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The petrochemistry of 2-naphthyl azide studies by employing ultrafast time-resolved spectroscopy and quantum chemical calculations is reported. The singlet nitrene intermediate with a barrier of approximately 5 kcal/mol for [N.sub.2] extrusion in the reaction is observed to have the shortest lifetime.

The cytochrome P450 acts as an oxidant of halogenated benzene derivatives mediated by Compound I. These reactions in which there is selective elimination of halogen atoms can be utilized to tackle the environmental pollution caused by the aromatic halogen derivatives.

The experimental laser flash photolysis and product studies along with molecular modeling shows that azide undergoes both [alpha]-cleavage and energy transfer to form benzoyl radical and triplet alkylnitrene while azide selectively forms triplet alkylnitrene. Based on the results it is concluded that benzoyl radicals could be formed from nitrenes and nitrene also undergo [alpha]-photocleavage to form benzoyl radicals and iminyl radicals.

The photochemistry of para- and ortho-biphenylyl azides and 1-naphthyl azide is studied by ultrafast spectroscopy. In each case the singlet azide second excited states are observed by transient absorption spectroscopy and are found to have lifetimes of hundreds of femtoseconds. The lifetime of singlet ortho-biphenynitrene is 16 ps in acetonitrite and is not affected by perdeutration of the aryl ring.

The iron(IV)-oxo porphyrin p-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis. [ABSTRACT FROM AUTHOR]

High-valent oxo-metal complexes exhibit correlated electronic behavior on dense, low-lying electronic state manifolds, presenting challenging systems for electronic structure methods. Among these species, the iron-oxo (IV) porphyrin denoted Compound I occupies a privileged position, serving a broad spectrum of catalytic roles. The most reactive members of this family bear a thiolate axial ligand, exhibiting high activity toward molecular oxygen activation and substrate oxidation. The default approach to such systems has entailed the use of hybrid density functionals or multi-configurational/multireference methods to treat electronic correlation. An alternative approach is presented based on the GGA+U approximation to density functional theory, in which a generalized gradient approximation (GGA) functional is supplemented with a localization correction to treat on-site correlation as inspired by the Hubbard model. The electronic structure of thiolate-ligated iron-oxo (IV) porphyrin and corresponding Coulomb repulsion U are determined both empirically and self-consistently, yielding spin-distributions, state level splittings, and electronic densities of states consistent with prior hybrid functional calculations. Comparison of this detailed electronic structure with model Hamiltonian calculations suggests that the localized 3d iron moments induce correlation in the surrounding electron gas, strengthening local moment formation. This behavior is analogous to strongly correlated electronic systems such as Mott insulators, in which the GGA+U scheme serves as an effective single-particle representation for the full, correlated many-body problem. [ABSTRACT FROM AUTHOR]

Deformylation mechanisms of the sterol 14[alpha]-demethylase (CYP51) from Mycobacterium tuberculosis are analyzed by using molecular dynamics simulations, density functional theory and hybrid quantum mechanics/molecular mechanics methods. The studies have provided a unified mechanistic view at understanding the deformylation reaction and also into the catalytic mechanisms of several biosynthetic and xenobiotic-oxidizing CYP enzyme of biomedical importance.

The first observation of aryl azide excited singlet states and the formation of the related singlet nitrenes is reported. Aryl azide excited states decompose to form singlet aryl nitrenes whose growth can be observed by ultrafast spectroscopy.

A model system of the cytochrome P450 active site and truncated steroid substrates are studied with density functional theory to unravel the third step of aromatase catalysis. The calculations support a dehydrogenase behavior of aromatase in the final catalytic step.

Abstract: This report seeks to address the role of hydrogen bonding with Brønsted acids and bases in proton-coupled electron transfer (PCET) as it pertains to concerted or stepwise pathways of quinone (Q) and hydroquinone (QH2) electrochemistry. This study was performed using a series of techniques that included cyclic voltammetry (CV), digital simulations, computational chemistry and 1H NMR. Hydrogen bonding was inferred by a decrease in diffusion coefficient (D) values measured using a pulsed gradient echo- (PGE-) 1H NMR technique. Changes of 40.8% and 37.9% in D values were only noted after the addition of two equivalents of acetate to 1,4-hydroquinone (1,4-QH2) and catechol (1,2-QH2), respectively. In contrast, the D values for the addition of selected amines (pyridine, N,N-diisopropylethylamine and triethylamine) changed only 3.2% on average. Quantum mechanical calculations were conducted to determine the pK a of all quinoid species to serve as a starting point for the determination of equilibrium constants in voltammetric simulations. Simulations indicate that 1,4-benzoquinone undergoes stepwise electron–proton transfer upon addition of acetic acid, N-ethyldiisopropylammonium perchlorate and pyridinium nitrate and were simulated without the presence of hydrogen bonds. The QH2 compounds show stepwise proton–electron transfers after addition of the both the conjugate amines and acetate. [ABSTRACT FROM AUTHOR]

Aromatase (CYP19A1), the enzyme that converts androgens to estrogens, is of significant mechanistic and therapeutic interest. Crystal structures and computational studies of this enzyme shed light on the critical role of Asp309 in substrate binding and catalysis. These studies predicted an elevated pKa for Asp309 and proposed that protonation of this residue was required for function. In this study, UV-visible absorption, circular dichroism, resonance Raman spectroscopy, and enzyme kinetics were used to study the impact of pH on aromatase structure and androstenedione binding. Spectroscopic studies demonstrate that androstenedione binding is pH-dependent, whereas, in contrast, the D309N mutant retains its ability to bind to androstenedione across the entire pH range studied. Neither pH nor mutation perturbed the secondary structure or heme environment. The origin of the observed pH dependence was further narrowed to the protonation equilibria of Asp309 with a parallel set of spectroscopic studies using exemestane and anastrozole. Because exemestane interacts with Asp309 based on its co-crystal structure with the enzyme, its binding is pH-dependent. Aromatase binding to anastrozole is pH-independent, consistent with the hypothesis that this ligand exploits a distinct set of interactions in the active site. In summary, we assign the apparent pKa of 8.2 observed for androstenedione binding to the side chain of Asp309. To our knowledge, this work represents the first experimental assignment of a pKa value to a residue in a cytochrome P450. This value is in agreement with theoretical calculations (7.7-8.1) despite the reliance of the computational methods on the conformational snapshots provided by crystal structures. [ABSTRACT FROM AUTHOR]