Specific Cationic Antimicrobial Peptides Enhance the Recovery of Low-Load Quiescent Mycobacterium tuberculosis in Routine Diagnostics

The culture confirmation of Mycobacterium tuberculosis (MTB) remains the gold standard for the diagnosis of Tuberculosis (TB) with culture conversion representing proof of cure. However, over 40% of TB samples fail to isolate MTB even though many patients remain infectious due to the presence of viable non-culturable forms. Previously, we have shown that two short cationic peptides, T14D and TB08L, induce a hormetic response at low concentrations, leading to a stimulation of growth in MTB and the related animal pathogen Mycobacterium bovis (bTB). Here, we examine these peptides showing they can influence the mycobacterial membrane integrity and function through membrane potential reduction. We also show this disruption is associated with an abnormal reduction in transcriptomic signalling from specific mycobacterial membrane sensors that normally monitor the immediate cellular environment and maintain the non-growing phenotype. We observe that exposing MTB or bTB to these peptides at optimal concentrations rapidly represses signalling mechanisms maintaining dormancy phenotypes, which leads to the promotion of aerobic metabolism and conversion into a replicative phenotype. We further show a practical application of these peptides as reagents able to enhance conventional routine culture methods by stimulating mycobacterial growth. We evaluated the ability of a peptide-supplemented sample preparation and culture protocol to isolate the MTB against a gold standard routine method tested in parallel on 255 samples from 155 patients with suspected TB. The peptide enhancement increased the sample positivity rate by 46% and decreased the average time to sample positivity of respiratory/faecal sampling by seven days. The most significant improvements in isolation rates were from sputum smear-negative low-load samples and faeces. The peptide enhancement increased sampling test sensitivity by 19%, recovery in samples from patients with a previously culture-confirmed TB by 20%, and those empirically treated for TB by 21%. We conclude that sample decontamination and culture enhancement with D-enantiomer peptides offer good potential for the much-needed improvement of the culture confirmation of TB.

Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of theTCNQF n 0 / 1 - ${{{\rm {TCNQF}}}_{{\rm {n}}}^{{\rm {0/1-}}}}$ andTCNQF n 1 - / 2 - ${{{\rm {TCNQF}}}_{{\rm {n}}}^{{\rm {1-/2-}}}}$ (n=0, 2, 4) processes. WithTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalystTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ is reduced toTCNQF 4 2 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {2-}}}}$ when reacting withS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ which is oxidised toS 4 O 6 2 - ${{{\rm {S}}}_{{\rm {4}}}{{\rm {O}}}_{{\rm {6}}}^{{\rm {2-}}}}$ . Subsequently,TCNQF 4 2 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {2-}}}}$ reacts with[ Fe ( CN ) 6 ]​ 3 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {3-}}}}$ to form[ Fe ( CN ) 6 ]​ 4 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {4-}}}}$ and reform theTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ catalyst, in another thermodynamically favoured process. An analogous mechanism applies withTCNQF 2 1 - ${{{\rm {TCNQF}}}_{{\rm {2}}}^{{\rm {1-}}}}$ as a catalyst. In contrast, since the reaction ofS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ withTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition ofTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ , reduction of[ Fe ( CN ) 6 ]​ 3 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {3-}}}}$ to[ Fe ( CN ) 6 ]​ 4 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {4-}}}}$ occurs with concomitant oxidation ofTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ toTCNQ 0 ${{{\rm {TCNQ}}}^{{\rm {0}}}}$ , which then acts as the catalyst forS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms.CuTCNQF n ${{{\rm {CuTCNQF}}}_{{\rm {n}}}}$ (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, theseCuTCNQF n ${{{\rm {CuTCNQF}}}_{{\rm {n}}}}$ (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.

Membrane interaction and selectivity of novel alternating cationic lipid-nanodisc assembling polymers

Synthetic polymer nanodiscs are self-assembled structures formed from amphipathic copolymers encapsulating membrane proteins and surrounding phospholipids into water soluble discs. These nanostructures have served as an analytical tool for the detergent free solubilisation and structural study of membrane proteins (MPs) in their native lipid environment. We established the polymer-lipid nanodisc forming ability of a novel class of amphipathic copolymer comprised of an alternating sequence of N-alkyl functionalised maleimide (AlkylM) of systematically varied hydrocarbon chain length, and cationic N-methyl-4-vinyl pyridinium iodide (MVP). Using a combination of physicochemical techniques, the solubilisation efficiency, size, structure and shape of DMPC lipid containing poly(MVP-co-AlkylM) nanodiscs were determined. Lipid solubilisation increased with AlkylM hydrocarbon chain length from methyl (MM), ethyl (EtM), n-propyl (PM), iso-butyl (IBM) through to n-butyl (BM) maleimide bearing polymers. More hydrophobic derivatives formed smaller sized nanodiscs and lipid ordering within poly(MVP-co-AlkylM) nanodiscs was affected by nanodisc size. In dye-release assays, shorter N-alkyl substituted polymers, particularly poly(MVP-co-EtM), exhibited low activities against eukaryotic mimetic POPC membrane and increased their liposome disruption as POPC : POPG membrane mixtures increased in their anionic POPG component, resembling the charge profile of bacterial membranes. These trends in membrane selectivity were transferred towards native cell systems in which gram-positive Staphylococcus aureus and gram-negative Acenobacter baumannii bacterial strains were relatively susceptible to disruption by hydrophobic n-butyl- and n-propyl-poly(MVP-co-AlkylM) derivatives compared to human red blood cells (HRBCs), with a more pronounced selectivity resulting from poly(MVP-co-PM). Such selective membrane interaction by less hydrophobic polymers provides a framework for polymer design towards applications including selective membrane component solubilisation, biosensing and antimicrobial development.

