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Open GYK Dissertation Final.Pdf The Pennsylvania State University The Graduate School College of Agricultural Sciences REACTION MECHANISMS OF TRANSITION METALS WITH HYDROGEN SULFIDE AND THIOLS IN WINE A Dissertation in Food Science by Gal Y. Kreitman 2016 Gal Y. Kreitman Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2016 The dissertation of Gal Y. Kreitman was reviewed and approved* by the following: Ryan J. Elias Associate Professor of Food Science Dissertation Advisor Chair of Committee Joshua D. Lambert Associate Professor of Food Science John N. Coupland Professor of Food Science Michela Centinari Assistant Professor of Horticulture David W. Jeffery Senior Lecturer in Wine Science Special Member John C. Danilewicz Special Signatory Robert F. Roberts Professor of Food Science Head of the Department of Food Science *Signatures are on file in the Graduate School ii ABSTRACT Sulfidic off-odors due to hydrogen sulfide (H2S) and low molecular weight thiols are commonly encountered in wine production. These odors are a serious quality issue in wine and may result in consumer rejection. Therefore, sulfidic off-odors are generally controlled prior to bottling, and are frequently removed by the process of Cu(II) fining – a process that remains poorly understood. Cu(II) is effective at binding with sulfhydryl functionalities and forming nonvolatile complexes thereby removing aroma associated with the compound. However, this technique leaves residual copper in the wine which catalyzes non-enzymatic wine oxidations. Furthermore, elevated copper concentrations are usually associated with increased sulfidic off-odors under anaerobic aging conditions. In this work, I elucidated the underlying mechanisms by which Cu(II) interacts with H2S and thiol compounds under wine-like conditions. Adding Cu(II) sulfate to air saturated model wine containing H2S, cysteine (Cys), 6-sulfanylhexan-1-ol (6SH), or 3-sulfanylhexan-1-ol (3SH) led to a rapid formation of ~1.4:1 H2S:Cu and ~2:1 thiol:Cu complexes. This resulted in the oxidation of H2S and thiols, and reduction of Cu(II) to Cu(I) without oxygen uptake. Both H2S and thiols resulted in the formation of Cu(I)-SR complexes, and subsequent reactions with oxygen led to the oxidation of H2S rather than the formation of insoluble copper sulfide, which has been previously assumed. The proposed reaction mechanisms provide an insight into the extent to which H2S can be selectively removed in the presence of thiols in wine. The interaction of iron and copper is also known to play an important synergistic role in mediating non-enzymatic wine oxidation. Therefore, I assessed the interaction of these two metals in the oxidation of H2S and thiols (Cys, 6SH, and 3SH) under wine-like conditions. H2S and thiols were shown to be slowly oxidized in the presence of Fe(III) alone, and were not bound to Fe(III) under model wine conditions. However, Cu(II) added to model wine containing Fe(III) was quickly iii reduced by H2S and thiols to form Cu(I)-complexes, which then rapidly reduced Fe(III) to Fe(II). Oxidation of Fe(II) in the presence of oxygen regenerated Fe(III) and completed the iron redox cycle. This work clearly demonstrated a synergistic effect between Fe and Cu during the oxidation of H2S and thiols. In addition, sulfur-derived oxidation products were observed, and the formation of organic polysulfanes was demonstrated for the first time under wine-like conditions. Manganese has a modest activity in catalyzing polyphenol and sulfite oxidation in wine. Furthermore, manganese is known to have a catalytic activity at mediating thiol and H2S oxidation in aquatic systems. Thus, the interaction of manganese with iron and copper was investigated in relation to thiol and H2S oxidation in model wine. The reaction of thiols with Mn alone or in combination with Fe resulted in radical chain reaction paired with large oxygen uptake and generation of sulfur oxyanions. H2S did not generate free thiyl radicals, and had minimal interaction with Mn(II). When Cu(II) was introduced, Cu-mediated oxidation dominated in all treatments and Mn-mediated radical reaction was limited. Mn demonstrated a different reaction mechanism with thiols compared to Cu and Fe, and may generate transient thiyl radicals during wine oxidation. Demonstrating that Cu(II) addition to model systems containing H2S and thiols resulted in the generation of polysulfanes led to an investigation of the formation of mixed disulfides and polysulfanes in model and white wine samples. I found that at relatively low concentrations of H2S and methanethiol (MeSH, 100 µg/L each), Cu(II)-fining resulted in the generation of MeSH- glutathione disulfide and trisulfane in white wine. The reduction of the resulting nonvolatile disulfides may then play a role in the generation