Mechanistic investigation of sustainable heme-inspired biocatalytic synthesis of cyclopropanes for challenging substrates.

Engineered heme proteins exhibit excellent sustainable catalytic carbene transfer reactivities toward olefins for value-added cyclopropanes. However, unactivated and electron-deficient olefins remain challenging in such reactions. To help design efficient heme-inspired biocatalysts for these difficult situations, a systematic quantum chemical mechanistic study was performed to investigate effects of olefin substituents, non-native amino acid axial ligands, and natural and non-natural macrocycles with the widely used ethyl diazoacetate.

One-Electron NO to NO Pathways via Heme Models and Lewis Acid: Metal Effects and Differences from the Enzymatic Reaction.

Some pathogens use heme-containing nitric oxide reductases (NORs) to reduce NO to NO as their defense mechanism to detoxify NO and reduce nitrosative stress. This reduction is also significant in the global N cycle. Our previous experimental work showed that Fe and Co porphyrin NO complexes can couple with external NO to form NO when activated by the Lewis acid BF.

Mechanistic manifold in a hemoprotein-catalyzed cyclopropanation reaction with diazoketone.

Hemoproteins have recently emerged as promising biocatalysts for new-to-nature carbene transfer reactions. However, mechanistic understanding of the interplay between productive and unproductive pathways in these processes is limited. Using spectroscopic, structural, and computational methods, we investigate the mechanism of a myoglobin-catalyzed cyclopropanation reaction with diazoketones.

Liposome-Templated Green Synthesis of Mesoporous Metal Nanostructures with Universal Composition for Biomedical Application.

Porous noble metal nanoparticles have received particular attention recently for their unique optical, thermal, and catalytic functions in biomedicine. However, limited progress has been made to synthesize such porous metallic nanostructures with large mesopores (≥25 nm). Here, a green yet facile synthesis strategy using biocompatible liposomes as templates to mediate the formation of mesoporous metallic nanostructures in a controllable fashion is reported.

Insights into the Observed -Bond Length Variations upon NO Binding to Ferric and Ferrous Porphyrins with Neutral Axial Ligands.

NO is well-known for its effect. NO binding to ferrous hemes of the form (por)Fe(L) (L = neutral N-based ligand) to give the {FeNO} (por)Fe(NO)(L) product results in a lengthening of the axial Fe-L bond. In contrast, NO binding to the ferric center in [(por)Fe(L)] to give the {FeNO} [(por)Fe(NO)(L)] product results in a shortening of the Fe-L bond.

Stepwise nitrosylation of the nonheme iron site in an engineered azurin and a molecular basis for nitric oxide signaling mediated by nonheme iron proteins.

Mononitrosyl and dinitrosyl iron species, such as {FeNO}, {FeNO} and {Fe(NO)}, have been proposed to play pivotal roles in the nitrosylation processes of nonheme iron centers in biological systems. Despite their importance, it has been difficult to capture and characterize them in the same scaffold of either native enzymes or their synthetic analogs due to the distinct structural requirements of the three species, using redox reagents compatible with biomolecules under physiological conditions. Here, we report the realization of stepwise nitrosylation of a mononuclear nonheme iron site in an engineered azurin under such conditions.

Insight into the preferential N-binding O-binding of nitrosoarenes to ferrous and ferric heme centers.

Nitrosoarenes (ArNOs) are toxic metabolic intermediates that bind to heme proteins to inhibit their functions. Although much of their biological functions involve coordination to the Fe centers of hemes, the factors that determine N-binding or O-binding of these ArNOs have not been determined. We utilize X-ray crystallography and density functional theory (DFT) analyses of new representative ferrous and ferric ArNO compounds to provide the first theoretical insight into preferential N-binding versus O-binding of ArNOs to hemes.

NTO Sensing by Fluorescence Quenching of a Pyoverdine Siderophore-A Mechanistic Approach.

In this study, a siderophore, pyoverdine (PVD), has been isolated from Pseudomonas sp. and used to develop a fluorescence quenching-based sensor for efficient detection of nitrotriazolone (NTO) in aqueous media, in contrast to other explosives such as research department explosive (RDX), picric acid, and trinitrotoulene (TNT). The siderophore PVD exhibited enhanced fluorescence quenching above 50% at 470 nm for a minimal concentration (38 nM) of NTO.

Not Limited to Iron: A Cobalt Heme-NO Model Facilitates N-N Coupling with External NO in the Presence of a Lewis Acid to Generate N O.

