Biological Thiols and Carbon Disulfide: The Formation and Decay of Trithiocarbonates under Physiologically Relevant Conditions

Carbon disulfide is an environmental toxin, but there are suggestions in the literature that it may also have regulatory and/or therapeutic roles in mammalian physiology. Thiols or thiolates would be likely biological targets for an electrophile, such as CS₂, and in this context, the present study examines the dynamics of CS₂ reactions with various thiols (RSH) in physiologically relevant near-neutral aqueous media to form the respective trithiocarbonate anions (TTC^–, also known as “thioxanthate anions”). The rates of TTC^– formation are markedly pH-dependent, indicating that the reactive form of RSH is the conjugate base RS^–. The rates of the reverse reaction, that is, decay of TTC^– anions to release CS₂, is pH-independent, with rates roughly antiparallel to the basicities of the RS^– conjugate base. These observations indicate that the rate-limiting step of decay is simple CS₂ dissociation from RS^–, and according to microscopic reversibility, the transition state of TTC^– formation would be simple addition of the RS^– nucleophile to the CS₂ electrophile. At pH 7.4 and 37 °C, cysteine and glutathione react with CS₂ at a similar rate but the trithiocarbonate product undergoes a slow cyclization to give 2-thiothiazolidine-4-carboxylic acid. The potential biological relevance of these observations is briefly discussed

Six-Coordinate Ferrous Nitrosyl Complex Fe^II(TTP)(PMe₃)(NO) (TTP = <i>meso</i>-Tetra‑<i>p</i>‑tolylporphyrinato Dianion)

Low-temperature in situ Fourier transform infrared and UV–vis measurements show that trimethylphosphine (PMe₃) reacts with microporous layers of Fe^II(TTP)­(NO) (TTP = <i>meso</i>-tetra-<i>p</i>-tolylporphyrinato dianion; NO = nitric oxide) to form the previously unknown six-coordinate complex Fe^II(TTP)­(PMe₃)­(NO). Upon warming this compound to room temperature in the presence of excess phosphine, the NO ligand is completely replaced by phosphine, resulting in formation of the bis­(trimethylphosphine) complex Fe^II(TTP)­(PMe₃)₂. Simultaneously, the NO released oxidizes free PMe₃ to the corresponding phosphine oxide (OPMe₃) with concomitant formation of nitrous oxide (N₂O)

Correction to “A Luminescent and Biocompatible PhotoCORM”

Correction to “A Luminescent and Biocompatible PhotoCORM

A Luminescent and Biocompatible PhotoCORM

The water-soluble rhenium­(I) complex <i>fac</i>-[Re­(bpy)­(CO)₃(thp)]⁺ (<b>1</b>) [CF₃SO₃^– salt; bpy = 2,2′-bipyridine, thp = tris­(hydroxymethyl)­phosphine] is both strongly luminescent and photoactive toward carbon monoxide release. It is stable in aerated aqueous media, is incorporated into cells from the human prostatic carcinoma cell line PPC-1, and shows no apparent cytotoxicity. Furthermore, the solvated Re­(I) photoproduct of CO release (<b>2</b>) is also luminescent, a feature that allows one to track the transformation of <b>1</b> to <b>2</b> inside such cells using confocal fluorescence microscopy. In this context, <b>1</b> is a very promising candidate as a photoactivated CO releasing moiety (photoCORM) with potential therapeutic applications

Lanthanide Modification of CdSe/ZnS Core/Shell Quantum Dots

Lanthanide-modified CdSe quantum dots (CdSe­(Ln) QDs) have been prepared by heating a solution of Cd­(oleate)₂, SeO₂, and Ln­(bipy)­(S₂CNEt₂)₃ (bipy = 2,2′-bipyridine) to 180–190 °C for 10–15 min. The elemental compositions of the resulting CdSe­(Ln) cores and CdSe­(Ln)/ZnS core/shell QDs show this route to be highly reproducible. The optical absorption spectra of these composite materials are similar to those of the unmodified nanocrystals, but the QD-centered band edge photoluminescence (PL) is partially quenched. The time-gated emission and excitation spectra of the CdSe­(Ln) cores display sensitized lanthanide-centered PL upon higher energy excitation of the nanocrystal host but not upon excitation at the lowest energy QD absorption band. Growth of the ZnS shell led to the depletion of about 60% of the lanthanide ions present together with depletion of nearly all of the lanthanide-centered PL. On these bases, we conclude that the lanthanide-centered PL from the CdSe­(Ln) cores originates with Ln³⁺-related trap states associated with the QD surface

