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46557-AC2
Redox Reactions of Iron and Manganese that Chemically Transform Natural Organic Matter Within Sediments

Alan T. Stone, Johns Hopkins University

����������� Dihydroxybenzenes (XH₂) and benzoquinones (Z), believed to be the principal redox-active moieties within natural organic matter (NOM), are linked through half-reactions, e.g.:

XH₂/Z interconversion within NOM is usually assumed to be reversible. �Nevertheless, Michael-Type nucleophilic addition by bisulfide ion (HS^-), which renders the reaction irreversible, has been documented:

Do weaker sulfur-, nitrogen-, oxygen-, and halogen-donor atom nucleophiles form Michael-Type adducts?� Our hypothesis is that unactivated benzoquinones, i.e. those with only alkyl substituents, cannot engage in reactions with weak nucleophiles, while activated benzoquinones possessing electron-withdrawing carboxylate substituents can.� Carboxylate substituents are abundant in lignin, an important NOM precursor biomolecule.� A number of physical methods have demonstrated that NOM is rich in carboxylate groups.� Carboxylate groups make dihydroxybenzenes more resistant towards oxidation.� In order for this means of weak nucleophile incorporation to be viable, oxidants strong enough to act upon carboxylate-substituted dihydroxybenzenes must exist in marine environments.

����������� Enrichments of manganese(III,IV) (hydr)oxides have been found at the oxic/anoxic interface within the water column of the Black Sea, within marine sediments, and in a numerous other relevant environments.� Mn^III,IV (hydr)oxides are strong enough in a thermodynamic sense and reactive enough in a kinetic sense to oxidize carboxylate-substituted dihydroxybenzenes to activated benzoquinones.� We have investigated the oxidation of more than sixteen phenols and dihydroxybenzenes by Mn^IVO₂(s, pyrolusite).� Here are a few examples: catechol (2.1x10^-3 s^-1) > hydroquinone (6.1 x10^-4 s^-1) > 2,5-dihydroxybenzoic acid (3.3x10^-4 s^-1) > 4,4'-biphenol (2.0x10^-4 s^-1) > 2,5-dihydroxyterephthalic acid (1.7x10^-4 s^-1) > 2,2'-biphenol (7.2x10^-8 s^-1).� These values reflect initial rates of Mn^II solubilization divided by the dihydroxybenzene substrate concentration, measured in a 10 mM MES medium (pH 6.5).

����������� Next, we investigated adducts formed through attack by OH^-/H₂O, by the nitrogen-donor nucleophile 4-methylimidazole, and by two sulfur-donor nucleophiles, thiosulfate and sulfite.� Our efforts focused on HPLC with electrospray ionization (ES) mass spectrometric (MS) detection, because of its' ability to identify specific adducts.� Capillary electrophoresis and HPLC with UV detection yielded a similar number of peaks, but detection limits were lower, and structure assignments were not possible.

����������� As expected, p-benzoquinone, classified as an unactivated electrophile, formed an adduct with the representative sulfur-donor nucleophile thiosulfate, but not with OH^-/H₂O or with 4-methylimidazole.� p-Benzoquinone is stable enough for a stock solution to be prepared from an authentic standard.� Thiosulfate or 4-methylimidazole was then added.� With ESI-MS, the ionization process is strong enough to generates the molecular ion from ^-O₃S-S-XH₂ (the thiosulfate adduct), but not from the p-benzoquinone substrate. �HPLC with UV detection was used to monitor concentrations of p-benzoquinone.

����������� 2,5-Dihydroxybenzoic acid oxidation by MnO₂(s) yields the activated electrophile carboxy-p-benzoquinone.� In simple aqueous media (beginning pH ~ 5.6), MnO₂(s) oxidation followed by OH^-/H₂O nucleophilic attack generated three addition products, and one product of oxidative coupling, all shown to the left of the line, below.

4-Methylimidazole is not subject to oxidation by MnO₂(s), so it can be added along with the 2,5-dihydroxybenzoic acid substrate.� 50, 100, 200, 300, and 400 mM concentrations were employed.� As the 4-methylimidazole concentration was increased, peaks collected in total ion mode for the left four species to decreased, and new peaks corresponding to the 4-methylimidazole adducts (to the right of the line) increased.� MS fragmentation patterns are consistent with the structural assignments.� Sulfite ion and iodide are subject to oxidation by MnO₂(s).� It was therefore necessary to first generate carboxy-p-benzoquinone using MnO₂(s), then remove unreacted MnO₂(s) prior to addition of nucleophile stock solution using microfiltration.� Sulfite ion yielded an adduct discernible by HPLC-ESI-MS; iodide did not.

����������� When 2,5-dihydroxyterephthalic acid is oxidized by MnO₂(s), a highly activated electrophile is generated, dicarboxy-p-benzoquinone.� With have conducted experiments employing 50, 100, 200, 300, and 400 mM 4-methylimidazole, and discerned the following products using HPLC-ESI-MS:

The rightmost product, a mixed hydroxide ion/4-methylimidazole adduct, is particularly noteworthy.� It's peak area reached a maximum value when 200 mM 4-methylimidazole was employed.�

����������� We have several important objectives for the second and final year of this project.� We will expand our structural determinations to include oxygen-donor and halogen-donor nucleophiles representative of marine sedimentary environments.� We are also interested in whether other transient species generated by oxidation, e.g. quinone methides, provide additional opportunities for adduct formation.� We will employ reactions performed at higher concentrations (~ 10 mM) to generate enough quantities of each product for isolation and purification to be conducted.� Once we have authentic standard prepared, we can quantify production rates and compare rate constants for OH^-/H₂O adduct formation with rate constants obtained with nucleophile amendments.� We are currently testing "Good's Buffers" such as MES and MOPS to see if they are sufficiently nucleophilic to generate Michael Addition products.� Once we have identified an inert buffer system, we can carefully work out rates of competing adduct formation reactions within a range of pH appropriate for marine sedimentary environments.

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