The origins of handedness in life

first_img © 2014 Phys.org Finding supports model on cause of DNA’s right-handed double helix Handedness is a complicated business. To simply say life is left-handed doesn’t even begin to capture the blooming hierarchy of binary refinements it continues to evolve. Over the years there have been numerous imaginative theories for how life’s amino acids, nucleic acids, and sugars came to favor one orientation over another. Everything from circularly polarized UV light, magnetism of the Earth, oriented clays or quartzes, to the weak nuclear force itself has been considered, but none has yet to securely emerge into realm of plausibility. A recent paper by Dreiling and Gay in Physical Review Letters has now thrown a life jacket to the weak force making it a theoretically viable possibility. To fully vet the author’s conception a little work needs to be done. Effort well spent we might say, because to understand where and when the handedness of life’s molecules originated is to know the origin of life. Citation: The origins of handedness in life (2014, October 1) retrieved 18 August 2019 from https://phys.org/news/2014-10-handedness-life.html Journal information: Physical Review Letters Explore further The capital “L” forms of aminos acids are the chiral orientations mostly preferred in our proteins. They are said to be left-handed when referenced to the optical activity of an L-glyceraldehyde molecule from which they could in theory be derived. However nine of our nineteen L-amino acids are actually dextrorotatory with a lowercase d (rotate polarized light to the right), when measured at the standard optical wavelength of 589 nm. Similarly the D orientation of glucose, the dextrose of life, rotates light to the right, while D-fructose actually rotates it to the left. The handedness of the DNA helix is more straightforward to assign than values for optical rotation of constituent molecules with multiple chiral centers. The A-DNA helix which life mostly employs, threads to the right when viewed from either direction as sure as a nut threads in either direction on the machinist’s screw. While the debate continues on the exact order in which the many key molecules of life first appeared, amino acids have been the center of attention. One possible explanation for their chirality is that circularly polarized (CP) light preferentially destroyed one amino acid enantiomer over the other, potentially giving it a head start. This idea gained some support when CP radiation in the infrared band was discovered in the Orion Nebula. The main problem with this idea is that CP also destroys much of the “correct” amino acid form as well. Moreover, the magnitude and orientation preference for the effect depends on the frequency of the light. The desireable bias—L selection for the narrow UV light band—would be swamped by competing broadband effects with the result that any long term amplification would grow asymptotically small.Experimentatlly, the CP theory of handedness is not completely dead, only weak. The best result to date has been the creation of 20% optically pure camphor in the lab. Unfortunately this was only obtained after 99% of the original stock was destroyed. If the D form amino acids are life’s cancer, than CP light hardly seems to be the most effective chemotherapy. The weak nuclear force, one the other hand, may be a bit more interesting for the origins of chirality. It is one of the four fundamental forces of nature and governs a particular kind of radioactive decay known as β-decay. The weak force has a peculiar handedness, called parity violation, which preferentially produces left-handed electrons during β-decay. For electrons with a left-handed “helicity”, the directions its of spin and motion are opposite to each other.last_img read more