Revisiting the TCNQF 4 0 / 1 - / 2 - ${{\bf TCNQF}_{\bf 4}^{{\bf 0/1 - /2 - }} }$ Catalysis Mechanism for the F e C N 6 3 - / 4 - ${\left[ {{\bf Fe}\left( {{\bf CN}} \right)_{\bf 6} } \right]^{{\bf 3 - /4 - }} }$ - S 2 O 6 2 - ${{\bf S}_{\bf 2} {\bf O}_{\bf 6}^{{\bf 2 - }} }$ / S 4 O 6 2 - ${{\bf S}_{\bf 4} {\bf O}_{\bf 6}^{{\bf 2 - }} }$ Redox Reaction

The front cover artwork was done by Michelle Farrelly, a member of the Martin group at Monash University. The image represents a perspective of a cuvette in which the catalysis of the thiosulfate-ferricyanide reaction was achieved by a TCNQF₄ -based redox reaction in aqueous solution. The primary method used to monitor these reactions was spectrophotometry. Read the full text of the Research Article at 10.1002/cphc.202200942.

Comparative yields of antimicrobial peptides released from human and cow milk proteins under infant digestion conditions predicted by in silico methodology

Mammalian milk proteins are known to encrypt antimicrobial peptides (AMPs) which can be passively released and exert bioactivity in the gastrointestinal and cardiovascular systems pre- or post-absorption, respectively. However, the contribution of 'passive' food-derived AMPs to the pool of endogenous and microbial AMPs has not been differentiated in previous research. Insight into the consequences of protein digestion and peptide bioactivity can be gained using in silico tools. The aim of this investigation was to use in silico methods to characterise the yields of AMPs released from major proteins in human and cow milk under infant digestion conditions, as relevant to early nutrition. The profiles of major proteins in human and cow milk from UniProtKB/Swiss-Prot, were subjected to in silico digestion by ExPASy-PeptideCutter, and the AMP activity of resulting peptides (≥4 amino acids, AAs) evaluated with the CAMP_R3-RF predictive tool. The mass yields and counts of absorbing (≤10 AAs) and non-absorbing (>10 AAs) AMPs, as found in human, cow and 'humanised' ratios of cow milk proteins, were quantified. The results indicated that major whey proteins from both human and cow milks displayed a higher degree of hydrolysis than caseins, consistent with their known 'fast' digestion properties. Larger albumin and lactoferrin proteins generated relatively more and/or longer peptides. Yields of AMPs from cow milk were higher than from human milk, even after standardising the ratio of whey to casein and total protein concentration, as practiced in formulations manufactured for human newborn babies. Whereas alpha-lactalbumin (2.65 g L^-1) and lactoferrin (1.75 g L^-1) provided the major yields of AMPs in human milk whey proteins; beta-lactoglobulin, which is unique to cow milk, released the highest yield of AMPs in cow milk (3.25 g L^-1 or 19.9% w/w of total whey protein), which may represent an important and overlooked biological function of this protein in cow milk.

Revisiting the TCNQF4 0/1-/2- Catalysis Mechanism for the [Fe(CN)6 ]3-/4- -S2 O3 2- /S4 O6 2- Redox Reaction

Published data suggest that sparingly soluble metal complexes of TCNQF n 1 - ${{\rm{TCNQF}}_{\rm{n}}^{{\rm{1 - }}} }$ , where n=0, 1, 2, 4, can act as heterogeneous catalysts for the kinetically very slow [ Fe ( CN ) 6 ]​ 3 - / 4 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - /4 - }}} }$ - S 2 O 3 2 - ${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ / S 4 O 6 2 - ${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ reaction in aqueous solution. This study shows that the coordination polymer CuTCNQF 4 ${{\rm{CuTCNQF}}_{\rm{4}} }$ , participates as a homogeneous catalyst via an extremely small concentration of dissolved TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ . This finding suggests that the generally accepted mechanism of catalysis by TCNQF 4 ${{\rm{TCNQF}}_{\rm{4}} }$ based solids needs to be revisited to ascertain the role of homogeneous pathways. In the present study, UV-visible spectrophotometry was used to examine the catalysis of the aqueous redox reaction of [ Fe ( CN ) 6 ]​ 3 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - }}} }$ (1.0 mM) with S 2 O 3 2 - ${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ (100 mM) in the presence of (i) a precursor catalyst, TCNQF 4 0 ${{\rm{TCNQF}}_{\rm{4}}^{\rm{0}} }$ ; (ii) the catalyst, TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ , as the water soluble Li⁺ salt; and (iii) CuTCNQF 4 ${{\rm{CuTCNQF}}_{\rm{4}} }$ . A homogeneous reaction scheme that utilises the TCNQF 4 1 - / 2 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - /2 - }}} }$ couple is provided. In the case of TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ derived from highly soluble LiTCNQF 4 ${{\rm{LiTCNQF}}_{\rm{4}} }$ , quantitative conversion of 1.0 mM S 2 O 3 2 - ${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ to 0.50 mM S 4 O 6 2 - ${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ occurs with complete reduction of [ Fe ( CN ) 6 ]​ 3 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - }}} }$ to [ Fe ( CN ) 6 ]​ 4 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{4 - }}} }$ being rapidly accelerated by sub-micomolar concentrations of TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ . TCNQF 4 2 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{2 - }}} }$ generated in the catalytic cycle, reacts with [ Fe ( CN ) 6 ]​ 3 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - }}} }$ to reform TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ and produce [ Fe ( CN ) 6 ]​ 4 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{4 - }}} }$ . Along with the rapid catalytic reaction, the sluggish competing reaction between TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ and S 2 O 3 2 - ${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ occurs to give TCNQF 4 2 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{2 - }}} }$ , which is protonated to HTCNQF 4 1 - ${{\rm{\;HTCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ , along with a trace amount of S 4 O 6 2 - ${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ . On addition of the precursor catalyst, TCNQF 4 0 ${{\rm{TCNQF}}_{\rm{4}}^{\rm{0}} }$ , rapid reduction with S 2 O 3 2 - ${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ occurs to form TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ - the active catalyst. CuTCNQF 4 ${{\rm{CuTCNQF}}_{\rm{4}} }$ added to water is shown to be sufficiently soluble to provide adequate TCNQF 4 1 - ${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ to act as the catalyst for the [ Fe ( CN ) 6 ]​ 3 - / 4 - ${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - /4 - }}} }$ - S 2 O 3 2 - ${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ / S 4 O 6 2 - ${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ reaction.

