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Chiral gold complexes have been applied in homogeneous catalytic reactions since 1986, in some cases with high enantioselectivity. Acyclic diaminocarbene (ADC) ligands are acyclic analogues of N-heterocyclic carbenes (NHCs) that have larger N-CCarbene-N angles and stronger donating ability. ADCs have been developed as alternatives to phosphine and NHC ligands in homogeneous gold catalysis. In 2012, a new series of chiral gold(I) ADCs were first developed by Slaughter's group and were shown to give remarkable enantioselectivities in some reactions. Because of the hindered rotation of the N-CCarbene bonds of ADC, chiral ADC substituents can easily get close to the metal center in some conformations, although two rotameric structures are formed if the chiral amine is nonsymmetric. The selective of specific ADC conformations was the initial focus of this study. Formational selectivity of one diastereomer of an ADC ligand during synthesis was examines by measuring the relative rates of diastereomer formation in a 1H NMR kinetic study. The potential for converting multiple conformational isomers of ADCs into a single conformation, or at least a simpler mixture, was examined. This study used the analogy that anti- isomer has electronic and structural similarity with urea/thiourea, raising the possibility that 1,8-naphthyridine can be used to favor certain conformations through a self-assembled hydrogen-bonding complex. Gold(I) is a soft carbophilic Lewis acid able to active C-C π bonds to nucleophilic attack, and ADC-gold complexes are potentially useful in this regard. Therefore, biaryl gold(I) ADC complexes were examine with silver salt additives in catalytic 1,6-enyne cyclization reaction. A detailed study found that the counteranion affects the regioselectivities of these reactions more than substituents on the ancillary ADC ligands.

Complexes with pincer ligand moieties have garnered much attention in the past few decades. They have been shown to be highly active catalysts in several known transition metal-catalyzed organic reactions as well as some unprecedented organic transformations. At the same time, the use of computational organometallic chemistry to aid in the understanding of the mechanisms in organometallic catalysis for the development of improved catalysts is on the rise. While it was common in earlier studies to reduce computational cost by truncating donor group substituents on complexes such as tertbutyl or isopropyl groups to hydrogen or methyl groups, recent advancements in the processing capabilities of computer clusters and codes have streamlined the time required for calculations. As the full modeling of complexes become increasingly popular, a commonly overlooked aspect, especially in the case of complexes bearing isopropyl substituents, is the conformational analysis of complexes. Isopropyl groups generate a different conformer with each 120 ° rotation (rotamer), and it has been found that each rotamer typically resides in its own potential energy well in density functional theory studies. As a result, it can be challenging to select the most appropriate structure for a theoretical study, as the adjustment of isopropyl substituents from a higher-energy rotamer to the lowest-energy rotamer usually does not occur during structure optimization. In this report, the influence of the arrangement of isopropyl substituents in pincer complexes on calculated complex structure energies as well as a case study on the mechanism of the isomerization of an iPrPCP-Fe complex is covered. It was found that as many as 324 rotamers can be generated for a single complex, as in the case of an iPrPCP-Ni formato complex, with the energy difference between the global minimum and the highest local minimum being as large as 16.5 kcalmol-1. In the isomerization of a iPrPCP-Fe complex, it was found that the isopropyl substituents not only influence the calculated structure energies, but they dictate the mechanism of isomerization with the rotation of isopropyl substituents from the arrangement in the starting material complex to the arrangement in the product complex being the rate-determining step.

