Diversifying the functions of heme proteins with non-porphyrin cofactors

Heme proteins perform diverse biochemical functions using a single iron porphyrin cofactor. This versatility makes them attractive platforms for the development of new functional proteins. While directed evolution and metal substitution have expanded the properties, reactivity, and applications of heme proteins, the incorporation of porphyrin analogs remains an underexplored approach. This review discusses the replacement of heme with non-porphyrin cofactors, such as porphycene, corrole, tetradehydrocorrin, phthalocyanine, and salophen, and the attendant properties of these conjugates. While structurally similar, each ligand exhibits distinct optical and redox properties, as well as unique chemical reactivity. These hybrids serve as model systems to elucidate the effects of the protein environment on the electronic structure, redox potentials, optical properties, or other features of the porphyrin analog. Protein encapsulation can confer distinct chemical reactivity or selectivity of artificial metalloenzymes that cannot be achieved with the small molecule catalyst alone. Additionally, these conjugates can interfere with heme acquisition and uptake in pathogenic bacteria, providing an inroad to innovative antibiotic strategies. Together, these examples illustrate the diverse functionality that can be achieved by cofactor substitution. The further expansion of this approach will access unexplored chemical space, enabling the development of superior catalysts and the creation of heme proteins with emergent properties.

Ag(III)···Ag(III) Argentophilic Interaction in a Cofacial Corrole Dyad

Metallophilic interactions between closed-shell metal centers are exemplified by d¹⁰ ions, with Au(I) aurophilic interactions as the archetype. Such an interaction extends to d⁸ species, and examples involving Au(III) are prevalent. Conversely, Ag(III) argentophilic interactions are uncommon. Here, we identify argentophilic interactions in silver corroles, which are authentic Ag(III) species. The crystal structure of a monomeric silver corrole is a dimer in the solid state, and the macrocycle exhibits an atypical domed conformation. In order to evaluate whether this represents an authentic metallophilic interaction or a crystal-packing artifact, the analogous cofacial or "pacman" corrole was prepared. The conformation of the monomer was recapitulated in the silver pacman corrole, exhibiting a short 3.67 Å distance between metal centers and a significant compression of the xanthene backbone. Theoretical calculations support the presence of a rare Ag(III)···Ag(III) argentophilic interaction in the pacman complex.

Ratiometric Oxygen Sensing with H-NOX Protein Conjugates

Ratiometric sensors are self-referencing constructs that are functional in cells and tissues, and the read-out is independent of sensor concentration. One strategy for ratiometric sensing is to utilize two-color emission, where one component possesses analyte-dependent emission and the other is independent of analyte concentration, serving as an internal standard. In this way, the intensity ratio of the two components is a quantitative measure of the analyte. In this study, protein-based ratiometric oxygen sensors are prepared using the heme nitric oxide/oxygen-binding protein (H-NOX) from the thermophilic bacterium Caldanaerobacter subterraneus. The native heme cofactor is replaced with a Pd(II) or Pt(II) porphyrin as the oxygen-responsive phosphor. Mutagenesis is performed to incorporate a cysteine residue on the protein surface for thiol/maleimide coupling of the oxygen-insensitive dye, which serves as a Förster resonance energy transfer (FRET) donor for the porphyrin. While both Pd(II)- and Pt(II)-based sensors are responsive over biologically relevant ranges, the Pd sensor exhibits greater sensitivity at lower oxygen concentrations. Together, these sensors represent a new class of protein-based ratiometric oxygen sensors, and the modular platform allows the oxygen sensitivity to be tailored for a specific application. This proof-of-principle study has identified the key considerations and optimal methodologies to develop and subsequently refine protein-based ratiometric oxygen sensors.

Corrole–protein interactions in H-NOX and HasA

Mutagenesis was utilised to reveal corrole–protein interactions in H-NOX and HasA. The key interaction is a hydrogen bond between the PO unit of the corrole and a protonated histidine residue.