Lipid oxidation controls peptide self-assembly near membranes through a surface attraction mechanism

The self-assembly of peptides into supramolecular structures has been linked to neurodegenerative diseases but has also been observed in functional roles. Peptides are physiologically exposed to crowded environments of biomacromolecules, and particularly cellular membrane lipids. Previous research has shown that membranes can both accelerate and inhibit peptide self-assembly. Here, we studied the impact of membrane models that mimic cellular oxidative stress and compared this to mammalian and bacterial membranes. Using molecular dynamics simulations and experiments, we propose a model that explains how changes in peptide-membrane binding, electrostatics, and peptide secondary structure stabilization determine the nature of peptide self-assembly. We explored the influence of zwitterionic (POPC), anionic (POPG) and oxidized (PazePC) phospholipids, as well as cholesterol, and mixtures thereof, on the self-assembly kinetics of the amyloid β (1-40) peptide (Aβ40), linked to Alzheimer's disease, and the amyloid-forming antimicrobial peptide uperin 3.5 (U3.5). We show that the presence of an oxidized lipid had similar effects on peptide self-assembly as the bacterial mimetic membrane. While Aβ40 fibril formation was accelerated, U3.5 aggregation was inhibited by the same lipids at the same peptide-to-lipid ratio. We attribute these findings and peptide-specific effects to differences in peptide-membrane adsorption with U3.5 being more strongly bound to the membrane surface and stabilized in an α-helical conformation compared to Aβ40. Different peptide-to-lipid ratios resulted in different effects. We found that electrostatic interactions are a primary driving force for peptide-membrane interaction, enabling us to propose a model for predicting how cellular changes might impact peptide self-assembly in vivo.

Peptide Self-Assembly into Amyloid Fibrils at Hard and Soft Interfaces-From Corona Formation to Membrane Activity

Peptides and proteins are exposed to a variety of interfaces in a physiological environment, such as cell membranes, protein nanoparticles (NPs), or viruses. These interfaces have a significant impact on the interaction, self-assembly, and aggregation mechanisms of biomolecular systems. Peptide self-assembly, particularly amyloid fibril formation, is associated with a wide range of functions; however, there is a link with neurodegenerative diseases, such as Alzheimer's disease. This review highlights how interfaces affect peptide structure and the kinetics of aggregation leading to fibril formation. In nature, many surfaces are nanostructures, such as liposomes, viruses, or synthetic NPs. Once exposed to a biological medium, nanostructures are coated with a corona, which then determines their activity. Both accelerating and inhibiting effects on peptide self-assembly have been observed. When amyloid peptides adsorb to a surface, they typically concentrate locally, which promotes aggregation into insoluble fibrils. Starting from a combined experimental and theoretical approach, models that allow for a better understanding of peptide self-assembly near hard and soft matter interfaces are introduced and reviewed. Research results from recent years are presented and relationships between biological interfaces, such as membranes and viruses, and amyloid fibril formation are proposed.

Live bio-nano-sonosensitizer targets malignant tumors in synergistic therapy

Sonosensitizers that can increase the concentration of reactive oxygen species (ROS) within a tumor microenvironment is a high priority for sonodynamic therapy (SDT). In this study, a functionalized, smart nanosonosensitizer based on Au-RuO₂ nanoparticles (NPs) and selenium nanoparticles (Se NPs) that were electrostatically self-assembled onto the surface of Listeria innocua (LI) was used to create Bac@ARS. Au NPs provided the core in which RuO₂ was deposited to form Au-RuO₂ NPs. Additionally, the underlying properties of the Au NPs and Se NPs were used to optimize the sonosensitivity performance. Compared with pristine RuO₂ NPs, Bac@ARS exhibits highly efficient ROS-producing activity. Furthermore, Bac@ARS remodeled the hypoxic tumor microenvironment, enabling overproduction of ROS. Importantly, Bac@ARS exploits the natural tropism of LI to selectively accumulate in tumors, which improved the treatment precision at hypoxic tumor sites after sonodynamic activation. However, the activity of LI was greatly reduced after ultrasound (US) irradiation, ensuring the biosafety of Bac@ARS. Bac@ARS was also used to monitor tumors, in real time, using photoacoustic imaging of the gold-based nanoparticles. Therefore, Bac@ARS is a promising microbial sonosensitizer providing a new platform for the optimization of sonosensitizers for tumor treatment. STATEMENT OF SIGNIFICANCE: A bio-nano-sonosensitizer was designed using a Au nanoparticle (NP) core modified with RuO₂ NPs. The Au-RuO₂ NPs together with Se-NPs are attached via electrostatic adsorption to a live bacterium Listeria innocua (LI), creating Bac@ARS. The role of the NPs was to optimize the sonosensitivity performance at the target tumor site. Bac@ARS reshaped the tumor microenvironment and overcame tumor hypoxia leading to ROS overproduction. This activated a potent ICD-mediated cellular immunity and anti-tumor activity. Importantly, Bac@ARS exploited the natural tropism of LI to selectively accumulate in tumors, resulting in more precise delivery of the therapeutic effect while exhibiting reduced effects on healthy tissues.

