He worked out how nature’s catalysts, proteins known as enzymes, speed up biochemical reactions. They bind to the transition states of a substrate molecule and so lower the energy of the highest energy point on a reaction pathway, which means that the reaction can proceed at much greater speed, often millions of times faster than the uncatalyzed reaction in fact.

Chemists have borrowed this in the design of organic catalysts and in making artificial enzymes, for their non-biological reactions. It is not a
complete description of catalytic behavior of enzymes of course, for that you might turn to Nanda and Koder in Nature Chemistry.

However, in a new paper from Simón and Goodman, they reveal a simple system which is common in both enzymic catalysis and organocatalysis, that does not conform to this simple idea of transition state binding. The reaction of carbonyls with a nucleophile to form an oxyanion can be catalyzed by hydrogen bonding, they explain, and there are many examples of this type of process using enzymes and using organocatalysts. The enzymes, however, do not use the arrangement of hydrogen bonds that binds the transition state best.

Instead, the hydrogen bonds are twisted around the carbonyl axis by about ninety degrees. This is less effective for transition state binding, but much less effective for ground state binding. The energy barrier for the reaction is lowered most effectively by arranging the hydrogen bonds to minimize the energy difference between the bound ground state and the bound transition state, and not by maximizing transition state binding.

Enzymes do not bind to transition states; they bind to minimize the energy difference between the ground state and the transition state.

“This has implications for the design of both artificial enzymes and
organocatalysts,” says Goodman.

Simo?n, L., & Goodman, J. (2009). Enzyme Catalysis by Hydrogen Bonds: The Balance between Transition State Binding and Substrate Binding in Oxyanion Holes The Journal of Organic Chemistry DOI: 10.1021/jo901503d

This post adapted from materials provided by Dr Goodman.

Three current supposedly “green” initiatives highlight the problem, Pike says.

First, Brits are being encouraged to drive five miles less each week. But a back of an envelope calculation shows that this will reduce the UK’s carbon footprint by just 0.3% not the significant amount the government claims.

“Each car travels around 10,000 miles a year, or 200 miles each week, so that the reduction in fuel is about 2.5%. However, passenger cars represent only around one-eighth of the country’s carbon footprint, which accounts for the very small overall saving,” Pike explains.

A second example is the manufacture of gas-to-liquid (GTL) kerosene from natural gas for air transport. This will reduce sulfur and particulate emissions, but the process is incredibly energy-intensive.

“Typically, for every tonne of carbon dioxide emitted by a plane, another 0.7 tonnes rises over the Middle East where the GTL fuel is made, so that the total global effect is over one and a half times the emissions of conventional jet fuel,” points out Pike.

Thirdly, average tailpipe emissions of carbon dioxide from new vehicles are to be capped at 130 grams per kilometre (g/km) from 2012. But, for manufacturers to plan their models and production lines, electric cars will be deemed to have zero emissions, even if (as in the UK) most of the electricity used will have been generated from fossil fuels.

Claims for the green credentials of all three examples of green initiatives, and there are many others are often folly. Initiatives must be assessed across the whole life cycle and across all energy and resource inputs and outputs. Pike adds that political decisions made with inadequate scientific or behavioral evidence will inevitably lead to unintended consequences. The same perspective should be applied to the talks in Copenhagen. Let’s just hope they’ve got a team of scientific advisers willing to stand up to the political shenanigans.

Ironically, as plans to maintain Silverstone as the home of the British motor racing Formula 1 Grand Prix and a major guzzler of gas talk of a seventeen-year deal, might we expect the politicians in Denmark to take such a long view?

“Without the right direction and regulatory framework, backed by an understanding of the science and non-science issues, vested interests will continue to provide solutions, unchallenged, over the forthcoming decades that seem persuasive but are actually unsustainable or have little impact.

More discussion at the RSC’s blog – http://www.rsc.org/blog

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