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N O. We previously reported that a heme Fe-NO model engages in this N-N bond-forming reaction with NO. We now demonstrate that (OEP)Co (NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX (X=F, C F ) to generate N O.

Mechanistic Investigation of Biocatalytic Heme Carbenoid Si-H Insertions.

Recent studies reported the development of biocatalytic heme carbenoid Si-H insertions for the selective formation of carbon-silicon bonds, but many mechanistic questions remain unaddressed. To this end, a DFT mechanistic investigation was performed which reveals an Fe-based concerted hydride transfer mechanism with early transition state feature. The results from these computational analyses are consistent with experimental data of radical trapping, kinetic isotope effects, and structure-reactivity data using engineered variants of hemoproteins.

Biocatalytic Strategy for Highly Diastereo- and Enantioselective Synthesis of 2,3-Dihydrobenzofuran-Based Tricyclic Scaffolds.

2,3-Dihydrobenzofurans are key pharmacophores in many natural and synthetic bioactive molecules. A biocatalytic strategy is reported here for the highly diastereo- and enantioselective construction of stereochemically rich 2,3-dihydrobenzofurans in high enantiopurity (>99.9% de and ee), high yields, and on a preparative scale via benzofuran cyclopropanation with engineered myoglobins.

Catalytic Role of Conserved Asparagine, Glutamine, Serine, and Tyrosine Residues in Isoprenoid Biosynthesis Enzymes.

We report the results of an investigation into the catalytic role of highly conserved amide (asparagine, glutamine) and OH-containing (serine, tyrosine) residues in several prenyltransferases. We first obtained the X-ray structure of cyclolavandulyl diphosphate synthase containing two molecules of the substrate analog dimethylallyl ()-thiolodiphosphate (DMASPP). The two molecules have similar diphosphate group orientations to those seen in other ζ-fold (- head-to-tail and head-to-middle) prenyltransferases with one diphosphate moiety forming a bidentate chelate with Mg in the so-called S1 site (which is typically the allylic binding site in ζ-fold proteins) while the second diphosphate binds to Mg in the so-called S2 site (which is typically the homoallylic binding site in ζ-fold proteins) a single P1O1 oxygen.

Excited State Interaction of Ruthenium (II) Imidazole Phenanthroline Complex [Ru(bpy) ipH] with 1,4-Benzoquinone: Simple Electron Transfer or Proton-Coupled Electron Transfer?

The unidirectional proton coupled electron transfer (PCET) from the excited state of Ru(II) imidazole phenanthroline complex [Ru(bpy) ipH] to 1,4-benzoquinone, was studied by steady-state (SS) and time-resolved (TR) fluorescence and transient absorption (TA) measurements. The pK (9.7) and pK * (8.

Bisphosphonate-Generated ATP-Analogs Inhibit Cell Signaling Pathways.

Bisphosphonates are a major class of drugs used to treat osteoporosis, Paget's disease, and cancer. They have been proposed to act by inhibiting one or more targets including protein prenylation, the epidermal growth factor receptor, or the adenine nucleotide translocase. Inhibition of the latter is due to formation in cells of analogs of ATP: the isopentenyl ester of ATP (ApppI) or an AppXp-type analog of ATP, such as AMP-clodronate (AppCClp).

Lewis Acid Activation of the Ferrous Heme-NO Fragment toward the N-N Coupling Reaction with NO To Generate NO.

Bacterial NO reductase (bacNOR) enzymes utilize a heme/non-heme active site to couple two NO molecules to NO. We show that BF coordination to the nitrosyl O-atom in (OEP)Fe(NO) activates it toward N-N bond formation with NO to generate NO. N-isotopic labeling reveals a reversible nitrosyl exchange reaction and follow-up N-O bond cleavage in the NO formation step.

C-H Insertions by Iron Porphyrin Carbene: Basic Mechanism and Origin of Substrate Selectivity.

Recent experimental reports of heme carbene C-H insertions show promising results for sustainable chemistry due to good yield and selectivity, low cost of iron, and low/no toxicity of hemes. But mechanistic details are mostly unknown. Despite structural similarity and isoelectronic nature between heme carbene and the Fe =O intermediate, our quantum chemical studies with detailed geometric and electronic information for the first time reveal an Fe -based, concerted, hydride-transfer mechanism, which is different from the Fe -based stepwise hydrogen atom transfer mechanism for C-H functionalization by native heme enzymes.

HNO-Binding in Heme Proteins: Effects of Iron Oxidation State, Axial Ligand, and Protein Environment.

