Supernovas might be directing development of life

[LionElJonson]

http://io9.com/5614372/supernovas-could ... e=true&s=i

A basic property of Earth's organic molecules could be caused by supernova explosions. That means massive stellar explosions indirectly governed the building blocks of life on Earth...and could be doing it elsewhere in the cosmos.

In chemistry, molecules are said to be chiral if they lack symmetry - in other words, if a molecule and its mirror image can't be superimposed one on top of the other, then that molecule is chiral. Because our hands are the most obvious example of chirality in the human experience, chemists informally call chiral molecules "left-handed" and "right-handed", depending on their orientation.

Many chiral molecules are the result of an asymmetric carbon atom, which forces the rest of the molecule's atoms into two mirrored configurations. Since carbon is also the most crucial element to life as we know it, a lot of vital organic molecules happen to be chiral. None of that is particularly strange. What is mysterious is the fact that virtually all of Earth's amino acid molecules are left-handed when one chiral configuration should be just as common as the other. In fact, amino acids found on the Murchison meteorite show precisely the same overabundance of left-handed molecules, suggesting this bias is found throughout the universe.

Now astrophysicists think they might have the answer, and it's all to do with supernovas. When these stellar explosions occur, a lot of different particles are released in powerful bursts. One such particle is the antineutrino, the antimatter counterpart of the neutrino. Antineutrinos are themselves fairly mysterious particles, but we do know one thing for a fact - every antineutrino we have ever observed had a rightward spin, and that means it wants to interact with other particles that have a similarly rightward spin.

That's where right-handed amino acids reenter the picture. All amino acids feature not only a carbon atom but also a nitrogen atom, and the spin of that atom is determined by the chiral configuration of the amino acid as a whole. In left-handed amino acids, the nitrogen has a leftward spin, while it has a rightward spin in right-handed amino acids. Those rightward-spinning nitrogen atoms are the perfect candidates to align with antineutrinos as they shoot away from the supernova.

This alignment between antineutrinos and nitrogen atoms causes the nitrogen to be converted into carbon, which in the process destroys the original amino acid. This antineutrino wave would provide a natural sorting mechanism that breaks apart right-handed amino acids while leave the left-handed amino acids intact, leading to the eventual dominance of left-handed amino acids - precisely what's happened here on Earth.

You might wonder if there's a similar process that can destroy left-handed amino acids in much the same way. There is, and it's caused by antineutrino's opposite number, the neutrino. Supernovas can also emit bursts of neutrinos, and these left-spinning particles would convert nitrogen into oxygen in left-handed amino acids. However, this process requires way more energy, and so it would naturally happen far, far less often than the right-handed process.

Of course, the range of these antineutrino waves wouldn't be infinite. Most likely, the antineutrinos would only destroy the right-handed amino acids in a few molecular clouds that happened to be near the supernova. But then those molecular clouds would mix throughout the galaxy, diffusing a small but still noticeable bias toward left-handed amino acids. Even if the imbalance started as just 1 more left-handed molecule in every million, that would be enough to lead to their eventual dominance throughout space, including on any planet where life developed.

If this theory is true - and it happens to be the best and only explanation for why we see a left-handed imbalance on Earth - then that means the original building blocks of life on this planet must have originated elsewhere. It's possible that these supernova-altered molecules got thrown into the initial mixture of materials that formed our solar system, or perhaps meteorites brought these amino acids from the molecular clouds to Earth.

Either way, the left-handed nature of our amino acids puts some fairly fundamental constraints on what life on this planet can look like, and it's possible that any alien life might operate under similar guidelines. As nuclear astrophysicist Richard Boyd puts it:

"I find it really mind-boggling that the same constraints that exist on our chemicals of life might also exist for every other entity in the universe. If other entities are out there, the constraints on their chemistry appear to be sufficiently similar to ours that we may have lots of things in common with them."

Quite interesting, if it happens to be true.

[Starman7]

I'm not incredibly well-versed on the origin of life on Earth, so I'd like to know:

Is there a non-biological process that would have amplified the preponderance towards left-handed amino acids?
Was all modern life derived from a single ancestor that just happened to use left-handed amino acids?
If not, could it just have been that our ancestor(s) out-competed the organisms which used right-handed amino acids?

I'm trying to figure out how such a tiny preponderance towards one chirality could have mandated that organisms develop in that chirality.

[Twoyboy]

Ummm... what? Maybe I'm missing something here, and if so I'm going to look mighty stupid, but...

Left hand/right hard stereochemistry has nothing to do with the spin of fundamental particles. So I'm thinking someone saw the words right-hand spin neutrinos and left-hand chiral amino acids and put 2 and 2 together and got 5.

I'm not incredibly well-versed on the origin of life on Earth, so I'd like to know:

Is there a non-biological process that would have amplified the preponderance towards left-handed amino acids?
Was all modern life derived from a single ancestor that just happened to use left-handed amino acids?
If not, could it just have been that our ancestor(s) out-competed the organisms which used right-handed amino acids?

I'm trying to figure out how such a tiny preponderance towards one chirality could have mandated that organisms develop in that chirality.

My chemistry knowledge isn't what it used to be, but I'll give it a shot.

If I recall, some enzyme reactions actually DO behave differently with different stereochemistry. If you have a molecule which reacts with more than one functional group on a chiral carbon, it may favour a certain configuration. I don't know if it would have made such a huge difference, but it's something to think about.