Helical Intermediate Formation and Its Role in Amyloid of an Amphibian Antimicrobial Peptide

Helical intermediates appear to be crucial in amyloid formation of several amyloidogenic peptides, including Aβ, that are implicated in different neurodegenerative diseases. Intermediate species have been reported to be more...

Self-Assembly of Linear, Natural Antimicrobial Peptides: An Evolutionary Perspective

Antimicrobial peptides are an ancient and innate system of host defence against a wide range of microbial assailants. Mechanistically, unstructured peptides undergo a secondary structure transition into amphipathic α-helices, upon contact with membrane surfaces. This leads to peptide binding and removal of the membrane components in a detergent-like manner or via self-organisation into trans-membrane pores (either barrel-stave or toroidal pore) thereby destroying the microbe. Self-assembly of antimicrobial peptides into oligomers and ultimately amyloid has been mostly examined in parallel, however recent findings link diseases, such as Alzheimer's disease as an aberrant activity of a protective neuropeptide with antimicrobial activity. These self-assembled oligomers can also interact with membranes. Here, we review those antimicrobial peptides reported to self-assemble into amyloid, where supported by structural evidence. We consider their membrane activities as antimicrobial peptides and present evidence of consistent self-assembly patterns across major evolutionary groups. Trends are apparent across these groups, supporting the mounting data that self-assembly of antimicrobial peptides into amyloid should be considered as synergistic to the antimicrobial peptide response.

Mechanistic insights into the size-dependent effects of nanoparticles on inhibiting and accelerating amyloid fibril formation

The aggregation of peptides into amyloid fibrils has been linked to ageing-related diseases, such as Alzheimer's and type 2 diabetes. Interfaces, particularly those with large nanostructured surfaces, can affect the kinetics of peptide aggregation, which ranges from complete inhibition to strong acceleration. While a number of physiochemical parameters determine interfacial effects, we focus here on the role of nanoparticle (NP) size and curvature. We used thioflavin T (ThT) fluorescence assays to demonstrate the size-dependent effects of NPs on amyloid fibril formation for the peptides Aβ₄₀, NNFGAIL, GNNQQNY and VQIYVK. While 5 nm gold NPs (AuNP-5) retarded or inhibited the aggregation of all peptides except NNFGAIL, larger 20 nm gold NPs (AuNP-20) tended to accelerate or not influence peptide aggregation. Differences in the NP effects for the peptides resulted from the different peptide properties (size, tendency to aggregate) and associated surface binding affinities. Additional dynamic light scattering (DLS), electron microscopy, and atomic force microscopy (AFM) experiments with the Aβ₄₀ peptide confirmed size-dependent NP effects on peptide aggregation, and also suggested a structural influence on the formed fibrils. NPs can serve as a surface for the adsorption of peptide monomers and enable nucleation to oligomers and fibril formation. However, molecular dynamics (MD) simulations showed that peptide oligomers were less stable at smaller NPs. High surface curvatures destabilized prefibrillar structures, which provides a possible explanation for inhibitory effects on fibril growth, provided that peptide-NP surface binding was relevant for fibril formation. These mechanistic insights can support the design of future nanostructured materials.

Polymer Nanodiscs and Their Bioanalytical Potential

Membrane proteins (MPs) play a pivotal role in cellular function and are therefore predominant pharmaceutical targets. Although detailed understanding of MP structure and mechanistic activity is invaluable for rational drug design, challenges are associated with the purification and study of MPs. This review delves into the historical developments that became the prelude to currently available membrane mimetic technologies before shining a spotlight on polymer nanodiscs. These are soluble nanosized particles capable of encompassing MPs embedded in a phospholipid ring. The expanding range of reported amphipathic polymer nanodisc materials is presented and discussed in terms of their tolerance to different solution conditions and their nanodisc properties. Finally, the analytical scope of polymer nanodiscs is considered in both the demonstration of basic nanodisc parameters as well as in the elucidation of structures, lipid-protein interactions, and the functional mechanisms of reconstituted membrane proteins. The final emphasis is given to the unique benefits and applications demonstrated for native nanodiscs accessed through a detergent free process.

Crystalline ruthenium polypyridine nanoparticles: a targeted treatment of bacterial infection with multifunctional antibacterial, adhesion and surface-anchoring photosensitizer properties

Photodynamic antibacterial therapy employs nanocomposites as an alternative to traditional antibiotics for the treatment of bacterial infections. However, many of these antibacterial materials are less effective towards bacteria than traditional drugs, either due to poor specificity or antibacterial activity. This can result in needless and excessive drug use in treatments. This paper describes a multifunctional drug delivery nanoparticle (MDD-NP), Sph-Ru-MMT@PZ, based on the nanostructured-form of [Ru(bpy)2dppz] (PF6)2 (Sph-Ru), which has adhesive properties towards its microbial targets as well as surface-anchoring photosensitizer effects. The design and construction of MDD-NP is based on the adhesive properties of the outer layers of montmorillonite (MMT), which allows Sph-Ru-MMT@PZ to successfully reach its bacterial target; the outer layer of the E. coli. In addition, under 670 nm red irradiation therapy (R-IT), the surface-anchoring properties use the photosensitizer phthalocyanine zinc (PZ) to destroy the bacteria by producing reactive oxygen species (ROS) which causes cell lysis of E. coli. More importantly, Sph-Ru-MMT@PZ has no fluorescence response to live E. coli with intact cell membranes but selectively stained and demonstrated fluorescence during membrane damage of early-stage cells as well as exposure of nuclear materials at late-stage of cell lysis. Sph-Ru-MMT@PZ showed beneficial and synergistic anti-infective effects in vivo by inhibiting the E. coli infection-induced inflammatory response and eventually promoting wound healing in mice. This new strategy for high precision antibacterial therapy towards specific targets, provides an exciting opportunity for the application of multifunctional nanocomposites towards microbial infections.

