20 research outputs found
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 CS<sub>2</sub>, and
in this context, the present study examines the dynamics of CS<sub>2</sub> reactions with various thiols (RSH) in physiologically relevant
near-neutral aqueous media to form the respective trithiocarbonate
anions (TTC<sup>ā</sup>, also known as āthioxanthate
anionsā). The rates of TTC<sup>ā</sup> formation are
markedly pH-dependent, indicating that the reactive form of RSH is
the conjugate base RS<sup>ā</sup>. The rates of the reverse
reaction, that is, decay of TTC<sup>ā</sup> anions to release
CS<sub>2</sub>, is pH-independent, with rates roughly antiparallel
to the basicities of the RS<sup>ā</sup> conjugate base. These
observations indicate that the rate-limiting step of decay is simple
CS<sub>2</sub> dissociation from RS<sup>ā</sup>, and according
to microscopic reversibility, the transition state of TTC<sup>ā</sup> formation would be simple addition of the RS<sup>ā</sup> nucleophile
to the CS<sub>2</sub> electrophile. At pH 7.4 and 37 Ā°C, cysteine
and glutathione react with CS<sub>2</sub> 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<sup>II</sup>(TTP)(PMe<sub>3</sub>)(NO) (TTP = <i>meso</i>-Tetraā<i>p</i>ātolylporphyrinato Dianion)
Low-temperature in situ Fourier transform
infrared and UVāvis measurements show that trimethylphosphine
(PMe<sub>3</sub>) reacts with microporous layers of Fe<sup>II</sup>(TTP)Ā(NO) (TTP = <i>meso</i>-tetra-<i>p</i>-tolylporphyrinato
dianion; NO = nitric oxide) to form the previously unknown six-coordinate
complex Fe<sup>II</sup>(TTP)Ā(PMe<sub>3</sub>)Ā(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 bisĀ(trimethylphosphine) complex Fe<sup>II</sup>(TTP)Ā(PMe<sub>3</sub>)<sub>2</sub>. Simultaneously, the NO released oxidizes free
PMe<sub>3</sub> to the corresponding phosphine oxide (OPMe<sub>3</sub>) with concomitant formation of nitrous oxide (N<sub>2</sub>O)
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Correction to āA Luminescent and Biocompatible PhotoCORMā
Correction
to āA Luminescent and Biocompatible
PhotoCORM
A Luminescent and Biocompatible PhotoCORM
The water-soluble rheniumĀ(I) complex <i>fac</i>-[ReĀ(bpy)Ā(CO)<sub>3</sub>(thp)]<sup>+</sup> (<b>1</b>) [CF<sub>3</sub>SO<sub>3</sub><sup>ā</sup> salt; bpy = 2,2ā²-bipyridine,
thp
= trisĀ(hydroxymethyl)Āphosphine] 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
ReĀ(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 (CdSeĀ(Ln) QDs)
have been
prepared by heating a solution of CdĀ(oleate)<sub>2</sub>, SeO<sub>2</sub>, and LnĀ(bipy)Ā(S<sub>2</sub>CNEt<sub>2</sub>)<sub>3</sub> (bipy
= 2,2ā²-bipyridine) to 180ā190 Ā°C for 10ā15
min. The elemental compositions of the resulting CdSeĀ(Ln) cores and
CdSeĀ(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 CdSeĀ(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 CdSeĀ(Ln) cores originates with Ln<sup>3+</sup>-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 (<sup>15</sup>NO) with amorphous layers of RuĀ(II) carbonyl porphyrin
(RuĀ(TPP)Ā(CO), TPP<sup>2ā</sup> = <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 Ī½Ā(CO) at 2001 cm<sup>ā1</sup> and Ī½Ā(NO)
at 1810 cm<sup>ā1</sup> (1777 cm<sup>ā1</sup> for <sup>15</sup>NO isotopomer) was readily assigned to the mixed carbonylānitrosyl
complex RuĀ(TPP)Ā(CO)Ā(NO), which is the logical precursor to CO labilization.
Remarkably, RuĀ(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 RuĀ(TPP)Ā(NO)<sub>2</sub>, 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
Ā°C. The first three model compounds represent the Ī²-O-4
and Ī±-O-4 linkages in lignin as well as the furan group commonly
found in the Ī²-5 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 280ā320 Ā°C since it is this process that generates
the reducing equivalents for this catalytic system. In the absence
of substrate, the gaseous products H<sub>2</sub>, CO, and CO<sub>2</sub> were formed in ratios stoichiometrically consistent with catalyzed
methanol reformation and water gas shift reactions. The latter studies
suggest that the H<sub>2</sub> production ceases to be rate limiting
early in batch reactor experiments but also suggest that H<sub>2</sub> 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
FeSO<sub>4</sub>, cysteine (CysSH), and NO demonstrate that both the
rates and the outcomes are markedly pH dependent. The dinuclear DNIC
Fe<sub>2</sub>(Ī¼-CysS)<sub>2</sub>(NO)<sub>4</sub>, a Roussinās
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 FeĀ(NO)<sub>2</sub>(CysS)<sub>2</sub><sup>ā</sup> (<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 FeĀ(NO)Ā(CysS)<sub>2</sub><sup>ā</sup>. <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 FeĀ(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><sub>NO</sub> = 6.9 Ć
10<sup>7</sup> M<sup>ā1</sup> s<sup>ā1</sup>. In contrast,
photolysis of the mononuclear-DNIC species <b>Cys-DNIC</b> formed
from FeĀ(II)/NO/cysteine mixtures in anaerobic pH 10.0 solution did
not labilize NO but instead apparently led to release of the CysS<sup>ā¢</sup> 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 SolāGel Sulfated Zirconia: A Quantitative StructureāReactivity 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 solāgel synthesis. The catalyst preparation
allows for precise surface area control (morphology) and tunable catalytic
activity. The S/Zr ratio (0.1ā2.0) and calcination temperature
(425ā625 Ā°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 structureāreactivity 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 BrĆønsted 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, 14090ā14097). The models studied
are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether,
biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics
of the Ī±-O-4, Ī²-O-4, 4-O-5, 5-5, and Ī²-5 linkages
characteristic of lignin. Also, briefly investigated as a substrate
is poplar organosolv lignin. The catalyst modifications included added
samariumĀ(III) (both homogeneous and heterogeneous) or formic acid.
The highest activity for the hydrogenolysis of aryl ether linkages
was noted for catalysts with SmĀ(III) incorporated into the solid matrix
of the PMO structure. In contrast, simply adding Sm<sup>3+</sup> 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 Ī²-5 C-C bonds that represent
a small, but significant, percentage of the linkages between monolignol
units in lignins