One of the great outstanding mysteries in science is why biological molecules share a certain handedness or homochirality. Biologists have long known that life on Earth is built almost exclusively out of left-handed amino acids and right-handed sugars. What they can’t figure out is how this state of affairs arose.
One idea covered by this blog recently is that polarised starlight might have biased conditions in the cloud of dust and gas from which the Earth formed, leading to an excess of one amino acid entantiomer over another.
Today, F L Falcon at the University of Havana in Cuba suggests a mechanism that lies much closer to home
Behind Falcon’s idea is the observation that crystals of the amino acid l-alanine are ever so slightly bigger than crystals of d-alanine.
Falcon imagines the following scenario in which d and l-alanine is dissolved in water with a thermal gradient, perhaps close to a geothermal vent for example.
Thermal currents carry the alanine up to cooler waters where it precipitates in crystalline form. However, the larger l-alinine crystals get carried higher because they offer a greater cross section to the currents. So the l-alinine crystals end up in colder waters, growing to even larger sizes.
Eventually the crystals’ size causes them to sink and dissolve. However, the larger crystals take longer to dissolve and so last longer, effectively converting a portion of the l-alinine from asolute to a crystal form, a process known as Oswald ripening.
The imbalance in solution causes a process known as epimerisation in which d-alinine is converted into l-alinine.
And the cycle begins again. In this way the d-alinine is slowly converted into l-alinine, leaving a homochiral solution.
The beauty of Falcon’s approach is that it assumes only that there is a thermal gradient in the water and that the dissolved amino acids have a certain composition.
But there is also an interesting link between Falcon’s ideas and the Miller-Urey experiments in the 1950s, in which the eponymous scientists recreated the early Earth’s atmosphere in a test tube, zapped it with lightning and found to their amazement that this process generated a primordial goo of amino acids.
It is exactly this Urey-Miller composition of amino acids that Falcon says leads to homochirality.
Of course, nobody need take Falcon’s word for it. The process that Falcon describes ought to be easily reproducible in the lab. Any biochemists out there with some spare time on their hands?
Ref: arxiv.org/abs/1005.4142: A Natural Mechanism For l-Homochiralization Of Prebiotic Aminoacids