Designer Heme Proteins: Achieving Novel Function with Abiological Heme Analogues

Heme proteins have proven to be a convenient platform for the development of designer proteins with novel functionalities. This is achieved by substituting the native iron porphyrin cofactor with a heme analogue that possesses the desired properties. Replacing the iron center of the porphyrin with another metal provides one inroad to novel protein function. A less explored approach is substitution of the porphyrin cofactor with an alternative tetrapyrrole macrocycle or a related ligand. In general, these ligands exhibit chemical properties and reactivity that are distinct from those of porphyrins. While these techniques have most prominently been utilized to develop artificial metalloenzymes, there are many other applications of this methodology to problems in biochemistry, health, and medicine. Incorporation of synthetic cofactors into protein environments represents a facile way to impart water solubility and biocompatibility. It circumvents the laborious synthesis of water-soluble cofactors, which often introduces substantial charge that leads to undesired bioaccumulation. To this end, the incorporation of unnatural cofactors in heme proteins has enabled the development of designer proteins as optical oxygen sensors, MRI contrast agents, spectroscopic probes, tools to interrogate protein function, antibiotics, and fluorescent proteins.Incorporation of an artificial cofactor is frequently accomplished by denaturing the holoprotein with removal of the heme; the refolded apoprotein is then reconstituted with the artificial cofactor. This process often results in substantial protein loss and does not necessarily guarantee that the refolded protein adopts the native structure. To circumvent these issues, our laboratory has pioneered the use of the RP523 strain of E. coli to incorporate artificial cofactors into heme proteins using expression-based methods. This strain lacks the ability to biosynthesize heme, and the bacterial cell wall is permeable to heme and related molecules. In this way, heme analogues supplemented in the growth medium are incorporated into heme proteins. This approach can also be leveraged for the direct expression of the apoprotein for subsequent reconstitution.These methodologies have been exploited to incorporate non-native cofactors into heme proteins that are resistant to harsh environmental conditions: the heme nitric oxide/oxygen binding protein (H-NOX) from Caldanaerobacter subterraneus (Cs) and the heme acquisition system protein A (HasA) from Pseudomonas aeruginosa (Pa). The exceptional stability of these proteins makes them ideal scaffolds for biomedical applications. Optical oxygen sensing has been accomplished using a phosphorescent ruthenium porphyrin as the artificial heme cofactor. Paramagnetic manganese and gadolinium porphyrins yield high-relaxivity, protein-based MRI contrast agents. A fluorescent phosphorus corrole serves as a heme analogue to produce fluorescent proteins. Iron complexes of nonporphyrin cofactors bound to HasA inhibit the growth of pathogenic bacteria. Moreover, HasA can deliver a gallium phthalocyanine into the bacterial cytosol to serve as a sensitizer for photochemical sterilization. Together, these examples illustrate the potential for designer heme proteins to address burgeoning problems in the areas of health and medicine. The concepts and methodologies presented in this Account can be extended to the development of next-generation biomedical sensing and imaging agents to identify and quantify clinically relevant metabolites and other key disease biomarkers.

Corrole-Substituted Fluorescent Heme Proteins

Although fluorescent proteins have been utilized for a variety of biological applications, they have several optical limitations, namely weak red and near-infrared emission and exceptionally broad (>200 nm) emission profiles. The photophysical properties of fluorescent proteins can be enhanced through the incorporation of novel cofactors with the desired properties into a stable protein scaffold. To this end, a fluorescent phosphorus corrole that is structurally similar to the native heme cofactor is incorporated into two exceptionally stable heme proteins: H-NOX from Caldanaerobacter subterraneus and heme acquisition system protein A (HasA) from Pseudomonas aeruginosa. These yellow-orange emitting protein conjugates are examined by steady-state and time-resolved optical spectroscopy. The HasA conjugate exhibits enhanced fluorescence, whereas emission from the H-NOX conjugate is quenched relative to the free corrole. Despite the low fluorescence quantum yields, these corrole-substituted proteins exhibit more intense fluorescence in a narrower spectral profile than traditional fluorescent proteins that emit in the same spectral window. This study demonstrates that fluorescent corrole complexes are readily incorporated into heme proteins and provides an inroad for the development of novel fluorescent proteins.

