Synthesis, characterization, and reactivity of rhenium dithiodiolate and monothiodiolate complexes

The rhenium dithiodiolate, Hydrido-tris-(3 ,5-dimethyl- 1 -pyrazol yl) borato(ethane- 1 ,2-dithiodiolato)(oxo)rheni um(V), and Hydrido-tris-(3 ,5-dimethyl- 1 -pyrazolyl)borato(phenylethanedithiodiolato)(oxo)rhenium(V), were synthesized by reductive cyclocondensation of alkane-1,2-dithiols with Tp'Re0₃ in a one pot fashion. The rhemium monothiodiolate, Hydrido-tris-(3 ,5-dimethyl- 1 -pyrazolyl) borato(ethane- 1 ,2-monothiodiolato)(oxo)rhenium(V), and Hydrido-tris-(3 ,5- dimethyl- 1 -pyrazolyl)borato(propanemonothiodiolato)(oxo)rhenium(V), were also synthesized by using the same procedure. The syn and anti isomers of rhenium phenylethanedithiodiolate and rhenium propanemonothiodiolate were characterized by using COSY, nOe, and HSQC experiments. An X-ray crystal structure of the rhenium ethanedithiodiolate was obtained, and an unusually small dihedral angle (S-C-C-S) of about 12° was observed. The solution conformation of these compounds was investigated by using a Karplus relationship between vicinal coupling constants and dihedral angle. The dihedral angle for the ethanedithiodiolate appeared to be 38° indicating a staggered geometry for the ring. All of these complexes failed to cyclorevert to any detectable extent at 120°C after 7 days. This observation reflects the thermal stability of rhenium dithiodiolate and monothiodiolate complexes. Energetics of ethylene addition to tetrathioperrhenate anion (ReS₄⁻) and addition of hydrogen sulfide to the alkene adduct were calculated by DFT calculation using LACVP** basis set with B3LYP functionals. The heat of reaction of ethylene addition to ReS₄⁻ was 15.4 kcal/mol, and 111.1 kcal/mol for addition of hydrogen sulfide to the alkene adduct. These results are consistent with the stability of the dithiodiolate complex toward cycloreversion reaction. Reaction of Tp'Re0₃ and ethylene sulfide led to the rhenium ethanedithiolate complex both with and without acid catalysis. This observation led to a proposed multi step mechanism

The asparagine-transamidosome from Helicobacter pylori: a dual-kinetic mode in non-discriminating aspartyl-tRNA synthetase safeguards the genetic code

Helicobacter pylori catalyzes Asn-tRNAAsn formation by use of the indirect pathway that involves charging of Asp onto tRNAAsn by a non-discriminating aspartyl-tRNA synthetase (ND-AspRS), followed by conversion of the mischarged Asp into Asn by the GatCAB amidotransferase. We show that the partners of asparaginylation assemble into a dynamic Asn-transamidosome, which uses a different strategy than the Gln-transamidosome to prevent the release of the mischarged aminoacyl-tRNA intermediate. The complex is described by gel-filtration, dynamic light scattering and kinetic measurements. Two strategies for asparaginylation are shown: (i) tRNAAsn binds GatCAB first, allowing aminoacylation and immediate transamidation once ND-AspRS joins the complex; (ii) tRNAAsn is bound by ND-AspRS which releases the Asp-tRNAAsn product much slower than the cognate Asp-tRNAAsp; this kinetic peculiarity allows GatCAB to bind and transamidate Asp-tRNAAsn before its release by the ND-AspRS. These results are discussed in the context of the interrelation between the Asn and Gln-transamidosomes which use the same GatCAB in H. pylori, and shed light on a kinetic mechanism that ensures faithful codon reassignment for Asn

Lipid Chain Selectivity by Outer Membrane Phospholipase A

Outer membrane phospholipase A (OMPLA) is a unique, integral membrane enzyme found in Gram-negative bacteria and is an important virulence factor for pathogens such as Helicobacter pylori. This broadspecificity lipase degrades a variety of lipid substrates, and it plays a direct role in adjusting the composition and permeability of bacterial membranes under conditions of stress. Interestingly, OMPLA shows little preference for the lipid headgroup and, instead, the length of the hydrophobic acyl chain is the strongest determinant for substrate selection by OMPLA, with the enzyme strongly preferring substrates with chains equal to or longer than 14 carbon atoms. The question remains as to how a hydrophobic protein like OMPLA can achieve this specificity, particularly when the shorter chains can be accommodated in the binding pocket. Using a series of sulfonyl fluoride inhibitors with various lengths of acyl chain, we show here that the thermodynamics of substrate-induced OMPLA dimerization are guided by the acyl chain length, demonstrating that OMPLA uses a unique biophysical mechanism to select its phospholipid substrate

Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of <i>rac</i>-Lactide: Experimental and Theoretical Studies

A series of bis­(pyrrolidene) Schiff base aluminum complexes (<b>1</b>–<b>7</b>) were synthesized and characterized by NMR spectroscopy and elemental analysis. All complexes were efficient initiators for the ring-opening polymerizations of l-LA and <i>rac</i>-LA in toluene at 70 °C. Kinetic studies revealed first-order kinetics in monomer and the rates of l-LA and <i>rac</i>-LA polymerizations decreased in the order of 1,2-benzylene (<b>4</b>) ≫ 1,3-propylene (<b>2</b>) > 2,2-dimethyl-1,3-propylene (<b>3</b>) > 1,4-butylene (<b>5</b>) > <i>rac</i>-1,2-cyclohexylene (<b>7</b>) > 1,2-ethylene (<b>1</b>) ≫ 1,2-phenylene (<b>6</b>). Microstructure analyses of the resulting polylactides by homonuclear decoupled ¹H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except <b>5</b>. Isotactic stereoblock polylactide with a high <i>P</i>_m value of 0.80 was produced by <b>3</b>. A systematic DFT study on the <i>rac</i>-lactide ring-opening mechanism initiated by the initiators synthesized in this study revealed the correlation between the structure of backbone linker and the polymerization activity and stereoselectivity

Regioselectivity of Larock Heteroannulation: A Contribution from Electronic Properties of Diarylacetylenes

A series of 2,3-diarylindoles were synthesized from 2-iodoaniline and unsymmetrical diarylacetylenes using the Larock heteroannulation. Diarylacetylenes bearing electron-withdrawing substituents lead to 2,3-diarylindoles with substituted phenyl moieties at the 2-position as major products, while those with electron-donating groups preferably yield indole products with substituted phenyl moieties at the 3-position. The regioisomeric product ratios exhibit a clear correlation with Hammett σ_p values. DFT calculations reveal the origin of this effect, displaying smaller activation energy barriers for those pathways leading to the major regioisomer