Photophysical Exploration of Two Isomers of Octaethyltrioxopyrrocorphin.

The introduction of three ß-oxosubstituents to octaethylporphyrin by means of an oxidation/rearrangement reaction generates the trioxopyrrocorphin chromophore. Pyrrocorphins (hexahydroporphyrins) are generally nonaromatic, but we recently demonstrated trioxopyrrocorphins to possess considerable aromatic character. This contribution explores the photophysical characteristics of these unusual chromophores. In agreement with density functional theory modeling, the UV-vis and magnetic circular dichroism spectra of the two─out of the four possible─triketone regioisomers investigated conform to the Gouterman model of porphyrinoid optical spectra, in alignment with their aromaticity. Their excited-state dynamics shed further light on the degree to which ß-oxo substitutions tune the photophysical properties of porphyrinoids. Introduction of ß-oxo functionalities increases the rate and yield of intersystem crossing and shortens the triplet state lifetime. Unexpectedly, the singlet oxygen generation yield of both pyrrocorphins remains relatively high, with modes of distortion from planarity likely enhancing triplet energy transfer. This work thus expands our understanding of a rare class of porphyrinoids and further characterizes them as sustaining aromatic porphyrinic π-systems. Our findings suggest triple ß-oxo substitution as a viable route toward the development of novel, high-singlet oxygen yield porphyrinic photosensitizers.

From Chemotherapy to Phototherapy - Changing the Therapeutic Action of a Metallo-Intercalating RuII -ReI Luminescent System by Switching its Sub-Cellular Location.

The synthesis of a new heterodinuclear ReI RuII metallointercalator containing RuII (dppz) and ReI (dppn) moieties is reported. Cell-free studies reveal that the complex has similar photophysical properties to its homoleptic M(dppz) analogue and it also binds to DNA with a similar affinity. However, the newly reported complex has very different in-cell properties to its parent. In complete contrast to the homoleptic system, the RuII (dppz)/ReI (dppn) complex is not intrinsically cytotoxic but displays appreciable phototoxic, despite both complexes displaying very similar quantum yields for singlet oxygen sensitization. Optical microscopy suggests that the reason for these contrasting biological effects is that whereas the homoleptic complex localises in the nuclei of cells, the RuII (dppz)/ReI (dppn) complex preferentially accumulates in mitochondria. These observations illustrate how even small structural changes in metal based therapeutic leads can modulate their mechanism of action.

Tailoring the Intersystem Crossing and Triplet Dynamics of Free-Base Octaalkyl-ß-oxo-Substituted Porphyrins: Competing Effects of Spin-Vibronic and NH Tautomerism Relaxation Channels.

We demonstrate that ß-oxo-substitution provides effective fine-tuning of both steady-state and transient electronic properties of octaalkyl-ß-mono-oxochlorin and all isomers of the ß,ß'-dioxo-substituted chromophores. The addition of a carbonyl group increases the Qy oscillator strength and red-shifts the absorption spectra. Each oxo-substitution results in a 2-fold increase in the singlet to triplet state intersystem crossing (ISC) rates, resulting in a 4-fold ISC rate increase for the dioxo-substituted chromophores. The effects of oxo-substitution on the ISC rate are thus additive. The progressive increase in the ISC rates correlates directly with the spin-vibronic channels provided by the C═O out-of-plane distortion modes, as evidenced by density functional theory (DFT) modeling. The triplet states, however, were not evenly affected by ß-oxo-substitution, and reduction in the triplet lifetime seems to be influenced instead by the presence of NH tautomers in the dioxoisobacteriochlorins.

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl-ß,ß'-dioxo-bacterio- and -isobacteriochlorin Series.

It is well-known that treatment of ß-octaethylporphyrin with H2 O2 /conc. H2 SO4 converts it to a ß-oxochlorin as well as all five constitutional isomers of the corresponding ß,ß'-dioxo-derivatives: two bacteriochlorin-type isomers (ß-oxo groups at opposite pyrrolic building blocks) and three isobacteriochlorin-type isomers (ß-oxo-groups at adjacent pyrrolic building blocks). By virtue of the presence of the strongly electronically coupled ß-oxo auxochromes, none of the chromophores are archetypical chlorins, bacteriochlorins, or isobacteriochlorins. Here the authors present, inter alia, the single crystal X-ray structures of all free-base diketone isomers and a comparative description of their UV-vis absorption spectra in neutral and acidic solutions, and fluorescence emission and singlet oxygen photosensitization properties, Magnetic Circular Dichroism (MCD) spectra, and singlet excited state lifetimes. DFT computations uncover underlying tautomeric equilibria and electronic interactions controlling their electronic properties, adding to the understanding of porphyrinoids carrying ß-oxo functionalities. This comparative study lays the basis for their further study and utilization.

Deciphering the Mechanisms of Bacterial Inactivation on HiPIMS Sputtered CuxO-FeOx-PET Surfaces: From Light Absorption to Catalytic Bacterial Death.