Abstract: Pincer compounds are organometallic complexes with intriguing tunable reactivities. In this work we explore their unique properties and reactivities through spectroscopic and computational investigations, with a focus on the PN3P pincer platform. First, we conducted a computational study on five pincer complexes with stereogenic phosphine arms that have multiple well-defined rotamers. Significant energy differences could be found between the lowest and highest energy rotamer in each set of pincer complexes. All rotamers for reactant, transition state, and product, were evaluated in a reaction energy profile of a CO2 reduction by a pincer nickel hydride, and we found that this reaction could be found either favorable or unfavorable, depending on the choice of rotamer. A software to generate rotamers has been developed and applied to the work presented in this part. The zwitterionic aromatic resonance form has a large contribution in the dearomatized PN3P* nickel pincer complexes, which is demonstrated by the imine arm's ability to act as an organic σ-donor, similar to NHC catalysts. Related to this property, as well as the pincer compound's ability to undergo metal-ligand cooperation catalysis, is the basicity (or acidity) of pincer ligand spacer arms. Therefore, we have determined the Brønsted basicity of the imine arm in three PN3P* nickel pincer complexes in THF. The relative basicity was found to be strongly influenced by the X ligand trans to the PN3P* ligand, and less by alkyl groups on phosphine donor arms. Finally, we explored the reactivity between a PN3P* rhodium carbonyl pincer complex and dioxygen at room temperature in solution, and at elevated temperature in the solid state. Intriguingly, the singlet PN3P* rhodium carbonyl complex reacts with the triplet dioxygen both in solution and in the solid state to afford oxidation on the ligand backbone. This is possible due to the ligands ability to do a single-electron transfer to dioxygen. The solid state reaction was studied with in situ rhodium K-edge X-ray absorption spectroscopy under dioxygen flow, where an isobestic point was observed, and simulation studies support formation of a Rh-O2 adduct. In situ FTIR studies in a static dioxygen environment revealed that the PN3P* rhodium carbonyl complex is able to facilitate the incorporation of O2 into CO and CO2.

Molecular typing of strains among the highly diverse population of Helicobacter pylori (H. pylori) is an important approach for both basic and clinical studies. Genomic DNAs prepared from 18 gastric biopsy specimens, 21 H. pylori clinical isolates obtained from gastric biopsy specimens, and five isolates collected from a single patient at weekly intervals, were subjected to a combined single- and double-stranded conformational polymorphism (SDSCP) assay. The results showed that 19 of 21 isolates tested were discriminated by SDSCP analysis. SDSCP analysis of five H. pylori isolates collected from the same patient at different times resulted in five identical profiles, suggesting the reproducibility of the method. When DNA preparations from 18 gastric biopsy specimens were subjected to SDSCP analysis, 18 unique profiles were generated that matched those of their corresponding cultured H. pylori isolates from each patient. For comparison, polymerase chain reaction (PCR)-based restriction fragment length polymorphism analysis yielded only nine profiles for 20 strains. The data suggest that SDSCP analysis may be an effective and reliable method for differentiation of H. pylori strains directly from gastric biopsy specimens without requiring isolation of the organisms by culture.

Organic co-solvent-induced secondary conformation of α17-40 of human hemoglobin facilitates the splicing of E30-R31 in a mixture of its complementary segments by V8 protease. The amino acid sequence of α17-40 has been conceptualized by the general structure FR1-EALER-FRII and the pentapeptide sequence EALER playing a major role in inducing the α-helical conformation. The primary structure of α17-40 has been engineered in multiple ways to perturb one, two, or all three regions and the influence of the organic co-solvent-induced conformation and the concomitant resistance of E30-R31 peptide bond to V8 protease digestion has been investigated. The central pentapeptide (EALER), referred to here as splicedon,3 appears to dictate a primary role in facilitating the splicing reaction. When the same flanking regions are used, (1) splicedons that carry amino acid residues of low α-helical potential, for example G at position 2 or 3 of the splicedon, generate a conformational trap of very low thermodynamic stability, giving an equilibrium yield of only 3%-5%; (2) splicedons with amino acid residues of good α-helical potential generate a conformational trap of medium thermodynamic stability and give an equilibrium yield of 20%-25%; (3) the splicedons with amino residues of good α-helical potential and also an amino acid that can generate an i, i + 4 side-chain carboxylate-guanidino (amino) interaction, a conformational trap of maximum thermodynamic stability is generated, giving an equilibrium yield of 45%-50%; and (4) the thermodynamic stability of the conformational trap of the spliced peptide is also influenced by the amino acid composition of the flanking regions. The V8 protease resistance of the spliced peptide bond is not a direct correlate of the amount of α-helical conformation induced into the product. The results of this study reflect the unique role of the splicedon in translating the organic co-solvent-induced product conformation as a site-specific stabilization of the spliced peptide bond. It is speculated that the splicedon with higher α-helical potential as compared to either one of the flanking regions achieves this by integrating its potential with that of the flanking region(s). Exchange of flanking regions with the products of other V8 protease-catalyzed splicing reactions will help to establish the general primary structural requirements of this class of splicing reactions and facilitate their application in modular construction of proteins.

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