Wednesday, 1 October 2014

Does your right hand know what your left hand is doing? Some thoughts on the importance of Physics for Life Sciences.

A commentary published this week in the Journal Nature, entitled 

Force of nature gave life its asymmetry

'Left-handed' electrons destroy certain organic molecules faster than their mirror versions.
made me think about the discussions relating to the possible role of epigenetic factors in the development of unique fingerprint patterns. It also prompted the thought that whilst we appreciate the aesthetic of symmetry (facial beauty is generally correlated with beauty), our existence as living organisms,  depends upon asymmetry at both the molecular and cellular levels. Thus, many biosynthetic precursors and metabolites, including glucose and most biological amino acids, are ‘chiral’. This means that one of the two mirror-image forms are preferred in vivo (think of your left and right hands, as I have discussed before, Jimi Hendrix and Paul McCartney both played guitar left handed: notice the controls on Hendrix's guitar are upside down, since he is playing a right handed Fender Stratocaster). In the same way, precursors (or building blocks) lead to a preferred "handedness" in for example,the DNA double helix or protein structural elements like the alpha helix. 
The origins of bilateral symmetry in living organisms is easy to see in the   image of a butterfly, with the dotted line indicating the symmetry axis (RHS). In the case of human embryogenesis, the initial division of the fertilised egg generates two identical cells, positioned as mirror images. From here on in, the line of symmetry is drawn and, through simple physical principles relating to molecular diffusion, coupled with differential gene expression (and a sprinkling of epigenetic factors), the body plan of many organisms is laid down. (You may want to look also at radial symmetry in Nature). Handedness is of fundamental importance in Biology, from sexual evolution through to drug interactions with protein targets. This is why it is so important to understand its origins, as well as appreciating its significance. 
So what has been discovered? It has been known for some time that molecules can sometimes adopt left or right handed forms, which both possess the same chemical formulae. We refer to this as chirality. It has been demonstrated by the authors of this work that the rate of decay of left and right forms are not identical and therefore an intrinsic level of preference may be possible in the selection of right over left in the early stages of the evolution of life forms. As descried in Nature (with some minor alterations):
Two physicists, Gay and Dreiling, at the University of Nebraska–Lincoln, fired low-energy, spin-polarized electrons at a gas of the organic compound and bromocamphor, used in some parts of the world as a sedative. In the resulting reaction, some electrons were captured by the molecules, in a manner similar to the capture of electromagnetic (think of my explanation of colour in solutions of dyes) radiation, which then were raised to an excited state. The molecules then degrade, producing bromide ions and other reactive compounds. By measuring the flow of ions produced, the researchers could see how often the reaction occurred for each handedness of electron.
The researchers found that left-handed bromocamphor was just slightly more likely to react with right-handed electrons than with left-handed ones. The converse was true when they used right-handed bromocamphor molecules. At the lowest energies, the direction of the preference flipped, causing an opposite asymmetry. In all cases the asymmetry was tiny, but consistent, like tossing a biased coin. “The scale of the asymmetry is small, but as in all matters evolutionary, significant: out of 20,000 coins tossed again and again, on average, 10,003 of them land on heads while 9,997 land on tails,” says Dreiling.Their research was published in Physical Review Letters on the 12th September 2014. 
You may wish to look at my previous post on thalidomide as an example of how (racemic)mixtures of different forms of a molecule can have a profound effect on human development. But for today, I want you to appreciate how experiments in physical chemistry can impact on Life Scientists (illustrated as a Venn diagram, left). Without a robust understanding of the molecular (and quantum) principles that underpin Life, we will surely make mistakes in designing drugs, developing new materials and generally translating our knowledge of Science into everyday use. 
This “duty” of all Scientists to “keep on top of the literature” is what the public expects and should, in my view be what the public demand. Those who are funded by taxes, to carry out scientific research, must be the best and must be prepared to keep abreast of all important developments in the wider sphere of Science. On the other hand, this is a daunting problem, since the number of research papers published every week is unmanageable. We are fortunate therefore, that the premier journals in Science (including Nature and Science), together with shrewd observers and commentators, pick out such breakthroughs on our behalf. As we approach Nobel Prize week, the handful of scientists who will be honoured, in my (limited) experience, will not only be experts in their own fields, but will also be able to demonstrate a much broader perspective on Science than most others.

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