Chemical Science Blog

I feel pretty safe saying that the development of effective antimicrobial drugs (looking at you penicillin!) was one of the most significant pharmacological events of recent history. Unfortunately, the widespread and often indiscriminate use of antibiotics has created an environment where bacteria with evolved drug resistances, colloquially known as “superbugs,” pose a serious threat to global health. While development of new small-molecule antimicrobial drugs is still ongoing, scientists are exploring alternative approaches as well. Of interest are antimicrobial peptides, found in plants and animals as a part of native immune systems. The most common are cationic antimicrobial peptides (CAMPs) comprised of 10-50 amino acids that possess both cationic and hydrophobic subunits and an overall positive charge. This allows them to strongly electrostatically interact with negatively charged bacterial cell membranes without disrupting neutral animal cells. Unfortunately, CAMPs have proved challenging to commercialize with generally unfavorable activities and issues with large-scale manufacture.

Figure 1. General synthetic scheme, structure, and key for components of metallohelices.

Researchers in the UK and Czech Republic recently developed a range of cationic metallohelices (Figure 1) that demonstrate structure-dependent activity against both Gram-positive and -negative bacteria. The diamine ligands form cleanly in the presence of [15]-crown-[5] and upon combination with 2-pyridinecarboxaldehyde and a metal salt self-assemble into the iron or zinc metallohelices. The metallohelices consist of a single enantiomer, as the ligands are optically pure, as determined by NMR and single-crystal X-ray diffraction when possible. Altering the aryl linker unit in the ligand the overall size and shape of the metallohelix in both zinc and iron derivatives. The iron metallohelices are water compatible, with lifetimes exceeding 10 days even in highly acidic conditions, and thus suitable for antimicrobial activity screening.

The researchers used ­in vitro studies to find minimum inhibitory concentrations (MICs), the lowest concentration necessary to see bacterial growth inhibition. While the metallohelices demonstrate the highest activity towards Gram-positive bacteria, some showed lethal effects on Gram-negative E. coli in 20-40 minutes. The 5b helices with a para-benzene bridging group acted most selectively on E. coli with the Λ-5b enantiomer acting as the champion compound and selected for further mechanistic study. The researchers exposed a notorious E. coli strain to inhibitory levels of Λ-5b in an attempt to isolate resistant mutants. The 17 isolates showed only slight tolerance increases rather than true resistance and could be classified into 4 genetic sub-types. Two of the sub-types developed mutations that altered the biophysical properties of their outer membrane, a third lost the ability to produce the vitamin B12 transporter protein, and the fourth lost the pO157 virulence plasmid (which makes this particularly E. coli strain particularly unpleasant). Taken in concert, these 4 sub-types show that tolerance can be derived from disrupting the ability of Λ-5b to interact with and cross the cellular membrane.

Figure 2. Fluorescence images of cells treated with Λ-5b coupled to a fluorescent probe, with arrows pointing to the localization of Λ-5b.

Given data suggesting the ability of Λ-5b to cross the cellular membrane, despite the relatively large of the metallohelices, the researchers used fluorescence microscopy to probe Λ-5b localization by coupling Λ-5b with a fluorescent label. The labeled metallohelix preferentially localized to regions in growing cells that contain anionic phospholipids (Figure 2). This indicates that Λ-5b can cross the cellular membrane and acts internally to the cell rather than simply acting via electrostatic interaction that disrupts the membrane. Overall, this work provides an exciting approach to developing novel anti-microbial drugs that mimic CAMPs with higher stability, activity, and easier synthesis.

To find out more please read:

Metallohelices that kill Gram-negative pathogens using intracellular antimicrobial peptide pathways

Daniel H. Simpson, Alexia Hapeshi, Nicola J. Rogers, Viktor Brabec, Guy J. Clarkson, David J. Fox, Ondrej Hrabina, Gemma L. Kay, Andrew K. King, Jaroslav Malina, Andrew D. Millard, John Moat, David I. Roper, Hualong Song, Nicholas R. Waterfield and Peter Scott

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

While nanobots are still a thing of science fiction, the development of the fundamental scientific concepts that could lead to highly complex molecular-scale machines has been an active area of research for over 2 decades. Molecular motors rely on isomerization through metastable intermediate states that leads to unidirectional rotation via a rotor portion of the molecule. Initial work utilized molecules that convert high-energy UV-light into this directional motion. However, these systems are limited in their future utility, particularly in biological applications, if they need UV-light for activation. Recent work focuses on driving molecular motors with visible light via a range of approaches, including altering the absorbance profile of the motor molecule itself. Unfortunately, changing the electronics of the molecule has previously substantially decreased the quantum yield while resulting in only slight red shifts.

Researchers in The Netherlands recently developed 2^nd generation molecular motors featuring a mixture of electron-donating and electron-withdrawing groups that exhibit substantial red-shifts. Density functional theory (DFT) calculations predicted that adding cyano- and methoxy- groups to opposite halves of the motor would shift the absorbance past 410 nm. The new molecules were synthesized via a general procedure shown in Figure 1, with yields ranging from 3 – 10%.

Figure 1. General synthetic route to cyano- and methoxy- substituted molecular motors and photoisomerization reaction that results in unidirectional motion.