Exploring the Role of Peptide Helical Stability in the Propensity of Uperin 3.x Peptides toward Beta-Aggregation

Antimicrobial peptides of the uperin 3.x family, obtained from the skin secretions of Uperoleia mjobergii, have an inherent ability to form amyloid with possible functional roles and can serve as model peptides to understand mechanistic aspects of amyloidogenesis. The substitution of a positively charged amino acid with a nonpolar alanine residue increased aggregation, fibril content, and propensity for β-sheet formation for the uperin 3.5 R7A variant when compared with the uperin 3.5 wild-type peptides. We use molecular dynamics (MD) simulations and circular dichroism (CD) measurements on three uperin 3.x peptides and their corresponding seventh position alanine variants to understand the effect of substitution of a positively charged amino acid with a nonpolar alanine residue on the process of β-aggregation. Both CD experiments and simulations show that the uperin 3.x wild-type peptides demonstrated lower β-sheet content and propensity than with the corresponding alanine variants. Significantly, simulations of helix-to-coil transitions in individual peptides show an inverse relationship between the helical stability of peptides and their propensity to form structures rich in β-sheets as observed in CD experiments. A simulation scheme based on a conformational search of helix-to-coil transition trajectories to select peptide conformers was used to assemble propagating peptide oligomers. Whereas octamers consisting of lower helical stability peptide conformers evolve into compact aggregates with a large β-sheet component, octamers composed of high helical stability conformers disintegrate and show the least amounts of β-sheet components. The highlight of the current work is that MD simulations are able to predict the correct order of β-sheet propensity among the six peptides derived from the CD experiments and indicate the importance of helical intermediates in the amyloidogenesis pathway for uperin 3.x peptides.

Sequence comparisons of cytochrome P450 aromatases from Australian animals predict differences in enzymatic activity and/or efficiency†

Aromatase (P450arom, CYP19A1) is the terminal enzyme in the synthesis of the steroid hormone family of estrogens. Not surprisingly, this enzyme has structural similarities between the limited number of species studied thus far. This study examined the structure of aromatases from four diverse Australian species including a marsupial (tammar wallaby; Macropus eugenii), monotreme (platypus; Ornithorhynchus anatinus), ratite (emu; Dromaius novaehollandiae) and lizard (bearded dragon; Pogona vitticeps). We successfully built homology models for each species, using the only crystallographically determined structure available, human aromatase. The amino acid sequences showed high amino acid sequence identity to the human aromatase: wallaby 81%, platypus 73%, emu 75% and bearded dragon at 74%. The overall structure was highly conserved among the five species, although there were non-secondary structures (loops and bends) that were variable and flexible that may result in some differences in catalytic activity. At the N-terminal regions, there were deletions and variations that suggest that functional distinctions may be found. We found that the active sites of all these proteins were identical, except for a slight variation in the emu. The electrostatic potential across the surfaces of these aromatases highlighted likely variations to the protein-protein interactions of these enzymes with both redox partner cytochrome P450 reductase and possibly homodimerization in the case of the platypus, which has been postulated for the human aromatase enzyme. Given the high natural selection pressures on reproductive strategies, the relatively high degree of conservation of aromatase sequence and structure across species suggests that there is biochemically very little scope for changes to have evolved without the loss of enzyme activity.

Electrochemically Directed Synthesis of Cobalt(II) and Nickel(II) TCNQF21–/2– Coordination Polymers: Solubility and Substituent Effects in the TCNQFn (n=0, 1, 2, 4) Series of Complexes

The reversible diffusion controlled cyclic voltammetry for the reduction of TCNQF­n0/1–/2– (where n=0, 1, 2, 4) changes significantly on addition of Co2+ and Ni2+ transition metal ions (M2+) because the kinetics associated with electrocrystallisation of the resulting coordination polymers [M(TCNQF2)2(H2O)2] and [M(TCNQF2)] are rapid on the voltammetric time scale. The voltammetry of solutions containing M2+ and TCNQF­2 was undertaken in acetonitrile (0.1M Bu4NPF6) at both GC and ITO electrodes. New one electron reduced TCNQF2 materials prepared via electrochemically directed synthesis were shown to have the formula [M(TCNQF2)2(H2O)2], assessed by vibrational (IR and Raman) spectroscopy, elemental analysis and thermogravimetric analysis. The solubility of [Ni(TCNQF2)2(H2O)2] (Ksp=8.29×10−11 M3) was significantly higher than the [Co(TCNQF2)2(H2O)2] (Ksp=1.43×10−11M3). Cyclic voltammetric data suggest the electrocrystallisation of two phases of [Ni(TCNQF2)2(H2O)2] occurs, which is not evident for [Co(TCNQF2)2(H2O)2]. Electrocrystallisation of the highly insoluble [M(TCNQF2)] was achieved at low M2+ and TCNQF2 concentrations. A comparison with published data on the voltammetry of TCNQF­n (n=0, 1, 2 and 4) for the series of TCNQF­n (n=0, 1, 2 and 4) containing M2+ is provided. An assessment of the electronic impact of the fluorine substituent of the underlying redox reactions also is established. Predictions are made for the voltammetric behaviour expected for the other transition metal cations with reduced TCNQFn derivatives.

