Electronic and Nuclear Structural Snapshots in Ligand Dissociation and Recombination Processes of Iron Porphyrin in Solution: A Combined Optical/X-ray Approach

X-ray snapshots reveal conformational influence on active site ligation during metalloprotein folding

Cytochrome c (cyt c) has long been utilized as a model system to study metalloprotein folding dynamics and the interplay between active site ligation and tertiary structure. However, recent reports regarding the weakness of the native Fe(ii)-S bond (Fe-Met80) call into question the role of the active site ligation in the protein folding process. In order to investigate the interplay between protein conformation and active site structures, we directly tracked the evolution of both during a photolysis-induced folding reaction using X-ray transient absorption spectroscopy and time-resolved X-ray solution scattering techniques. We observe an intermediate Fe-Met80 species appearing on ∼2 μs timescale, which should not be sustained without stabilization from the folded protein structure. We also observe the appearance of a new active site intermediate: a weakly interacting Fe-H2O state. As both intermediates require stabilization of weak metal-ligand interactions, we surmise the existence of a local structure within the unfolded protein that protects and limits the movement of the ligands, similar to the entatic state found in the native cyt c fold. Furthermore, we observe that in some of the unfolded ensemble, the local stabilizing structure is lost, leading to expansion of the unfolded protein structure and misligation to His26/His33 residues.

The surprisingly high ligation energy of CO to ruthenium porphyrins

A combined theoretical and experimental approach has been used to investigate the binding energy of a ruthenium metalloporphyrin ligated with CO, ruthenium tetraphenylporphyrin [RuII TPP], in the RuII oxidation degree. Measurements performed with VUV ionization using the DESIRS beamline at Synchrotron SOLEIL led to adiabatic ionization energies of [RuII TPP] and its complex with CO, [RuII TPP-CO], of 6.48 ± 0.03 eV and 6.60 ± 0.03 eV, respectively, while the ion dissociation threshold of [RuII TPP-CO]+ is measured to be 8.36 ± 0.03 eV using the ground-state neutral complex. These experimental data are used to derive the binding energies of the CO ligand in neutral and cationic complexes (1.88 ± 0.06 eV and 1.76 ± 0.06 eV, respectively) using a Born-Haber cycle. Density functional theory calculations, in very satisfactory agreement with the experimental results, help to get insights into the metal-ligand bond. Notably, the high ligation energies can be rationalized in terms of the ruthenium orbital structure, which is singular compared to that of the iron atom. Thus, beyond indications of a strengthening of the Ru-CO bond due to the decrease in the CO vibrational frequency in the complex as compared to the Fe-CO bond, high-level calculations are essential to accurately describe the metal ligand (CO) bond and show that the Ru-CO bond energy is strongly affected by the splitting of triplet and singlet spin states in uncomplexed [Ru TPP].

Three metalloporphyrin frameworks containing imidazole groups: synthesis, characterization and properties

Three metalloporphyrin frameworks containing imidazole groups were structurally characterized, revealing the moderate CO₂ adsorption capacity and high catalytic activity for ethylbenzene oxidation.

Dynamic structure elucidation of chemical reactivity by laser pulses and X-ray probes

Visualising chemical reactions by X-ray methods is a tantalising prospect. New light sources provide the prospect for studying atomic, electronic and energy transfers accompanying chemical change by X-ray spectroscopy and inelastic scattering. Here we assess how this adventure can illuminate inorganic and catalytic chemistry. In particular X-ray inelastic scattering provides a means of exploiting X-ray free electron lasers, as a parallel to laser Raman spectroscopy.

Subpicosecond and Sub-Angstrom Time and Space Studies by Means of Light, X-ray, and Electron Interaction with Matter

This Perspective article considers an experimental system that consists of ultrafast optical, electron, and X-ray time-resolved components. These techniques are used simultaneously on the same sample to study, in real time, the events that occur immediately upon disturbance with an ultrafast optical pulse. Excited states and metastable species are generated on the surface, and the electrical and mechanical waves propagating through the sample are recorded with subpicosecond and sub-Angstrom resolution. The characteristic of each technique is briefly described as a means of introducing the experimental system that intergrates these techniques. The processes evolved after femtosecond excitation of a Au single crystal have been monitored by these techniques. The data presented show changes with a resolution of 0.3 ± 0.1 ps in optical thermoreflectance, 1.0 ± 0.2 ps in electron Bragg diffraction, and 0.6 ± 0.1 ps in X-ray diffraction intensity accompanying shift and broadening.

Recent advances on ultrafast X-ray spectroscopy in the chemical sciences

Molecular snapshots obtained by ultrafast X-ray spectroscopy reveal new insight into fundamental reaction mechanisms at single electron and atomic levels.