Corrole photochemistry

The rapid expansion of photoredox catalysis and artificial photosynthesis has garnered renewed interest in the field of photochemistry. While porphyrins have been widely utilized for a variety of photochemical applications, corrole photochemistry remains underexplored, despite an exponential growth in corrole chemistry. Indeed, less than 4% of all corrole-related publications have studied the photochemistry of these molecules. Since corroles exhibit chemical properties that are distinct from porphyrins and related macrocycles, it is likely that this divergence would also be observed in their photochemical properties. This review provides a comprehensive summary of the extant corrole photochemistry literature. Corroles primarily serve as photosensitizers that transfer energy or an electron to molecular oxygen to form singlet oxygen or superoxide, respectively. While both of these reactive oxygen species can be used to drive chemical reactions, they can also be exploited for photodynamic therapy to treat cancer and other diseases. Although direct photochemical activation of metal–ligand bonds has been less explored, corroles mediate a variety of transformations, particularly oxygen atom transfer reactions. Together, these examples illustrate the diversity of corrole photochemistry and suggest that there are many additional applications yet to be discovered.

Multielectron C–H photoactivation with an Sb(v) oxo corrole

An Sb(v) bis-μ-oxo corrole dimer performs photochemical multielectron C–H activation, oxidising toluene to benzaldehyde in a four-electron process.

Optical oxygen sensing with quantum dot conjugates

The ability to track and quantify changes in oxygen concentration as a function of disease progression or therapy is crucial to advance targeted chemotherapeutics. New non-invasive sensors must be developed that are small enough to penetrate into tissue and monitor dynamic changes with high resolution in real time. One way to address this challenge is with the use of nanoparticle-based sensors. This review details the design, synthesis, and characterization of optical oxygen sensors that combine a fluorescent semiconductor quantum dot (QD) with an oxygen-responsive phosphorescent molecule. The QD may have multifaceted roles in these constructs, serving as an internal standard for ratiometric sensing, as an antenna for multiphoton absorption, and as an energy transfer donor for the attendant phosphorescent molecule. Solid-state devices may be prepared by embedding the two components in a polymer matrix. Alternatively, solution-phase sensors can be synthesized by covalent conjugation, self-assembly in organic solvents, or micelle encapsulation in aqueous media. Several sensors have been used for biological imaging and oxygen sensing, demonstrating that these constructs can quantify oxygen in biological systems.

Micelle-Encapsulated Quantum Dot-Porphyrin Assemblies as in Vivo Two-Photon Oxygen Sensors

Micelles have been employed to encapsulate the supramolecular assembly of quantum dots with palladium(II) porphyrins for the quantification of O2 levels in aqueous media and in vivo. Förster resonance energy transfer from the quantum dot (QD) to the palladium porphyrin provides a means for signal transduction under both one- and two-photon excitation. The palladium porphyrins are sensitive to O2 concentrations in the range of 0-160 Torr. The micelle-encapsulated QD-porphyrin assemblies have been employed for in vivo multiphoton imaging and lifetime-based oxygen measurements in mice with chronic dorsal skinfold chambers or cranial windows. Our results establish the utility of the QD-micelle approach for in vivo biological sensing applications.

Proton-coupled electron transfer chemistry of hangman macrocycles: Hydrogen and oxygen evolution reactions

The splitting of water into its constituent elements is an important solar fuels conversion reaction for the storage of renewable energy. For each of the half reactions of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), multiple protons and electrons must be coupled to avoid high-energy intermediates. To understand the mechanistic details of the PCET chemistry that underpins HER and OER, we have designed hangman porphyrin and corrole catalysts. In these hangman constructs, a pendant acid/base functionality within the secondary coordination sphere is "hung" above the macrocyclic redox platform on which substrate binds. The two critical thermodynamic properties of a PCET event, the redox potential and pKa may be tuned with the macrocycle and hanging group, respectively. This review outlines the synthesis of these catalysts, as well as the examination of the PCET kinetics of hydrogen and oxygen evolution by the hangman catalysts. The insights provided by these systems provide a guide for the design of future HER and OER catalysts that use a secondary coordination sphere to manage PCET.

