Mechanistic Studies of Aziridine Formation Catalyzed by Mononuclear Non-Heme Iron Enzymes.

Aziridines are compounds with a nitrogen-containing three-membered ring. When it is incorporated into natural products, the reactivity of the strained ring often drives the biological activities of aziridines. Despite its importance, the enzymes and biosynthetic strategies deployed to install this reactive moiety remain understudied. Herein, we report the use of in silico methods to identify enzymes with potential aziridine-installing (aziridinase) functionality. To validate candidates, we reconstitute enzymatic activity in vitro and demonstrate that an iron(IV)-oxo species initiates aziridine ring closure by the C-H bond cleavage. Furthermore, we divert the reaction pathway from aziridination to hydroxylation using mechanistic probes. This observation, isotope tracing experiments using H218O and 18O2, and quantitative product analysis, provide evidence for the polar capture of a carbocation species by the amine in the pathway to aziridine installation.

Evidence for Participation of 4f and 5d Orbitals in Lanthanide Metal-Ligand Bonding and That Y(III) Has Less of This Complex-Stabilizing Ability. A Thermodynamic, Spectroscopic, and DFT Study of Their Complexation by the Nitrogen Donor Ligand TPEN.

The formation constants (log K1) of lanthanide(III) (Ln) ions [except for Pm(III)] and the Y(III) cation have been measured with the ligand TPEN (N,N,N',N'-tetra-2-picolylethylenediamine). These log K1 values show a typical variation with ionic radius, with a local maximum at Sm(III) and a local minimum at Gd(III), with an overall increase in log K1 from La(III) to Lu(III) as the ionic radius decreases. The log K1 for the Y(III)/TPEN complex is much lower than expected from its ionic radius, while the literature log K1 for Am(III) is much higher. The latter effect is thought to be due to greater covalence in the M-L (metal-ligand) bond than for Ln(III) ions: the low log K1 for Y(III) is interpreted as being due to lower covalence. The f → f transitions in the Nd(III) and Pr(III) complexes were examined for effects that might indicate the participation of f orbitals in M-L bonding. The intensity of the f → f transitions in the Nd(III)/TPEN complex was greatly increased compared to that of the Nd3+ aqua ion, which appeared to be due to additional sharp peaks, possibly parity forbidden transitions where parity rules were broken by covalence in the M-L bond. The Pr(III)/TPEN complex showed that all of the f → f transitions shifted to longer wavelengths by some 5 nm, with modest increases in intensity. The effects seen in the f → f transitions of Nd(III) and Pr(III) with TPEN with its six nitrogen donors were present to a much smaller extent in the EDTA and other complexes with fewer nitrogen donors. The changes in the f → f transitions of the TPEN complexes of Er(III) and Ho(III) were small, suggesting a smaller contribution of f orbitals to M-L bonding in these heavier Ln(III) ions. The intense Laporte allowed f → d transitions in Ce(III) complexes show large shifts to longer wavelengths as complexes of, for example, EDTA with increasing numbers of nitrogen donors, suggesting the participation of both f and d orbitals, or either, in M-L bonding. The nature of M-L bonding in M(III)/TPEN complexes was further investigated via density functional theory calculations.

Design and testing of selective inactivators against an antifungal enzyme target.

Systemic infections from fungal organisms are becoming increasingly difficult to treat as drug resistance continues to emerge. To substantially expand the antifungal drug landscape new compounds must be identified and developed with novel modes of action against previously untested drug targets. Most drugs block the activity of their targets through reversible, noncovalent interactions. However, a significant number of drugs form irreversible, covalent bonds with their selected targets. While more challenging to develop, these irreversible inactivators offer some significant advantages as novel antifungal agents. Vinyl sulfones contain a potentially reactive functional group that could function as a selective enzyme inactivator, and members of this class of compounds are now being developed as inactivators against an antifungal drug target. The enzyme aspartate semialdehyde dehydrogenase (ASADH) catalyzes a key step in an essential microbial pathway and is essential for the survival of every microorganism examined. A series of vinyl sulfones have been designed, guided by molecular modeling and docking studies to enhance their affinity for fungal ASADHs. These newly synthesized compounds have been examined against this target enzyme from the pathogenic fungal organism Candida albicans. Vinyl sulfones containing complementary structural elements inhibit this enzyme with inhibition constants in the low-micromolar range. These inhibitors have also led to the rapid and irreversible inactivation of this enzyme, and show some initial selectivity when compared to the inactivation of a bacterial ASADH. The best inactivators will serve as lead compounds for the development of potent and selective antifungal agents.