HNO plays significant roles in many biological processes. Numerous heme proteins bind HNO, an important step for its biological functions. A systematic computational study was performed to provide the first detailed trends and origins of the effects of iron oxidation state, axial ligand, and protein environment on HNO binding.

Hydride Attack on a Coordinated Ferric Nitrosyl: Experimental and DFT Evidence for the Formation of a Heme Model-HNO Derivative.

Heme-HNO species are crucial intermediates in several biological processes. To date, no well-defined Fe heme-HNO model compounds have been reported. Hydride attack on the cationic ferric [(OEP)Fe(NO)(5-MeIm)]OTf (OEP = octaethylporphyrinato dianion) generates an Fe-HNO product that has been characterized by IR and (1)H NMR spectroscopy.

Catalytic and Biocatalytic Iron Porphyrin Carbene Formation: Effects of Binding Mode, Carbene Substituent, Porphyrin Substituent, and Protein Axial Ligand.

Iron porphyrin carbenes (IPCs) are important intermediates in various chemical reactions catalyzed by iron porphyrins and engineered heme proteins, as well as in the metabolism of various xenobiotics by cytochrome P450. However, there are no prior theoretical reports to help understand their formation mechanisms and identify key information governing the binding mode, formation feasibility, and stability/reactivity. A systematic quantum chemical study was performed to investigate the effects of carbene substituent, porphyrin substituent, and axial ligand on IPC formation pathways.

Solid-state ¹⁷O NMR spectroscopy of paramagnetic coordination compounds.

High-quality solid-state (17)O (I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers. For complexes containing V(III) (S=1), Cu(II) (S=1/2), and Mn(III) (S=2) metal centers, the (17)O isotropic paramagnetic shifts were found to span a range of more than 10,000 ppm. In several cases, high-resolution (17)O NMR spectra were recorded under very fast magic-angle spinning (MAS) conditions at 21.

A distonic radical-ion for detection of traces of adventitious molecular oxygen (O2) in collision gases used in tandem mass spectrometers.

We describe a diagnostic ion that enables rapid semiquantitative evaluation of the degree of oxygen contamination in the collision gases used in tandem mass spectrometers. Upon collision-induced dissociation (CID), the m/z 359 positive ion generated from the analgesic etoricoxib undergoes a facile loss of a methyl sulfone radical [(•)SO(2)(CH(3)); 79-Da] to produce a distonic radical cation of m/z 280. The product-ion spectrum of this m/z 280 ion, recorded under low-energy activation on tandem-in-space QqQ or QqTof mass spectrometers using nitrogen from a generator as the collision gas, or tandem-in-time ion-trap (LCQ, LTQ) mass spectrometers using purified helium as the buffer gas, showed two unexpected peaks at m/z 312 and 295.

Iron porphyrin carbenes as catalytic intermediates: structures, Mössbauer and NMR spectroscopic properties, and bonding.

Iron porphyrin carbenes (IPCs) are thought to be intermediates involved in the metabolism of various xenobiotics by cytochrome P450, as well as in chemical reactions catalyzed by metalloporphyrins and engineered P450s. While early work proposed IPCs to contain Fe(II), more recent work invokes a double-bond description of the iron-carbon bond, similar to that found in Fe(IV) porphyrin oxenes. Reported herein is the first quantum chemical investigation of IPC Mössbauer and NMR spectroscopic properties, as well as their electronic structures, together with comparisons to ferrous heme proteins and an Fe(IV) oxene model.

How does the urea dynamics differ from water dynamics inside the reverse micelle?

In this study, the urea dynamics inside AOT reverse micelle (RM) has been monitored without intervention of water using time-resolved fluorescence techniques from the picosecond to nanosecond time regime. It has been observed that urea dynamics inside the reverse micelle is severely retarded compared to water RM due to the formation of highly networked urea cluster inside the RM. Time-resolved fluorescence anisotropy study also confirms the existence of a confined environment around the dye at higher concentrations of urea inside the reverse micelle.

Structural, EPR superhyperfine, and NMR hyperfine properties of the Cu-octarepeat binding site in the prion protein.

Previous experimental and computational investigations show that the copper binding in the prion protein that is involved in a number of neurodegenerative diseases is complicated and the exact binding structures remain to be determined. To facilitate structural investigation in this field, we report a quantum chemical investigation of structural, EPR superhyperfine, and NMR hyperfine properties of various copper complexes of the octarepeat domain, which has several copies of highly conserved amino acid sequence of PHGGGWGQ. The predicted metal-ligand bond lengths of the X-ray structure of CuHGGGW, involving the central five residues in this domain, from the best method examined here, have a mean absolute deviation (MAD) of 0.