Adsorption of Amyloidogenic Peptides to Functionalized Surfaces Is Biased by Charge and Hydrophilicity

Surfaces are abundant in living systems, such as in the form of cellular membranes, and govern many biological processes. In this study, the adsorption of the amyloidogenic model peptides GNNQQNY, NNFGAIL, and VQIVYK as well as the amyloid-forming antimicrobial peptide uperin 3.5 (U3.5) were studied at low concentrations (100 μM) to different surfaces. The technique of a quartz crystal microbalance with dissipation monitoring (QCM-D) was applied as it enables the monitoring of mass binding to sensors at nanogram sensitivity. Gold-coated quartz sensors were used as unmodified gold surfaces or functionalized with self-assembled monolayers (SAMs) of alkanethiols (terminated as methyl, amino, carboxyl, and hydroxyl) resulting in different adsorption affinities of the peptides. Our objective was to evaluate the underlying role of the nature and feature of interfaces in biological systems which could concentrate peptides and impact or trigger peptide aggregation processes. In overall, the largely hydrophobic peptides adsorbed with preference to hydrophobic or countercharged surfaces. Further, the glycoprotein lubricin (LUB) was tested as an antiadhesive coating. Despite its hydrophilicity, the adsorption of peptides to LUB coated sensors was similar to the adsorption to unmodified gold surfaces, which indicates that some peptides diffused through the LUB layer to reach the underlying gold sensor surface. The LUB protein-antiadhesive is thus more effective as a biomaterial coating against larger biomolecules than small peptides under the conditions used here. This study provides directions toward a better understanding of amyloid peptide adsorption to biologically relevant interfaces, such as cellular membranes.

Interactions between Lubricin and Hyaluronic Acid Synergistically Enhance Antiadhesive Properties

Preventing the unwanted adsorption of proteins and cells at articular cartilage surfaces plays a critical role in maintaining healthy joints and avoiding degenerative diseases such as osteoarthritis. Immobilized at the surface of healthy articular cartilage is a thin, interfacial layer of macromolecules consisting mainly of hyaluronic acid (HA) and lubricin (LUB; a.k.a. PRG4) that is believed to form a co-adsorbed, composite film now known to exhibit synergistic tribological properties. Bioinspired by the composition of cartilage surfaces, composite layers of HA and LUB were grafted to Au surfaces and the antiadhesive properties were assessed using surface plasmon resonance and quartz crystal microbalance. A clear synergistic enhancement in antiadhesive properties was observed in the composite films relative to grafted HA and LUB layers alone. Atomic force microscopy (AFM) normal force measurements provide insight into the architecture of the HA/LUB composite layer and implicate a strong contribution of hydrophobic interactions in the binding of LUB end-domains directly to HA chains. These AFM force measurements indicate that the adhesion of LUB to HA is strong and indicate that the hydrophobic coupling of LUB to HA shields the hydrophobic domains in these molecules from interactions with other proteins or molecules.

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

The reaction of [FeII(L•)2][BF4]2 with LiTCNQF4 results in the formation of [FeII(L•)2][TCNQF4•−]2·2CH3CN (1) (L• is the neutral aminoxyl radical ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide; TCNQF4 is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). Single-crystal X-ray diffraction; Raman, Fourier-transform infrared (FTIR) and ultraviolet–visible spectroscopies; and electrochemical studies are all consistent with the presence of a low-spin FeII ion, the neutral radical form (L•) of the ligand, and the radical anion TCNQF4•−. 1 is largely diamagnetic and the electrochemistry shows five well-resolved, diffusion-controlled, reversible one-electron processes.

Impact of nanoparticles on amyloid peptide and protein aggregation: a review with a focus on gold nanoparticles

Society is increasingly exposed to nanoparticles as they are ubiquitous in nature and introduced as man-made air pollutants and as functional ingredients in cosmetic products as well as in nanomedicine. Nanoparticles differ in size, shape and material properties. In addition to their intended function, the side effects on biochemical processes in organisms remain unclear. Nanoparticles can significantly influence the nucleation and aggregation process of peptides. The development of several neurodegenerative diseases, such as Alzheimer's disease, is related to the aggregation of peptides into amyloid fibrils. However, there is no comprehensive or universal mechanism to predict or explain apparent acceleration or inhibition of these aggregation processes. In this work, selected studies and possible mechanisms for amyloid peptide nucleation and aggregation, in the presence of nanoparticles, are highlighted. These studies are discussed in the context of recent data from our group on the role of gold nanoparticles in amyloid peptide aggregation using experimental methods and large-scale molecular dynamics simulations. A complex interplay of the surface properties of the nanoparticles, the properties of the peptides, as well as the resulting forces between both the nanoparticles and the peptides, appear to determine whether amyloid peptide aggregation is influenced, catalysed or inhibited by the presence of nanoparticles.

