Readers’ thoughts and feedback
On the one hand…
Rachel Brazil’s inspiring reflections on homochirality in ‘Breaking the mirror’ may lead readers to an erroneous perception of a very basic stereochemistry fact. She writes: ‘(natural) sugars are exclusively right-handed, amino acids left-handed’. This is not true and comes from a nomenclature problem.
According to Emil Fischer’s classification, natural sugars belong to the (d)-series while natural amino series belong to the (l) series. The d/l descriptors come from the configuration of particular carbon atoms in the structures, but have nothing to do with the absolute chirality (or handedness), which is related to the optical activity of chiral compounds.
If plane polarised light is passed through such a sample and its plane is turned clockwise [denoted as (+)], then the compound is dextrorotatory (d), or right-handed. The enantiomer – its non-superimposable mirror image – will rotate the same plane polarised light by the same angle, but in the opposite direction [denoted as (–)] and is termed laevorotatory (l), or left-handed. As a result, there are natural ‘left-handed’ sugars, such as (d)-(–)-fructose, or ‘right-handed’ amino acids such as (l)-(+)-alanine. [Editor: Iupac recommends using the +/– notation, not d/l, because it causes confusion with the d/l notation of sugars and amino acids.]
Absolute stereochemistry is unequivocally characterised by the R/S descriptors introduced by Robert Cahn, Sir Christopher Ingold and Vladimir Prelog. Accordingly, the stereogenic centres in both (d)-glyceraldehyde (the simplest of the natural sugars) and (l)-alanine (the simplest chiral amino acid) are configured (R). All the (d)-sugars and protein-forming amino acids share this spatial configuration (although for cysteine it is denoted (S) owing to the higher priority of sulfur in the R/S naming system). In that sense, life on Earth truly is homochiral: there is only one ‘handedness’ for both natural sugars and amino acids.
Teodor Silviu Balaban
I read Rachel Brazil’s mini-review on the origins of homochirality with considerable interest. In my opinion it is not necessary to assume that life started with RNA in order to explain homochirality, nor to postulate any novel chemistry.
It is more likely that homochirality evolved as a consequence of the molecular copying process itself, and that it would happen regardless of which molecules were actually responsible for the beginning of life.
Any assembly of a copy onto a polymeric template must necessarily form a rough double helix, and all helices are necessarily asymmetric. The molecular copying system in which life began would have contained a racemic mixture of various irregular double helical structures. The effect of Darwinian evolution on the structures present would be to progressively produce more regular helices by eliminating chain branching and incorporating chiral monomers selectively into the appropriate helical structure.
Thus evolution would produce two increasingly distinct chiral molecular lineages. Eventually the differences between those two chiral lineages would become so great that the copying of genetic information between them would be impossible, and they would then be in direct competition. That competition would continue until one or other of the chiral molecular lineage became extinct.
There is no need to postulate any inherent superiority for the surviving chirality. ‘Time and chance happen to them all’ is a sufficient explanation of the outcome.
Laurence Payne CChem MRSC
I was delighted to read of physicist Richard Feynman’s awe of chemistry. In my student days organic chemistry was clearly disdained as ‘pot boiling’, but, as the classical methods of determining molecular structure became superseded by physical ones, and results were compared, the physicists had a change of heart. As Feynman put it: ‘lo and behold!, the chemists are almost always correct.’
Solving structures as complex as strychnine’s, with little other than glassware, brains and perseverance, was a supreme human achievement. I believe that its unfairly low rating has had an adverse effect on the course of science teaching. I’ve spent years with older children as they model small molecules with the ‘atoms’ issued to them, decide and enter the formulas into Google, then find the structures confirmed in Wikipedia. A short course leaves them with something very like the physicists’ awe of chemistry.
If students started secondary school chemistry with this introduction, they would want to learn more. A lifetime’s observation of school leavers’ chemical understanding has convinced me that molecular modelling is the only approach to chemistry worth considering. It would help the cause of science (not just chemistry) in schools.
It can never be too late for something as important as capturing young minds for science. This approach needs champions.
John Leisten OBE CChem FRSC
Mount Tamborine, Australia
R&D warning over EU exit
The recent case of public R&D spending in Switzerland falling by 7% after it was prevented from applying for EU research funding deserves as much publicity as possible as a response to those who wish the UK to secede from the EU.
The head of the UK Science and innovation network should also be closely questioned about the effects of UK secession.
As a British scientist who now works in France, I really appreciate the social and economic mobility afforded by the UK’s EU membership. The same must apply to numerous UK scientists working in French or other
EU member states’ companies, government establishments and research institutes.
Adrian Parr CSci CChem MRSC