Bioinspired reactivity and coordination chemistry

We are delighted to welcome Dalton Transactions readers to this special issue on bioinspired reactivity and coordination chemistry. Enthused by the “International Year of the Periodic Table of Chemical Elements (IYPT2019)”, which celebrates the 150^th anniversary of the establishment of the periodic table of chemical elements, we set out to compile a special issue highlighting the importance of inorganic compounds in biological chemistry and reactivity. Coordination chemistry plays a fundamental role in many biological processes, from respiration and photosynthesis to protein folding. Biological systems exhibit formidable control over reactivity through the choice of metal, coordinating ligand set, and redox environment. The diversity offered by biological coordination chemistry has inspired inorganic chemists amongst others to design and develop a multitude of small molecule biomimetics for various applications, and to study as well as manipulate biological processes using inorganic tools. In this special issue we bring together a diverse body of work from chemists from throughout the world (Europe, Asia, North and South America) that captures the current landscape of state-of-the-art biocoordination chemistry. More specifically, the special issue provides a focused overview of current progress in (i) the activation/reduction of O₂, CO₂, N₂ and other biologically relevant small molecules, (ii) inorganic mimics for electron transfer processes, (iii) proton coupled electron transfer (PCET) reactions with bioinspired enzyme mimetics, (iv) coordination chemistry of bioligands bound to metal ions, and (v) bioinspired reactivity leading to biological activity of metal complexes.

This issue presents three thought-provoking reviews, one by Marc Robert et al. on the recent advancements and future directions of O₂ activation and CO₂ reduction by iron porphyrins (DOI: 10.1039/C9DT00136K), another by Oliver Wenger et al. on the recent advancements in bioinspired PCET with a particular emphasis on light activated reactions (DOI: 10.1039/C8DT04373F), and last but not least, one by Yong Cheng et al. on the application of fundamental coordination chemistry to design (optical, electrochemical, and optical-electrochemical) DNA-based metal ion sensors (DOI: 10.1039/C8DT04733B). The role of second sphere hydrogen bonding in CO₂ reduction by iron porphyrins is highlighted in an experimental and theoretical study by Abhishek Dey et al. (DOI: 10.1039/C8DT03850C), and the electrochemical reduction of CO₂ by another macrocycle, nickel dithiacyclam, is reported by Ulf-Peter Apfel, Kallol Ray, and collaborators (DOI: 10.1039/C8DT04740E). The activation of N₂ is spectroscopically evaluated in the first molybdenum dinitrogen complex supported by a cyclohexane-based triphosphine ligand by Felix Tuczek et al. (DOI: 10.1039/C8DT04738C). Two studies on dinitrosyl iron complexes (DNICs) are reported. Tsai-Te Lu and collaborators report the first hydride-containing DNIC, which was elegantly reacted with CS₂ to produce the first DNIC featuring both linear/bent NO ligands and a back-bonding interaction involving iron and the activated CS₂ moiety (DOI: 10.1039/C8DT04714F). Another study on the ionic and covalent character of DNICs with chalcogenolate-containing ligands is reported by Wen-Feng Liaw et al. (DOI: 10.1039/C8DT04670K). The addition of hydrosulfide (HS⁻) from a thiol source to an iron complex and its subsequent oxidation and O₂ activation is described by Amit Majumdar et al. (DOI: 10.1039/C8DT04092C).

The hydrogenase enzyme (HydA) interconverts protons and H₂ with outstanding efficiency. At the heart of hydrogenase is a diiron unit linked to a tetrairon cluster. Inorganic chemists have been developing hydrogenase biomimics for decades, in the hope of better understanding the catalytic mechanism of hydrogenase and creating cleaner H₂-producing biological energy sources. Three impressive studies on hydrogenase are presented in this issue. Graeme Hogarth, Michael Richmond, and co-workers show that certain electron-accepting, synthetic diphosphines are unable to recreate the electron transfer capabilities of the tetrairon cluster (in HydA) (DOI: 10.1039/C8DT04906H). Ott et al. elegantly use terminal phosphine ligands to covalently link the bridging thiolate ligands, thereby reducing rotation about the iron–carbonyl unit and endowing kinetic stability of reactive intermediates in the hydrogenase catalytic cycle (DOI: 10.1039/C8DT05148H). Gustav Berggren et al. propose, with the aid of a semi-synthetic HydF protein (involved in the biosynthesis of the diiron subunit), that electron transfer occurs during H-cluster assembly, resulting in simultaneous oxidation of the diiron subunit and reduction of the tetrairon subunit in HydF (DOI: 10.1039/C8DT04294B).