The production of nontoxic, affordable, and efficient antibacterial surfaces is key to the well-being of our societies. In this aim, antibacterial thin films have been prepared using earth-abundant metals deposited using high-power impulse magnetron sputtering (HiPIMS). The sputtered FeOx, CuxO, and mixed CuxO-FeOx films exhibited fast bacterial inactivation properties under exposure to indoor light (340-720 nm) showing total bacterial inactivation within 180, 120, and 60 min, respectively. The photocatalytic mechanisms of these films were investigated, from the absorption of photons up to the bacteria's fate, by means of ultrafast transient spectroscopy, flow cytometry, and malondialdehyde (MDA) quantification justifying the cell wall disruption. The primary driving force leading to bacterial inactivation was found to be the oxidative stress at the interface between the sputtered thin films and the microorganism. This was justified by using engineered porinless bacteria disabling the possible ion diffusion leading to internal bacterial inactivation. Such stress is a direct consequence of the photogenerated electron-hole pairs at the interface of the sputtered layers. By diffuse reflectance spectroscopy, we found that both FeOx and CuxO present a band gap of ∼2.9 eV (>425 nm), while the mixed CuxO-FeOx thin film has a band gap below 2.3 eV (>540 nm). The structure and atomic composition of the films were characterized by energy-dispersive X-ray, X-ray photoelectron, and optical spectroscopy. While the composition and metal oxidation states are distinct in all three films, the difference in photocatalytic efficiency can, at first sight, be explained as the direct consequence of their absorbance and the unique interaction between Fe and Cu oxides in the composite film.

Ultrafast dynamics of the photo-excited hemes b and cn in the cytochrome b6f complex.

The dynamics of hemes b and cn within the cytochrome b6f complex are investigated by means of ultrafast broad-band transient absorption spectroscopy. On the one hand, the data reveal that, subsequent to visible light excitation, part of the b hemes undergoes pulse-limited photo-oxidation, with the liberated electron supposedly being transferred to one of the adjacent aromatic amino acids. Photo-oxidation is followed by charge recombination in about 8.2 ps. Subsequent to charge recombination, heme b is promoted to a vibrationally excited ground state that relaxes in about 4.6 ps. On the other hand, heme cn undergoes ultrafast ground state recovery in about 140 fs. Interestingly, the data also show that, in contrast to previous beliefs, Chl a is involved in the photochemistry of hemes. Indeed, subsequent to heme excitation, Chl a bleaches and recovers to its ground state in 90 fs and 650 fs, respectively. Chl a bleaching allegedly corresponds to the formation of a short lived Chl a anion. Beyond the previously suggested structural role, this study provides unique evidence that Chl a is directly involved in the photochemistry of the hemes.

Photo-induced oxidation of the uniquely liganded heme f in the cytochrome b6f complex of oxygenic photosynthesis.

The ultrafast behavior of the ferrous heme f from the cytochrome b6f complex of oxygenic photosynthesis is revealed by means of transient absorption spectroscopy. Benefiting from the use of microfluidic technologies for handling the sample as well as from a complementary frame-by-frame analysis of the heme dynamics, the different relaxation mechanisms from vibrationally excited states are disentangled and monitored via the shifts of the heme α-absorption band. Under 520 nm laser excitation, about 85% of the heme f undergoes pulse-limited photo-oxidation (<100 fs), with the electron acceptor being most probably one of the adjacent aromatic amino acid residues. After charge recombination in 5.3 ps, the residual excess energy is dissipated in 3.6 ps. In a parallel pathway, the remaining 15% of the hemes directly relax from their excited state in 2.5 ps. In contrast to a vast variety of heme-proteins, including the homologous heme c1 from the cytochrome bc1 complex, there is no evidence that heme f photo-dissociates from its axial ligands. Due to its unique binding, with histidine and an unusual tyrosine as axial ligands, the heme f exemplifies a dependence of ultrafast dynamics on the structural environment.

Note: Small anaerobic chamber for optical spectroscopy.

The study of oxygen-sensitive biological samples requires an effective control of the atmosphere in which they are housed. In this aim however, no commercial anaerobic chamber is adequate to solely enclose the sample and small enough to fit in a compact spectroscopic system with which analysis can be performed. Furthermore, spectroscopic analysis requires the probe beam to pass through the whole chamber, introducing a requirement for adequate windows. In response to these challenges, we present a 1 l anaerobic chamber that is suitable for broad-band spectroscopic analysis. This chamber has the advantage of (1) providing access, via a septum, to the sample and (2) allows the sample position to be adjusted while keeping the chamber fixed and hermetic during the experiment.

Photo-induced dynamics of the heme centers in cytochrome bc1.

The ultrafast response of cytochrome bc1 is investigated for the first time, via transient absorption spectroscopy. The distinct redox potentials of both c1- and b-hemes allow for a clear differentiation of their respective signals. We find that while the c1-heme photo-product exhibits the characteristics of a 5-coordinated species, the b-hemes presumably undergo photo-oxidation at a remarkably high quantum yield. The c1-heme iron-ligand recombination time is 5.4 ps, in agreement with previous reports on homologous cytochromes. The suggested photo-oxidized state of the b-hemes has a life-time of 6.8 ps. From this short life-time we infer that the electron acceptor must be within van der Walls contact with the heme, which points to the fact that the axial histidine residue is the electron acceptor. The different heme-responses illustrate the flexibility of the c1-heme ligation in contrast to the more rigid b-heme binding, as well as the higher electronic reactivity of the b-hemes within the bc1 complex. This study also demonstrates the remarkable connection between the heme local environment and its dynamics and, therefore, biological function.