A Focus Honoring Helmut Schwarz's Election to the National Academy of Sciences

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Role of the sulfhydryl group on the gas phase fragmentation reactions of protonated cysteine and cysteine containing peptides

AbstractThe gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G∗//HF/6-31G∗ level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH2O2]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH3]+ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH2O2]+ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]+ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH3 loss is observed, although the relative abundance of the resultant [M + H − NH3]+ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H2O is in competition with those channels leading to the formation of structurally relevant sequence ions

Dimethylcuprate-Catalyzed Decarboxylative Coupling of Allyl Acetate

Allylic alkylation of organocuprates represents an important class of C–C bond forming reactions. A drawback of these reactions is their stoichiometric nature. Although metal-catalyzed decarboxylative coupling reactions offer new opportunities for formation of C–C bonds, catalytic allylic alkylation reactions of organocuprates have not been previously reported. Here, multistage mass spectrometry experiments performed on ion trap mass spectrometers in conjunction with electronic structure calculations are used to demonstrate that the dimethylcuprate anion, [CH₃CuCH₃]⁻, can catalyze decarboxylative coupling of allyl acetate in the gas phase via a simple two-step catalytic cycle. In step 1, [CH₃CuCH₃]⁻ undergoes a cross-coupling reaction with allyl acetate to yield [CH₃CuO₂CCH₃]⁻ as the major product ion. Step 2 involves subjecting the product ion to collision-induced decarboxylation to re-form [CH₃CuCH₃]⁻, thereby closing the catalytic cycle. Since the details of step 2 have been previously described (Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069−1079), here the focus is on step 1. Comparison of the reactivity of dimethylcuprate toward allylic susbtrates reveals that while allyl acetate reacts around 70 times more slowly than allyl iodide, it is more selective for cross-coupling. This is rationalized by DFT calculations, which reveal that an increase in kinetic barrier is responsible for both reactivity trends. The lowest energy path was found to involve a stepwise π-oxidative addition proceeding via an η²-C₃H₅O₂CCH₃ intermediate and extrusion of a leaving group (LG) anion, followed by a reductive elimination where this LG is recomplexed to the copper center. Finally, DFT calculations were used to shed light on the role of leaving groups LG = CH₃CO₂^–, I^–, Br^–, Cl^– in the allylic alkylation. While selectivity is indeed achieved at the cost of reactivity, the LG effects were more complex than can be accounted for by a simple consideration of the anion proton affinity (APA) of the LG

Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt

Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSn) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal complexes [(phen)M(O2CR)]+], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms involving group 10 metals differ from long-known charge-remote fragmentation reactions

Dimethylcuprate-Mediated Transformation of Acetate to Dithioacetate

Dithiocarboxylic acids, RCS₂H, and their esters, RCS₂R′, are useful reagents that can be synthesized by the reaction of carbon disulfide with organometallic reagents. Here the coinage-metal-mediated transformation of acetate to dithioacetate is explored in the gas phase using multistage mass spectrometry experiments in a linear ion trap mass spectrometer in conjunction with density functional theory (DFT) calculations. The ion–molecule reactions between coinage-metal dimethylmetalate anions [CH₃MCH₃]⁻ (M = Au, Ag, Cu), formed via double decarboxylation of the metal acetate anions, [CH₃CO₂MO₂CCH₃]⁻, and carbon disulfide, were examined. Only [CH₃CuCH₃]⁻ reacts with CS₂ with a reaction efficiency of 0.8% of the collision rate to yield the adduct [CH₃CuS₂CCH₃]⁻ (77.5%) as well as CH₃CS₂^– (22.5%). Collision-induced dissociation (CID) of the adduct [CH₃CuS₂CCH₃]⁻ gives CH₃CS₂^– as the major product, with a small amount of [CuS₂CCH₂]⁻ being formed via loss of methane. DFT calculations reveal the following. (i) [CH₃CuCH₃]⁻ reacts via oxidative addition to form a Cu­(III) intermediate, followed by reductive elimination of CH₃CS₂^–, which is captured by Cu to form [CH₃CuS₂CCH₃]⁻. This energetic adduct can fragment via loss of CH₃CS₂^– or can be collisionally cooled by the helium bath gas used in the experiments. (ii) Loss of CH₄ from [CH₃CuS₂CCH₃]⁻ also involves a Cu­(III) intermediate and results in formation of the metalladithiolactone [Cu­(CH₂CS₂)]⁻