The addition of the cyano- and methoxy- groups shifted the absorbance maxima corresponding to the HOMO-LUMO transitions to from 422 – 453 nm, an increase of over 60 nm from the parent molecule. When irradiated, the molecules transitioned from their stable ground state to the metastable isomers. These were characterized by UV-Vis spectroscopy, with the emergence of further red-shifted features (Figure 2), and ¹H NMR.  The ¹H NMR spectra were obtained with in-situ irradiation (which sounds like a sweet experimental setup) at various wavelengths and the ratios of several specific protons on the rotor. The derivative 2 with a methoxy- group on the rotor and cyano- groups on the stator demonstrated activity with irradiation at wavelengths up to 530 nm.

Figure 2. UV-Vis absorption spectra of stable (solid) and metastable (dashed) isomers of the molecular motors.

The energy of activation of the rotation for all three derivatives was determined by Eyring analysis and corroborated by DFT calculations. All were around 90 kJ/mol, with 2 requiring the most energy to elongate the central alkene bond and isomerize. The quantum yields of the motors for the forward reaction range from 5.8 – 11.5%, comparable to state-of-the-art UV absorbing motors. The quantum yields for the back reactions were calculated to be significantly lower than those for the forward reaction, which corresponds to the excess of metastable isomers observed under active irradiation. These motors also exhibit high photostability, with no significant change in the ground state absorbance after irradiation and cycling. This is promising for smart materials applications where stability is crucial. This work pushes forward the design of molecular motor systems that utilize visible rather than UV light.

To find out more please read:

Photoefficient 2^nd generation molecular motors responsive to visible light

Lukas Pfeifer, Maximilian Scherübl, Maximilian Fellert, Wojciech Danowski, Jinling Cheng, Jasper Pol and Ben L. Feringa

Chem. Sci., 2019, 10, 8768-8773.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

As we transition to an energy future composed primarily of intermittent, renewable technologies, finding ways to store the excess generated charges will be critical. While the general public typically thinks of batteries, another area of massive interest is storing the electrons in a chemical bond. This can be accomplished by creating electrocatalysts that can couple an electric current with chemicals like water and oxygen to generate stable new bonds in molecules for later use as fuels. These renewably generated fuels would then be available whenever needed, like at night. However, finding efficient electrocatalysts composed of earth-abundant materials has proven challenging. Many researchers have turned to nature for inspiration by designing molecules that mimic the active sites of enzymes.

Researchers in the United States used this approach, focusing on creating high-valent iron-oxo species, which others previously identified as the key catalytic intermediates in multiple enzymatic reactions. These types of species have traditionally been synthesized by reacting a reduced iron complex with an oxygen transfer reagent, but the researchers developed a system to generate highly reactive species using electricity as the reaction driving force and water as the oxygen source. The studied catalyst is a commercially available iron(III)-aquo complex with a tetraamido macrocyclic ligand (TAML) as the ancillary ligand (Figure 1B).

Figure 1. A. Cyclic voltammagram of (TAML)Fe in acetonitrile. B. Structure of (TAML)Fe and cyclic voltammagram showing increased current upon the addition of ethylbenzene.

When analyzed by cyclic voltammetry, an electrochemical technique where you cycle the voltage between set points and measure the current output, the (TAML)Fe shows two redox events at around 650 and 1250 mV that the researchers attributed to generating the Fe^IV-OH and Fe^V(O) species (Figure 1). Addition of ethylbenzene, which should react with the Fe^V species, increased the current at voltages of 1250 mV and higher, indicating (TAML)Fe turnover. However, isolating the Fe^v(O) species proved challenging as it reacts rapidly with the (TAML)Fe to form an Fe^IV dimer. This also limits the efficiency of the overall system by decreasing the amount of the most reactive species in solution.

Figure 2. A, B. Products generated by oxidation of various substrates screened with (TAML)Fe for electrocatalysis with isolated yield and calculated conversion in parenthesis. C. Substrates that did not react with the (TAML)Fe complex.

Both the Fe^IV dimer and Fe^V(O) species proved capable of oxidizing C-H bonds in ethylbenzene, but the Fe^V(O) is much more reactive and increases the oxidation rate at high electrochemical potentials. The researchers tested the scope of (TAML)Fe reactivity using a series of compounds with benzylic C-H bonds (Figure 2). They found that the (TAML)Fe performed well with electron-rich and electron-neutral derivatives, with an electron-deficient nitro-substituted derivative showing lower reactivity. Several substrates with non-benzylic C-H bonds showed high selectivity for oxidation at the benzylic C-H bond. (TAML)Fe also showed high electrocatalytic activity for oxidizing alcohols and converted substrates as simple as cyclohexanol and as complex as a steroid to ketones in high yields (up to 97%).

This study of an earth-abundant, stable, and commercially available electrocatalyst acts as a baseline for further studies with other similar metal complexes. Despite the efficiency limits attributed to dimerization, the high stability and selectivity of the (TAML)Fe could lead to its use with a broader range of substrates with varied functional groups.

To find out more please read:

Electrochemical C–H oxygenation and alcohol dehydrogenation involving Fe-oxo species using water as the oxygen source

Amit Das, Jordan E. Nutting and Shannon S. Stahl

Chem. Sci., 2019, 10, 7542-7548

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.