Characterizing the Intermediates Compound I and II in the Cytochrome P450 Catalytic Cycle with Nonlinear X-ray Spectroscopy: A Simulation Study

Stimulated X-ray emission spectroscopy

We describe an emerging hard X-ray spectroscopy technique, stimulated X-ray emission spectroscopy (S-XES). S-XES has the potential to characterize the electronic structure of 3d transition metal complexes with spectral information currently not reachable and might lead to the development of new ultrafast X-ray sources with properties beyond the state of the art. S-XES has become possible with the emergence of X-ray free-electron lasers (XFELs) that provide intense femtosecond X-ray pulses that can be employed to generate a population inversion of core-hole excited states resulting in stimulated X-ray emission. We describe the instrumentation, the various types of S-XES, the potential applications, the experimental challenges, and the feasibility of applying S-XES to characterize dilute systems, including the Mn4Ca cluster in the oxygen evolving complex of photosystem II.

Enhanced Metabolic Activity of Cytochrome P450 via Carbon Nanocage-Based Photochemical Bionanoreactor

Recently, the early screening of the genotoxicity of new chemicals and drugs calls for the envelope of micro-/nanoreactors for metabolic study. Herein, a novel light-driven enzymatic bionanoreactor is designed with the gold nanoparticle (NP)-modified carbon nanocage (Au@CNC) as a nanoreactor and meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP) as a photosensitizer for cytochrome P450-mediated drug metabolism. By confining the cytochrome P450 3A4 (CYP3A4) enzyme and TCPP inside the pores of Au@CNC, a high metabolic activity is achieved by using 7-ethoxytrifluoromethyl coumarin as the substrate because of the three-dimensional hierarchical porous structure, large surface area, and fast electron transfer capacity of Au@CNC. It is noted that owing to the presence of AuNPs inside CNC, the surface hydrophilicity of CNC is much improved, which further promotes the catalytic activity of the CYP3A4 enzyme. To our knowledge, this is the first attempt to apply CNC as a bionanoreactor for NADPH-free and light-driven in vitro drug metabolism. In addition, the presented bionanoreactor exhibits a variety of advantages in terms of fast response, short assay time (10 min), high sensitivity, and good selectivity, which are expected to expedite drug screening and render potential advances in drug discovery and development.

Towards characterization of photo-excited electron transfer and catalysis in natural and artificial systems using XFELs

The ultra-bright femtosecond X-ray pulses provided by X-ray Free Electron Lasers (XFELs) open capabilities for studying the structure and dynamics of a wide variety of biological and inorganic systems beyond what is possible at synchrotron sources. Although the structure and chemistry at the catalytic sites have been studied intensively in both biological and inorganic systems, a full understanding of the atomic-scale chemistry requires new approaches beyond the steady state X-ray crystallography and X-ray spectroscopy at cryogenic temperatures. Following the dynamic changes in the geometric and electronic structure at ambient conditions, while overcoming X-ray damage to the redox active catalytic center, is key for deriving reaction mechanisms. Such studies become possible by using the intense and ultra-short femtosecond X-ray pulses from an XFEL, where sample is probed before it is damaged. We have developed methodology for simultaneously collecting X-ray diffraction data and X-ray emission spectra, using an energy dispersive spectrometer, at ambient conditions, and used this approach to study the room temperature structure and intermediate states of the photosynthetic water oxidizing metallo-protein, photosystem II. Moreover, we have also used this setup to simultaneously collect the X-ray emission spectra from multiple metals to follow the ultrafast dynamics of light-induced charge transfer between multiple metal sites. A Mn-Ti containing system was studied at an XFEL to demonstrate the efficacy and potential of this method.

Coherent control of long-range photoinduced electron transfer by stimulated X-ray Raman processes

We show that X-ray pulses resonant with selected core transitions can manipulate electron transfer (ET) in molecules with ultrafast and atomic selectivity. We present possible protocols for coherently controlling ET dynamics in donor-bridge-acceptor (DBA) systems by stimulated X-ray resonant Raman processes involving various transitions between the D, B, and A sites. Simulations presented for a Ru(II)-Co(III) model complex demonstrate how the shapes, phases and amplitudes of the X-ray pulses can be optimized to create charge on demand at selected atoms, by opening up otherwise blocked ET pathways.