Use of the TCNQF4 2- Dianion in the Spontaneous Redox Formation of [FeIII (L- )2 ][TCNQF4 ⋅- ]

The reaction of [Fe^II (L^. )₂ ](BF₄ )₂ with Li₂ TCNQF₄ results in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1) where L^. is the radical ligand, 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide and TCNQF₄ is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. This has been characterised by X-ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable-temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X-ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low-spin Fe^III ions, the anionic form (L^- ) of the ligand and the anionic radical form of TCNQF₄ ; viz. TCNQF₄ ^{. -} . Li₂ TCNQF₄ reduces the [Fe^II (L^. )₂ ]²⁺ dication, which undergoes a reductively induced oxidation to form the [Fe^III (L^- )₂ ]⁺ monocation resulting in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1), the electrochemistry of which revealed four well-separated, diffusion-controlled, one-electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [Fe^II (L^. )₂ ][TCNQF₄ ^2- ].

Leone Spiccia Memorial Issue

Leone Spiccia will be remembered as an outstanding scientist with a warm and generous personality. At the time of his death he was at the pinnacle of his scientific career with much more yet to be achieved. However, his legacy will continue to inspire new exciting science. Leone's scientific research was truly multidisciplinary and his friends and colleagues honour his memory with this special issue.

Nanocrystalline silicon optomechanical cavities

Silicon on insulator photonics has offered a versatile platform for the recent development of integrated optomechanical circuits. However, there are some constraints such as the high cost of the wafers and limitation to a single physical device level. In the present work we investigate nanocrystalline silicon as an alternative material for optomechanical devices. In particular, we demonstrate that optomechanical crystal cavities fabricated of nanocrystalline silicon have optical and mechanical properties enabling non-linear dynamical behaviour and effects such as thermo-optic/free-carrier-dispersion self-pulsing, phonon lasing and chaos, all at low input laser power and with typical frequencies as high as 0.3 GHz.

Critical effects of polar fluorescent probes on the interaction of DHA with POPC supported lipid bilayers

The understanding of lipid bilayer structure and function has been advanced by the application of molecular fluorophores. However, the effects of these probe molecules on the physicochemical properties of membranes being studied are poorly understood. A quartz crystal microbalance with dissipation monitoring instrument was used in this work to investigate the impact of two commonly used fluorescent probes, 1‑palmitoyl‑2‑{12‑[(7‑nitro‑2‑1,3‑benzoxadiazol‑4‑yl)amino]dodecanoyl}‑sn‑glycero‑3‑phosphocholine (NBD-PC) and 1,2‑dipalmitoyl‑sn‑glycero‑3‑phosphoethanolamine‑n‑(lissamine rhodamine‑B‑sulfonyl) (Lis-Rhod PE), on the formation and physicochemical properties of a 1‑palmitoyl‑2‑oleoyl‑sn‑glycero‑3‑phosphocholine supported lipid bilayer (POPC-SLB). The interaction of the POPC-SLB and fluorophore-modified POPC-SLB with docosahexaenoic acid, DHA, was evaluated. The incorporation of DHA into the POPC-SLB was observed to significantly decrease in the presence of the Lis-Rhod PE probe compared with the POPC-SLB. In addition, it was observed that the small concentration of DHA incorporated into the POPC:NBD-PC SLB can produce rearrangement processes followed by the lost not only of DHA but also of POPC or NBD-PC molecules or both during the washing step. This work has significant implications for the interpretation of data employing fluorescent reporter molecules within SLBs.

Solvent-, Cation- and Anion-Induced Structure Variations in Manganese-Based TCNQF4 Complexes: Synthesis, Crystal Structures, Electrochemistry and Their Catalytic Properties

The reaction of Mn(BF₄ )₂ ⋅x H₂ O with (Pr₄ N)₂ TCNQF₄ (TCNQF₄ =2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) in a mixture of CH₃ OH/CH₂ Cl₂ gives a 2:3 stoichiometric complex of (Pr₄ N)₂ [Mn₂ (TCNQF₄ )₃ (CH₃ OH)₂ ] (1). If the solvent system used for the crystallisation of 1 is changed to CH₃ OH/DMF, then a different product, [Mn(TCNQF₄ )(DMF)₂ ]⋅(CH₃ OH)₂ (2), is obtained. The use of Li₂ TCNQF₄ instead of (Pr₄ N)₂ TCNQF₄ leads to the generation of [Mn₂ (TCNQF₄ )₂ (DMF)₄ ]⋅3 DMF (3). An unexpected mixed oxidation state network with a composition of [Mn^II ₄ Mn^III ₁₆ O₁₀ (OH)₆ (OCH₃ )₂₄ (TCNQF₄ )₂ ](NO₃ )₂ ⋅24 CH₃ OH (4), is formed if Mn(NO₃ )₂ ⋅x H₂ O is used in place of Mn(BF₄ )₂ ⋅x H₂ O in the reaction that leads to the formation of 3. Compounds 1-3 have been characterised by X-ray crystallography; FTIR, Raman and UV/Vis spectroscopy; and electrochemistry. Compound 4 has only been analysed by X-ray crystallography and vibrational spectroscopy (Raman, FTIR), owing to rapid deterioration of the compound upon exposure to air. These results indicate that relatively minor changes in reaction conditions have the potential to yield products with vastly different structures. Compound 1 adopts an anionic 2D network with unusual π-stacked dimers of the TCNQF₄ ^2- dianion, whereas 2 and 3 are composed of similar neutral sheets of [Mn(TCNQF₄ )(DMF)₂ ]. Interestingly, the solvent has a significant influence on the stacking of the sheets in the structures of 2 and 3. In compound 4, clusters with a composition of [Mn^II ₄ Mn^III ₁₆ O₁₀ (OH)₆ (OCH₃ )₂₄ (CH₃ OH)₄ ]⁶⁺ serve as eight-connecting nodes, whereas TCNQF₄ ^2- ligands act as four-connecting nodes in a 3D network that has the same topology as fluorite. Compound 3 exhibits an exceptionally high super-catalytic activity for the electron-transfer reaction between ferricyanide and thiosulfate ions in aqueous media.