The clinical approval of the coordination complex cisplatin for the treatment of ovarian cancer in 1978 is widely regarded as one of the defining moments in the birth of modern-day bioinorganic chemistry. Coordination chemistry continues to play a significant role in the development of compounds with biological activity (for cancer and other diseases). In this issue, Dennis Gomez, Geneviève Pratviel, and collaborators present a thorough investigation of the interaction of a gold(III) porphyrin complex with G-quadruplex DNA, a secondary form of DNA related to oncogene expression and telomere regulation (DOI: 10.1039/C8DT04703K). Justin Wilson et al. report the biophysical properties (including reduction potential and ligand exchange) of cobalt(III) prodrugs and the implications of these traits for their anticancer or antiviral activities (DOI: 10.1039/C8DT04606A). Enrique Meléndez et al. show that chemical conjugation of ferrocene to estrogen yields bioconjugates with cancer cell potency comparable to cisplatin (DOI: 10.1039/C8DT01856A). Kogularamanan Suntharalingam, Leigh Aldous, and co-workers provide insight into the molecular level mechanism of action of cancer stem cell-potent copper coordination complexes (DOI: 10.1039/C8DT04706E). Two studies on the anticancer potential of ruthenium agents are reported; Elisângela Silveira-Lacerda, Alzir Batista, Koppe Grisolia and co-workers report a series of ruthenium complexes containing nitrogen, sulfur, and phosphorus ligands, with promising activity against triple negative breast cancer cells (and reduced in vivo toxicity) (DOI: 10.1039/C8DT03738H). Gilles Gasser, Santiago Gómez-Ruiz et al. report the synthesis and full characterisation of a nanomaterial based on mesoporous silica functionalised with a photoactive ruthenium complex, as well as its phototherapeutic activity against cervical cancer cells (DOI: 10.1039/C8DT02392A).

The interactions of metal ions with bioligands can have large biological implications, from tuning the catalytic activity of metalloenzymes, and structural control in biomolecule architectures, to underlying disease mechanisms. The role of aluminium in neurotoxicity and its proposed link with Alzheimer's disease (AD) has encouraged biophysical investigations to help understand these relationships better. Xabier Lopez and collaborators present a detailed study on the interaction of aluminium(III) ions with catecholamine-based neurotransmitters, and provide insight into whether this has a causative effect on neurodegenerative disease (DOI: 10.1039/C8DT04216K). Ivan Spasojević et al. provide insight into the interaction of copper(II) with biliverdin, a product of heme catabolism (DOI: 10.1039/C8DT04724C). A complete synthetic, structural, and computational study on three bioinspired dimanganese(II) complexes is reported by Vladimir Arion, Joshua Telser, and colleagues (DOI: 10.1039/C8DT04596H). Furthermore, Takashi Fujishiro et al. report the crystal structure of Bacillus subtilis with cobalt(II) – an enzyme that catalyses the insertion of iron(II) into sirohydrochlorin in siroheme biosynthesis (DOI: 10.1039/C8DT04727H).

Similarly, the interactions of bio-relevant ligands with metal ions can result in highly reactive complexes for catalysis. In this respect, Sonja Herres-Pawlis and co-workers showed that zinc complexes with guanidine ligands with a N,O donor can serve as highly potent catalysts for ring opening polymerisation of lactides, leading to new biodegradable plastics (DOI: 10.1039/C8DT04938F).

We believe that this exciting collection of manuscripts not only demonstrates the breadth of biocoordination chemistry at the present time but also highlights some of the future challenges in the field. We would like to thank all of the authors who have contributed to this special issue – without their hard work this would not have been possible. We are extremely grateful to all of the reviewers for taking the time to review the manuscripts. We would like to make a special acknowledgement to the Dalton Transactions team, especially Caroline Knapp, Michelle Canning, and Mike Andrews for helping coordinate this special issue. Finally, thank you for taking the time to read this special issue, we certainly hope you enjoy it as much as us!

---