Comparison of self-assembled and micelle encapsulated QD chemosensor constructs for biological sensing

Whereas a variety of covalent conjugation strategies have been utilized to prepare quantum dot (QD)-based nanosensors, supramolecular approaches of self-assembly have been underexplored. A major advantage of self-assembly is the ability to circumvent laborious synthetic efforts attendant to covalent conjugation of a chemosensor to functionalized QDs. Here, we combine a CdSe/ZnS core–shell QD with gold(iii) corroles using both self-assembly and micelle encapsulation to form QD nanosensors. Appreciable spectral overlap between QD emission and corrole absorption results in efficient Förster resonance energy transfer (FRET), which may be initiated by one- or two-photon excitation. The triplet state of the gold(iii) corroles is quenched by molecular oxygen, enabling these constructs to function as optical O₂ sensors, which is useful for the metabolic profiling of tumours. The photophysical properties, including QD and corrole lifetimes, FRET efficiency, and O₂ sensitivity, have been determined for each construct. The relative merits of each conjugation strategy are assessed with regard to their implementation as sensors.

Metabolic tumor profiling with pH, oxygen, and glucose chemosensors on a quantum dot scaffold

Acidity, hypoxia, and glucose levels characterize the tumor microenvironment rendering pH, pO2, and pGlucose, respectively, important indicators of tumor health. To this end, understanding how these parameters change can be a powerful tool for the development of novel and effective therapeutics. We have designed optical chemosensors that feature a quantum dot and an analyte-responsive dye. These noninvasive chemosensors permit pH, oxygen, and glucose to be monitored dynamically within the tumor microenvironment by using multiphoton imaging.

Two-photon oxygen sensing with quantum dot-porphyrin conjugates

Supramolecular assemblies of a quantum dot (QD) associated to palladium(II) porphyrins have been developed to detect oxygen (pO2) in organic solvents. Palladium porphyrins are sensitive in the 0-160 Torr range, making them ideal phosphors for in vivo biological oxygen quantification. Porphyrins with meso pyridyl substituents bind to the surface of the QD to produce self-assembled nanosensors. Appreciable overlap between QD emission and porphyrin absorption features results in efficient Förster resonance energy transfer (FRET) for signal transduction in these sensors. The QD serves as a photon antenna, enhancing porphyrin emission under both one- and two-photon excitation, demonstrating that QD-palladium porphyrin conjugates may be used for oxygen sensing over physiological oxygen ranges.

The synthesis, reactivity, and peripheral functionalization of corroles

Corroles are unusual macrocycles that often exhibit chemical reactivity that is distinct from their closely related porphyrin cousins. Standard organic transformations with corroles often result in the formation of unexpected products. A survey of synthetic methods for the preparation of both meso-substituted and β-substituted corroles will give an overview of the different synthetic strategies. This review provides a comprehensive description of the chemical reactivity and functionalization of corroles, focusing especially on reactions at the periphery of the macrocycle. Formylation, nitration, bromination and chlorosulfonation reactions will be explored in detail. In addition, demetalation processes, reactivity of the N-pyrrolic nitrogens and the lability of the macrocycle ring toward expansion and ring-opening reactions will be discussed. Finally, the synthesis of super-structured (picket-fence, capped, and strapped) corroles and isocorroles will be surveyed.

Porphyrin complexes of the period 6 main group and late transition metals

Metalloporphyrin complexes of the period six metals gold, mercury, thallium, lead and bismuth are often overlooked in favour of their lighter congeners. These complexes exhibit unusual coordination geometries, prominently featuring the metal centre residing out the porphyrin plane. Not only are these compounds chemically interesting, but several applications for these complexes are beginning to emerge. Gold and bismuth porphyrins have medicinal applications including novel chemotherapeutics and sensitizers for α-radiotherapy, while gold porphyrins have applications in materials chemistry and catalysis. This perspective serves to highlight trends in the synthesis and structure of these heavy metal complexes as well as illustrate the considerations necessary for rationally designing elaborate porphyrin architectures.