of undesirable sulfidic off-odors. Therefore,the ability of Fe and Cu in combination of bisulfite (SO2), ascorbic acid, and Cys to promote the catalytic scission of diethyl disulfide (DEDS). I found that the combination of SO2 along with Fe and Cu depleted more DEDS than the other treatments. Furthermore, a method for releasing volatile sulfur compounds from their precursors was investigated using tris(2-carboxyethyl)phosphine (a iv reducing agent) and bathocuproine disulfonic acid (a chelator). The addition of the reagents successfully released H2S and MeSH from red and white wines that were free of reductive faults at the time of addition. I have demonstrated the underlying reaction mechanisms of H2S and thiols with Cu, Fe, and Mn under wine-like conditions. I showed that Cu(II) was readily reduced by H2S and thiols, and that this complex remained redox active and reduced oxygen. The reaction of Cu with H2S and thiols is further accelerated by the presence of Fe and Mn. While the initial Cu(II) fining process removed volatile sulfhydryl compounds, it generated disulfides, polysulfanes, and Cu(I)- SR complexes that remain in the wine. I showed that disulfide scission is accelerated by the presence of metals and reducing agents under wine conditions. Furthermore, I provided a strategy to quickly reduce or dissociate disulfides, polysulfanes, and metal complexes for the release of volatile sulfur compounds in both red and white wines. This can be used by winemakers to predict a wine’s potential to exhibit sulfidic odors and take further action. Overall, a better understanding of the underlying reaction mechanisms with H2S and thiols provided a foundation for future strategies to better control sulfidic off-odors in wine. v TABLE OF CONTENTS LIST OF FIGURES .................................................................................................................... x LIST OF TABLES .................................................................................................................... xv ACKNOWLEDGEMENTS .................................................................................................... xvii Chapter 1 Literature Review....................................................................................................... 1 1.1 Introduction .............................................................................................................. 1 1.2 Metal-catalyzed redox reactions ................................................................................ 6 1.2.1 Copper ......................................................................................................... 10 1.2.1.1 Copper fining ............................................................................................ 10 1.2.1.2 Redox cycling of copper ........................................................................ 11 1.2.2 Iron .............................................................................................................. 12 1.2.3 Manganese ................................................................................................... 14 1.2.4 Other transition metals ................................................................................. 15 1.2.5 Release of metal sulfide and metal thiol complexes ...................................... 16 1.3 Thiol/disulfide couple ............................................................................................. 18 1.3.1 Occurrence and oxidation of disulfides ......................................................... 18 1.3.2 Thiol-disulfide interchange ........................................................................... 21 1.3.3 Sulfitolysis ................................................................................................... 22 1.3.4 Metal catalyzed disulfide scission ........................................................................ 24 1.3.5 Ascorbic acid ............................................................................................... 26 1.4 Reactions of sulfhydryls with organic wine constituents .......................................... 28 1.5 Thioester hydrolysis................................................................................................ 29 1.6 Strecker degradation of amino acids ........................................................................ 30 1.7 Further reactions of sulfur containing compounds ................................................... 30 1.8 Research overview, significance, and hypotheses .................................................... 31 Chapter 2 Reaction Mechanisms of Metals with Hydrogen Sulfide and Thiols in Model Wine. Part 1: Copper Catalyzed Oxidation. ........................................................................................
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