Sunday, May 19, 2013

Why is life chiral?

Ingeniøren, the magazine of the Danish society for engineers, has a column where people submit questions, which are then farmed out to scientists who provide short answers.

The question send to me is (translated from Danish)
Chemical substances are found in two variants which are each other's mirror image. In an ordinary chemical reaction in the laboratory, there is formed the same amount of the two mirror image forms. In the natural products and organisms there are only one of the two mirror image forms. Why is there only one mirror-image form of natural substances and living organisms? 
Once you know the term for this (homochirality) there is plenty of information on the web, including a wiki page.  Here's me trying to get my head around some of it and explain it in layman's terms.  Comments welcome.

This is not known but there is some speculation, which can be divided into two general categories: When life began, there were (1) a greater concentration of biologically relevant molecules with the mirror image form that are now found in living organisms, and (2) equal concentrations of mirror-image forms, and the mirror-image form we see now in living organisms was chosen by chance and then copied.

Category 1: It came from outer space
Amino acids with a higher concentration of the mirror-image form that is now found in living organisms have been found in meteorites on Earth. So if a sizable part of life's building blocks came from space, this may be the answer.

The reason for this difference in concentration is likely to result from chemical reactions in space that are affected by "polarized" light from the stars. Light comes also two mirror-image forms and starlight has a little bit more of one kind.  This is called polarized light, which kan affect chemical reactions in such a way that as to form more of one mirror-image form than the other. The light must travel long distances to be polarized so that the sunlight that hits the Earth is not polarized.
Beta-decay, a sort of natural radioactive radiation, can also lead to polarized light here on earth, but it is uncertain whether the amount of light is enough to have a practical effect on chemical reactions.

Category 2: That's life
Because the genetic code is the same in all organisms, we assume life as we know it may lead back to a single "primordial cell", as it is unlikely that such a complex system can be created randomly twice. Whether the  the reason for this is that life originated only once, or whether this cell happened to survived where other cells (in other biochemical ways to copy object) died, is not known.

The same considerations apply to the molecules and reactions which underlie life. We know that many molecules of the same mirror-image form "clumps together" and that many self-replicating molecules retain their mirror image form. It is therefore possible that a group of molecules of the same mirror-image form by chance clumped together in such a way that they can make copies of themselves, and this is the basis of our primordial cell. If this only happened on one occasion, it may explain why we only see a mirror image form in living organisms.



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