Title

3-Mercaptopropionic acid: Abiotic formation of a novel thiol in organic-rich marine sediments

Date of Award

1989

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine and Atmospheric Chemistry

First Committee Member

Kenneth Mopper, Committee Chair

Abstract

The origins and distributions of organic sulfur compounds in marine sediments are largely unknown. Thiols are found in significant concentrations in anoxic marine sediments. A major thiol is 3-mercapto-propionic acid (3-MPA). In marine sediments this thiol is probably formed from dimethylsulfoniopropionate (DMSP), a plant osmolyte. Pathways involving bacterial and chemical mechanisms have been identified for the formation of 3-MPA from DMSP. The present study focused on the chemical mechanisms for 3-MPA formation from DMSP in order to obtain a better understanding of the geochemical mechanisms for the incorporation of H$\sb2$S-sulfur into the organic fraction of the sediments.Studies on the addition reaction with acrylic acid revealed that polysulfides were more reactive than HS$\sp-$ under conditions typical of marine sediments. Temperature, pH and salinity influenced the addition reaction. Variation of pH influenced the rate of reaction probably by its effect on the speciation of the reactants. High ionic strength accelerated the rate suggesting that hypersaline palaeoenvironmental conditions would have favored organosulfur formation by the addition mechanism.In sediment slurries containing added DMSP, bacterial demethylation was the dominant pathway for 3-MPA formation. However, with added water extracts of seagrasses (the main source of DMSP to the sediment) the abiotic pathway dominated. This study suggests that if DMSP cleavage occurs by enzymes of plant origin, acrylic acid is available for chemical addition because the rate of production is probably much higher than the removal rate, including bacterial consumption.Acrylic acid was determined in sediments using a new gas chromatographic method developed in this study. In sediment profiles acrylic acid concentrations have surface maxima, indicating seagrasses inhabiting the sediment surface as sources. The reaction of acrylic acid with polysulfides is probably responsible for the formation of 3-polysulfidopropionic acid in near surface sediments. Abiotic formation of 3-MPA (and its polysulfide counterpart 3-polysulfidopropionic acid) illustrates that addition of sulfur nucleophiles to activated unsaturated bonds in organic molecules via the Michael addition mechanism could represent a major pathway for the incorporation of sulfur into sedimentary organic matter during early diagenesis.

Keywords

Biogeochemistry; Environmental Sciences; Geochemistry

Link to Full Text

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