Date of Award

2010

Document Type

Honors Thesis (Colby Access Only)

Department

Colby College. Chemistry Dept.

Advisor(s)

D. Whitney King (Mentor)

Second Advisor

Thomas W. Shattuck (Reader)

Abstract

Superoxide (O₂⁻) is a reactive oxygen species formed by the one electron reduction of oxygen or the one electron oxidation of hydrogen peroxide. It is produced by photochemical reactions in the surface water of lakes and oceans, and is also released from the cells of marine plankton. Superoxide can both oxidize and reduce other molecules, making it a crucial part of trace metal cycling. It decays on a time scale of seconds to minutes through second order disproportionation reactions and first order redox cycling, forming hydrogen peroxide and oxygen. This rapid rate results in a challenge to the preparation of traditional standards for the calibration of instruments during analytical work.

Previous work focused on the development of standards in aqueous media. Alkaline solutions of acetone and ethanol were irradiated to produce a ketone triplet, which then reacts with ethanol to produce two carbon-centered radicals. These radicals can then react with dissolved oxygen, producing superoxide and regenerating acetone, continuing the cycle. Reaction conditions were optimized by determining reaction rates under varying pH and ethanol concentrations. Absorbance measurements, corrected for the concentration of hydrogen peroxide, were used to measure superoxide concentrations during and after photolysis.

Previously, the chemiluminescent reagents methyl Cypridina luciferin analog (MCLA, 2- methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[1,2-α]pyrazin-3-one) and acridinium ester (AE, 10-methyl-9-(p-formylphenyl) acridinium carboxylate trifluoromethanesulfonate) were used to detect superoxide and hydrogen peroxide, respectively. Hydrogen peroxide reacts with AE rapidly above a pH of 11, because the reactive species is the conjugate base of hydrogen peroxide (HO₂⁻). Surprisingly, it was determined that above a pH of 6 AE also reacts with superoxide to produce chemiluminescence. AE preferentially reacts with superoxide at pH 8 because it is a weaker base (stronger acid) than hydrogen peroxide and exists in its unprotonated form. We have proposed the mechanism by which AE and superoxide react, as well as the relative rate at which each step of the reaction occurs. A dual detection system for both superoxide and hydrogen peroxide in natural waters using AE is presented, but after achieving insufficient selectivity toward superoxide, MCLA is used instead for its measurement.

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Keywords

superoxide

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