Tunable Biogenic Manganese Oxides

Influence of the conditions for aerobic oxidation of Mn2+(aq) catalysed by the MnxEFG protein complex on the morphology, structure and reactivity of the resulting biogenic manganese oxides (MnO_x ) is explored. Physical characterisation of MnO_x includes scanning and transmission electron microscopy, and X-ray photoelectron and K-edge Mn, Fe X-ray absorption spectroscopy. This characterisation reveals that the MnO_x materials share the structural features of birnessite, yet differ in the degree of structural disorder. Importantly, these biogenic products exhibit strikingly different morphologies that can be easily controlled. Changing the substrate-to-protein ratio produces MnO_x either as nm-thin sheets, or rods with diameters below 20 nm, or a combination of the two. Mineralisation in solutions that contain Fe2+(aq) makes solids with significant disorder in the structure, while the presence of Ca2+(aq) facilitates formation of more ordered materials. The (photo)oxidation and (photo)electrocatalytic capacity of the MnO_x minerals is examined and correlated with their structural properties.

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Effects of guanidino modified aminoglycosides on mammalian membranes studied using a quartz crystal microbalance

The increase in bacterial and viral resistance to current therapeutics has led to intensive research for new antibacterial and antiviral agents. Among these, aminoglycosides and their guanidino derivatives are potent candidates targeting specific RNA sequences. It is necessary that these substances can pass across mammalian membranes in order to reach their intracellular targets. This study investigated the effects of the aminoglycosides kanamycin A and neomycin B and their guanidino derivatives on mammalian mimetic membranes using a quartz crystal microbalance with dissipation monitoring (QCM-D). Lipid bilayers as membrane models were deposited onto gold coated quartz crystals and aminoglycosides added afterwards. Notably, the guanidino derivatives exhibited an initial stiffening of the membrane layer indicating a quick insertion of the planar guanidino groups into the membrane. The guanidino derivatives also reached their maximum binding to the membrane at lower concentrations than the native compounds. Therefore, these modified aminoglycosides are promising agents for the development of new antimicrobial treatments.

Electrochemistry of cytochrome P450 17α-hydroxylase/17,20-lyase (P450c17)

Within the superfamily of cytochrome P450 enzymes (P450s), there is a small class which is functionally employed for steroid biosynthesis. The enzymes in this class appear to have a small active site to accommodate the steroid substrates specifically and snuggly, prior to the redox transformation or hydroxylation to form a product. Cytochrome P450c17 is one of these and is also a multi-functional P450, with two activities, the first 17α-hydroxylation of pregnenolone is followed by a subsequent 17,20-lyase transformation to dehydroepiandrosterone (DHEA) as the dominant pathways to cortisol precursors or androgens in humans, respectively. How P450c17 regulates these two redox reactions is of special interest. There is a paucity of direct electrochemical studies on steroidogenic P450s, and in this mini-review we provide an overview of these studies with P450c17. Historical consideration as to the difficulties in obtaining reliable electrochemistry due to issues of handling proteins on an electrode, together with advances in the electrochemical techniques are addressed. Recent work using Fourier transformed alternating current voltammetry is highlighted as this technique can provide both catalytic information simultaneously with the underlying redox transfer with the P450 haem.

Structural, Spectroscopic, and Electrochemical Characterization of Semi-Conducting, Solvated [Pt(NH3)4](TCNQ)2·(DMF)2 and Non-Solvated [Pt(NH3)4](TCNQ)2

The demand for catalysts that are highly active and stable for electron-transfer reactions has been boosted by the discovery that [Pt(NH3)4](TCNQF4)2 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is an efficient catalyst. In this work, we prepare and characterize the two related [Pt(NH3)4]2+ complexes, [Pt(NH3)4](TCNQ)2·(DMF)2 (1) and [Pt(NH3)4](TCNQ)2 (2). Reaction of [Pt(NH3)4](NO3)2 with LiTCNQ in a mixed solvent (methanol/dimethylformamide, 4 : 1 v/v) gives [Pt(NH3)4](TCNQ)2·(DMF)2 (1), whereas the same reaction in water affords [Pt(NH3)4](TCNQ)2 (2). 2 has been previously reported. Both 1 and 2 have now been characterized by single-crystal X-ray crystallography, Fourier-transform (FT)IR, Raman and UV-vis spectroscopy, and electrochemistry. Structurally, in 1, the TCNQ1− anions form infinite stacks with a separation between adjacent anions within the stack alternating between 3.12 and 3.42 Å. The solvated structure 1 differs from the non-solvated form 2 in that pairs of TCNQ1− anions are clearly displaced from each other. The conductivities of pressed pellets of 1 and 2 are both in the semi-conducting range at room temperature. 2 can be electrochemically synthesized by reduction of a TCNQ-modified electrode in contact with an aqueous solution of [Pt(NH3)4](NO3)2 via a nucleation growth mechanism. Interestingly, we discovered that 1 and 2 are not catalysts for the ferricyanide and thiosulfate reaction. Li+ and tetraalkylammonium salts of TCNQ1−/2− and TCNQF41−/2− were tested for potential catalytic activity towards ferricyanide and thiosulfate. Only TCNQF41−/2− salts were active, suggesting that the dianion redox level needs to be accessible for efficient catalytic activity and explaining why 1 and 2 are not good catalysts. Importantly, the origin of the catalytic activity of the highly active [Pt(NH3)4](TCNQF4)2 catalyst is now understood, enabling other families of catalysts to be developed for important electron-transfer reactions.