Tracking Reactive Intermediates by FTIR Monitoring of Reactions in Low-Temperature Sublimed Solids: Nitric Oxide Disproportionation Mediated by Ruthenium(II) Carbonyl Porphyrin Ru(TPP)(CO)

Interaction of NO (¹⁵NO) with amorphous layers of Ru­(II) carbonyl porphyrin (Ru­(TPP)­(CO), TPP^2‑ = <i>meso</i>-tetraphenylporphyrinato dianion) was monitored by FTIR spectroscopy from 80 K to room temperature. An intermediate spectrally characterized at very low temperatures (110 K) with ν­(CO) at 2001 cm^–1 and ν­(NO) at 1810 cm^–1 (1777 cm^–1 for ¹⁵NO isotopomer) was readily assigned to the mixed carbonyl–nitrosyl complex Ru­(TPP)­(CO)­(NO), which is the logical precursor to CO labilization. Remarkably, Ru­(TPP)-mediated disproportionation of NO is seen even at 110 K, an indication of how facile this reaction is. By varying the quantity of supplied NO, it was also demonstrated that the key intermediate responsible for NO disproportionation is the dinitrosyl complex Ru­(TPP)­(NO)₂, supporting the conclusion previously made from solution experiments

Temperature Tuning the Catalytic Reactivity of Cu-Doped Porous Metal Oxides with Lignin Models

Reported are the temperature dependencies of the temporal product evolution for lignin model compounds over copper-doped porous metal oxide (CuPMO) in supercritical-methanol (sc-MeOH). These studies investigated 1-phenylethanol (PPE), benzyl phenyl ether (BPE), dihydrobenzofuran (DHBF), and phenol over operating temperature ranges from 280 to 330 °C. The first three model compounds represent the β-O-4 and α-O-4 linkages in lignin as well as the furan group commonly found in the β-5 linkage. Phenol was investigated due to its key role in product proliferation as noted in earlier studies with this Earth-abundant catalyst. In general, the apparent activation energies for ether hydrogenolysis proved to be significantly lower than that for phenol hydrogenation, a major side reaction leading to product proliferation. Thus, temperature tuning is a promising strategy to preserve product aromaticity as demonstrated by the more selective conversion of BPE and PPE at lower temperatures. Rates of methanol reforming over CuPMO were also studied over the temperature range of 280–320 °C since it is this process that generates the reducing equivalents for this catalytic system. In the absence of substrate, the gaseous products H₂, CO, and CO₂ were formed in ratios stoichiometrically consistent with catalyzed methanol reformation and water gas shift reactions. The latter studies suggest that the H₂ production ceases to be rate limiting early in batch reactor experiments but also suggest that H₂ overproduction may contribute to product proliferation

Dinitrosyl Iron Complexes with Cysteine. Kinetics Studies of the Formation and Reactions of DNICs in Aqueous Solution

