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SCIENCE AND SOCIETY Catalysts for change: the science of making molecules

Last week, the Nobel prize in chemistry 2021 was awarded for ‘the development of asymmetric organocatalysis’ — ROX MIDDLETON, LIAM SHAW and JOEL HELLEWELL introduce a greener way to build molecules

MAKING molecules is easy for humans: within your body, billions of delicately balanced chemical processes are happening every day, moving groups of atoms around in shifting patterns.

As carbon-based life-forms, our internal chemistry is fantastically rich. Carbon can combine with other atoms so easily and in so many ways. It’s a toolbox that produces all of life as we know it. But we don’t know how most of these submicroscopic reactions work.

Natural molecular processes have evolved over billions of years, with complex pathways looping in dizzying complexity within the cell. These require sophisticated biochemistry to untangle.

If we want to make molecules at scale outside the body, that can be difficult. Learning how to manipulate and manage a single chemical process can be a life’s work. An important part of chemistry has always been building chemical narratives that culminate in a desired molecule.

This is known as synthesis. The destination is known, but the best path to reach it is unclear.

Alchemists working to transform or purify materials had the same aim and the reasoning is little changed.

It’s clearly possible to get to the target somehow — otherwise it would never exist — but doing so with manageable, scalable and cheap conditions is the challenge.

The difficulty of alchemy was that its targets were often precious metal elements: gold or silver. The very reason for their value was their inertness, their recalcitrance to getting involved in chemical reactions.

In contrast, the targets of chemistry today are typically large and complex molecules.

To get there, chemists think creatively. They link other molecules together in long daisy chains, wiring together outputs of one reaction into the inputs of another, an interwoven molecular story.

Many of these molecular stories have central roles in our society, but writing about chemistry is hard. Its protagonists are characters few of us have heard of, with long unwieldy names.

The twists and turns are difficult to explain to the non-specialist — the Science and Society team included!

A general rule is that many chemical reactions are possible but proceed slowly, if at all. However, they can be sped up if the right catalyst can be found. A catalyst is a molecule that isn’t itself affected by the reaction, but facilitates it with its presence.

Within living cells, proteins that speed up reactions are known as enzymes, of which millions exist.

Outside cells, metals can also be used as catalysts. It was thought these were the two possible categories of catalyst. But in 2000, chemists developed a new category of catalyst.

Last week, the Nobel prize for chemistry was awarded in that field.

Leaving aside the theatrics of the late-night calls from Stockholm which make science into a game show, the work itself is exciting: the development of “organocatalysis.”

To unpack the term, the “organo” relates to “organic” molecules.

Although these molecules are the basis of living things, the term has got nothing to do with the normal use of “organic.”

The word applies to any molecule that contains carbon-hydrogen bonds: everything from the compounds that make up living things, to the petrol and plastic made of hydrocarbons.

Organic chemistry contrasts with inorganic chemistry, which excludes any molecules with these carbon bonds. So organocatalysis is the use of organic molecules for speeding up reactions.

The specific form the prize was awarded for is known as “asymmetric” organocatalysis. Some molecules are symmetrical: a carbon atom bonded to four hydrogen atoms (a methane molecule) is the same as its mirror-image.

But if the carbon atom is bonded to four different groups of atoms, this symmetry breaks down.

Organic molecules usually have a “handedness” and their effects in the cell are different depending on it. Enzymes often have handedness and so can chaperone reactions in the right direction. The reaction turns the molecules around in one loop and it never goes the other way.

The handedness of molecules matters a lot in industrial chemistry. When making a target molecule, such as one that works as a drug, only one handedness is desired.

To give one example, the same component atoms in the same molecule can be either a painkiller or a cough medicine.

The pharmaceutical company that manufactured both called one “Darvon” and the other “Novrad” — its reversed image.

But selecting only one symmetry type of the target molecule is challenging. The way to do it is to use a catalyst which is already “asymmetric.”

Biology manages this with enzymes. Taking inspiration from this, in the past two decades chemists have discovered that instead of enzymes, other small organic molecules which are asymmetric can play this role.

One benefit of organocatalysis is the avoidance of inorganic molecules. Organocatalysis is an important development for “green chemistry” which seeks to use molecules that are benign and have minimal environmental impact, unlike some toxic compounds.

If organic molecules react or break down they make biological waste, such as carbon dioxide and methane, which can be biodegraded.

For inorganic molecules, the carbon could be replaced by fluorine or metal, among many other possibilities. These are more unusual molecules that require mining or synthesis and can produce more toxic waste.

But the real result is not just specific catalysts for a given reaction, but new sets of principles for chemical creativity.

One example is the way that organocatalysis has been linked into existing research into solar energy. Plants naturally photosynthesise, using light to produce molecules used for food.

To mimic this process, certain metals can be used to absorb light and then catalyse organic reactions. But the reactions involved often need heating to speed them up.

If organocatalysis is used, this means that reactions can proceed at low temperatures.

Here, more intermediate molecules are possible and so new possibilities emerge.

The resulting “photoredox catalysis” has already been used to speed up a range of chemical reactions, turning solar energy into useful work: literal photosynthesis. This work tries to mimic the work of biology using new organic molecules.

At school, children are taught that biology and chemistry are separate disciplines. But the field of organocatalysis demonstrates how artificial these boundaries can be.

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