compound and the other half with H2O2. But, typically, we use a scavenger to protect > 90% of the organic compound. That means that you would want to use hydrogen peroxide with a concentration of at least 100 mM. To protect 99%, we have to use 1.0 M

Typical hydroxyl radical scavengers are ethanol and t-butanol. However, we must always keep in mind what the reaction product of HO with the scavenger does. With H2O2, we form superoxide, with ethanol we get a reducing radical, and t-butanol is probably the best, because the t-butanol radical is pretty much inert.

Hope this helps.

Dear Prof. Willem H Koppenol,

This help me a lot.

Thank you very much!

I will use this argumentation to explain the scavenger effect of H2O2 when used at high concentrations (compared to low concentrations of organic pollutants) during electro-Fenton process.

Thanks!

Anna

it depends on the concentration of the compounds and their rate constants, I agree with Mr. Willem H Koppenol

Like many who mentioned earlier, peroxide does scavenge hydroxyl radicals. Although one must be cautious of the rate constants as mentioned by Willem H Koppenol

Since no context was mentioned, it can be assumed that this is used for Fenton's reaction. If that's the case this article might help. Peroxide is used in Fenton's reaction to produce hydroxyl radicals, which drive the degradation of organics.

However, when peroxide is in excess, would scavenge hydroxyl radicals leading to lower degradation performance. Hence in the context of Fenton's reaction, it can be explained as such.

Article Fluidized-bed Fenton technologies for recalcitrant industria...

Yes. Hydroxyl radical is scavenged by hydrogen peroxide.