Kinetics studies provide mechanistic insight regarding the formation of dinitrosyl iron complexes (DNICs) now viewed as playing important roles in the mammalian chemical biology of the ubiquitous bioregulator nitric oxide (NO). Reactions in deaerated aqueous solutions containing FeSO₄, cysteine (CysSH), and NO demonstrate that both the rates and the outcomes are markedly pH dependent. The dinuclear DNIC Fe₂(μ-CysS)₂(NO)₄, a Roussin’s red salt ester (<b>Cys-RSE</b>), is formed at pH 5.0 as well as at lower concentrations of cysteine in neutral pH solutions. The mononuclear DNIC Fe­(NO)₂(CysS)₂^– (<b>Cys-DNIC</b>) is produced from the same three components at pH 10.0 and at higher cysteine concentrations at neutral pH. The kinetics studies suggest that both <b>Cys-RSE</b> and <b>Cys-DNIC</b> are formed via a common intermediate Fe­(NO)­(CysS)₂^–. <b>Cys-DNIC</b> and <b>Cys-RSE</b> interconvert, and the rates of this process depend on the cysteine concentration and on the pH. Flash photolysis of the <b>Cys-RSE</b> formed from Fe­(II)/NO/cysteine mixtures in anaerobic pH 5.0 solution led to reversible NO dissociation and a rapid, second-order back reaction with a rate constant <i>k</i>_NO = 6.9 × 10⁷ M^–1 s^–1. In contrast, photolysis of the mononuclear-DNIC species <b>Cys-DNIC</b> formed from Fe­(II)/NO/cysteine mixtures in anaerobic pH 10.0 solution did not labilize NO but instead apparently led to release of the CysS^• radical. These studies illustrate the complicated reaction dynamics interconnecting the DNIC species and offer a mechanistic model for the key steps leading to these non-heme iron nitrosyl complexes

Markedly Improved Catalytic Dehydration of Sorbitol to Isosorbide by Sol–Gel Sulfated Zirconia: A Quantitative Structure–Reactivity Study

Isosorbide, a bicyclic C6 diol, has considerable value as a precursor for the production of bio-derived polymers. Current production of isosorbide from sorbitol utilizes homogeneous acid, commonly H2SO4, creating harmful waste and complicating separation. Thus, a heterogeneous acid catalyst capable of producing isosorbide from sorbitol in high yield under mild conditions would be desirable. Reported here is a quantitative investigation of the liquid-phase dehydration of neat sorbitol over sulfated zirconia (SZ) solid acid catalysts produced via sol–gel synthesis. The catalyst preparation allows for precise surface area control (morphology) and tunable catalytic activity. The S/Zr ratio (0.1–2.0) and calcination temperature (425–625 °C) were varied to evaluate their effects on morphology, acidity, and reaction kinetics and, as a result, to optimize the catalytic system for this transformation. With the optimal SZ catalyst, complete conversion of sorbitol occurred in <2 h under mild conditions to give isosorbide in 76% yield. Overall, the quantitative kinetics and structure–reactivity studies provided valuable insights into the parameters that govern product yields and SZ catalyst activity, central among these being the relative proportion of acid site types and Brønsted surface density

Probing the Lignin Disassembly Pathways with Modified Catalysts Based on Cu-Doped Porous Metal Oxides

Described are the selectivities observed for reactions of lignin model compounds with modifications of the copper-doped porous metal oxide (CuPMO) system previously shown to be a catalyst for lignin disassembly in supercritical methanol (Matson et al., <i>J. Amer. Chem. Soc</i>. 2011, 133, 14090–14097). The models studied are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether, biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics of the α-O-4, β-O-4, 4-O-5, 5-5, and β-5 linkages characteristic of lignin. Also, briefly investigated as a substrate is poplar organosolv lignin. The catalyst modifications included added samarium­(III) (both homogeneous and heterogeneous) or formic acid. The highest activity for the hydrogenolysis of aryl ether linkages was noted for catalysts with Sm­(III) incorporated into the solid matrix of the PMO structure. In contrast, simply adding Sm³⁺ salts to the solution suppressed the hydrogenolysis activity. Added formic acid suppressed aryl ether hydrogenolysis, presumably by neutralizing base sites on the PMO surface but at the same time improved the selectivity toward aromatic products. Acetic acid induced similar reactivity changes. While these materials were variously successful in catalyzing the hydrogenolysis of the different ethers, there was very little activity toward the cleavage of the 5-5 and β-5 C-C bonds that represent a small, but significant, percentage of the linkages between monolignol units in lignins