Replacing 6PPD in Tires and Naturally Occurring Nickel MOFs from Cable Bacteria

Overview

In this week’s Chemistry World podcast, Mariana Kneppers hosts two connected stories: the search for a safer replacement for the tire preservative 6PPD after toxic byproducts were linked to declines in some salmon species, and the discovery of living organisms that biosynthesize nickel-based metal–organic frameworks (MOFs) that conduct electricity, potentially guiding safer materials design. The episode also touches on related chemistry news and a chemistry history segment about Ingold's curly arrows that shaped reaction mechanism notation.

• 6PPD in tires and its toxic quinone byproducts
• Replacing 6PPD without creating new environmental problems
• Cable bacteria that biosynthesize conductive MOFs on the seafloor
• Implications for sustainable electronics and materials design
• Chemistry history: Ingold and the origin of curly arrows

Overview

The podcast opens with a look at several chemistry world news items before diving into two central stories. First, the tire industry faces a regulatory and practical challenge: incorporating six ppd (6PPD) in tires has been effective at preventing rubber degradation, but its breakdown products, particularly 6PPD quinone, are highly toxic to some salmon species when they wash into waterways. The discussion outlines how 6PPD functions chemically, moving through the rubber matrix to protect the surface from ozone attack, and why finding a replacement is nontrivial. The toxicology is not universal across all fish, and the regulatory landscape in the US, Canada, and Europe is shifting to address the broader PPD family. This raises the specter of regrettable substitution, where a new chemical might solve one problem but introduce others. A key point is that while 6PPD is not extremely persistent in air, it can linger in sediments and sediments-derived environments where oxygen is scarce, releasing the toxic quinone back into the river system when disturbed.

6PPD in Tires and Safer Substitutes

The episode explains that tire formulation is a delicate balancing act among many ingredients. The 6PPD family acts as a paraphenylene diamine (PPD) core with side chains that enable diffusion through the rubber and surface reactivity with ozone. Replacements must retain mobility, allow diffusion through the solid rubber matrix, and preserve performance at high speeds and in wet conditions, while avoiding new ecological hazards. Industry players are narrowing candidates to roughly 40 compounds, including other PPDs and non-PPD alternatives, with toxicology studies increasingly prioritised. The discussion cautions that any substitute must be evaluated for a broad range of aquatic species, as toxicity can vary widely across fish and other organisms. Europe and parts of North America are moving toward regulation of the entire PPD family, which could complicate replacements that rely on similar chemical motifs.

Cable Bacteria, MOFs, and Natural Conductivity

The second story centers on a surprising discovery: cable bacteria that traverse sediment with long filaments have been found to biosynthesize nickel-based metal–organic frameworks (MOFs). These MOFs twist into nanoribbons and function as electrical cables that transfer electrons from sulfide oxidation in the sediment up into the bacterial cells, enabling chemosynthesis-based energy metabolism in a light-starved environment. The MOFs in question involve nickel and dithiolene ligands, motifs familiar to MOF chemists, but here they are produced naturally rather than synthesized in a lab. The filaments act as conductive networks whose efficiency rivals some synthetic conductive polymers and MOFs. This natural example of MOFs raises questions about how to emulate such structures, scale production, and potentially integrate bio-inspired or bio-grown MOFs into sustainable electronics and sensing technologies.

Implications for Materials Design

From a design perspective, the bacterial MOFs suggest strategies for creating conductive, bio-inspired materials that are potentially more sustainable to produce. Researchers are exploring how to replicate the ribbon-like MOF architecture in laboratory synthesis, or alternatively engineer microbial or enzymatic routes to assemble similar materials, potentially enabling low-energy, bio-compatible electronics. The episode also highlights the convergent evolution seen in MOF-inspired conductive motifs, drawing connections to polymer science and bioinorganic chemistry where sulfur-containing ligands and nickel cofactors appear in both biological and synthetic contexts.

Chemistry History: Ingold and the Curly Arrows

Ingold’s historical segment explains how reaction mechanism thinking shifted from recipe-like descriptions to electron flow narratives. Partnering with Hughes, Ingold helped formalize the SN1 and SN2 distinctions and popularized the curly arrows used to depict electron movement. This history underpins modern chemical intuition and toolkits for teaching and research. The discussion notes the interplay of ideas and credit, including Robinson’s competing framework, which eventually yielded a widely accepted language of electron movement in solution-phase organic reactions.

Outlook and Wrap-Up

The podcast closes with reflections on how safer tire chemistry and biologically derived MOFs might influence the design of future materials and environmental policy. The topics illustrate how chemistry intersects with policy, industry, and ecology, and how new discoveries can inspire safer, more sustainable technologies while raising important questions about regulation, toxicity, and responsible innovation.