[Starman7]

If I recall, some enzyme reactions actually DO behave differently with different stereochemistry. If you have a molecule which reacts with more than one functional group on a chiral carbon, it may favour a certain configuration. I don't know if it would have made such a huge difference, but it's something to think about.

Thing is, if I'm remembering my O-chemistry and biochemistry correctly, enzymes never really developed until life did (the whole being-a-complex-protein thing), and generally chirally active inorganic catalysts are fairly rare. Plus, it's only the amino acids which were stilted towards one chirality, not the theoretical inorganic catalysts.

And as for the "anti-neutrino spin relation to chirality" bit, my knowledge of particle physics is dreadfully poor, so I cannot comment.

[Simon_Jester]

Since neutrinos are pretty much irrelevant to anything made out of atoms (as demonstrated by how hard it is to spot the slippery little bastards)... I'm going to go with "neutrinos don't make any damn difference as far as chirality is concerned."

[Eternal_Freedom]

Ummm. I wasnt aware we had actually observed ANY neutrinos, anti or otherwise. I certainly dont think we've measured spin on them

Its a nice thought I suppose. But supernovas do enough anyway, do we need another random effect?

[Serafina]

Ummm. I wasnt aware we had actually observed ANY neutrinos, anti or otherwise. I certainly dont think we've measured spin on them

Its a nice thought I suppose. But supernovas do enough anyway, do we need another random effect?

We observed their interaction with matter - which is extremely rare, so you need large quantities of transparent matter under conditions with no interference. That's why we use large depots of pure water deep underground and look into it with extremely sensitive sensors. Every now and then, a neutrino will interact with an atom and release a short flash of light.

But yes, unless i am mistaken, we never observed a neutrione "directly" - the above is all we have.

[Simon_Jester]

Ummm. I wasnt aware we had actually observed ANY neutrinos, anti or otherwise. I certainly dont think we've measured spin on them

Its a nice thought I suppose. But supernovas do enough anyway, do we need another random effect?

We observed their interaction with matter - which is extremely rare, so you need large quantities of transparent matter under conditions with no interference. That's why we use large depots of pure water deep underground and look into it with extremely sensitive sensors. Every now and then, a neutrino will interact with an atom and release a short flash of light.

But yes, unless i am mistaken, we never observed a neutrione "directly" - the above is all we have.

Well, by that standard we've never observed anything directly; we only ever perceive anything by an indirect chain reaction that leads to neurons firing in our brains...

In any case, neutrinos are a well understood aspect of particle physics, with the first observation occuring in 1956. Determining the neutrino's spin is comparatively trivial given other extremely well nailed down features of modern physics.

They're not hypothetical particles at all; they're just very, very slippery. The probability of a neutrino interacting with a particle it encounters is so low that neutrinos "ghost" through large volumes of solid matter easily. Thus, even a large detector observes only a tiny fraction of neutrinos passing by- and yet there are so many neutrinos out there that this tiny fraction is enough to tell us a lot about the neutrino and the physics of reactions that produce neutrinos- such as nuclear fusion in the sun.

[starslayer]

Since neutrinos are pretty much irrelevant to anything made out of atoms (as demonstrated by how hard it is to spot the slippery little bastards)... I'm going to go with "neutrinos don't make any damn difference as far as chirality is concerned."

Well, in supernovas they're really damned important, because the densities involved are so high, so they could conceivably affect the abundances of left and right spinning carbon atoms. However, as Twoyboy said, I don't think chirality has anything to do with spin angular momentum, so I'd guess this is bunk.

Simon, do neutrinos have the preferences the article ascribes to them anyway? My understanding of particle physics here isn't enough to say one way or the other, though since neutrinos are fermions, I'm inclined to think they don't.

[Spectre_nz]

Is there a non-biological process that would have amplified the preponderance towards left-handed amino acids?
Was all modern life derived from a single ancestor that just happened to use left-handed amino acids?
If not, could it just have been that our ancestor(s) out-competed the organisms which used right-handed amino acids?

I'm trying to figure out how such a tiny preponderance towards one chirality could have mandated that organisms develop in that chirality.

Chiral molecules tend to crystallize with other molecules of the same chirality, and if they're aggregating on a surface can aggregate into homo-chiral patches. The former is kosher, I'll have to dig out a reference to see if I'm recalling the latter correctly.

Chirality was first identified because L and D tartaric acid aggregate separately into L and D mirror image crystals.

The origins of chiral life are pretty much speculative. There is no law against racemic proteins, only they're much less predictable in their structure than an enantio-pure version. If you imagine each chiral amino acid as either a left twist or a right twist option, taking a 50/50 coin flip on which version you insert at any point means your 3D structure is much more erratic (in terms of blue-prints vs what is actually produced) than one built entirely of D or L.

Any movement towards a preference to one or other chirality favors an even greater preference for that chirality. Once organisms start becoming chiral, I'd expect them to very quickly be pushed to develop a total preference for D or L amino acids. Either option would out compete more racemic organisms. Maybe there were D and L organisms together at one time and the luck of the draw saw the D organisms die out leaving the L organisms to take over. Maybe the L organisms got a foothold first and beat out all the other racemic organisms before the D organisms ever got going. It's a mystery.

If L and D amino acids were present in equal abundance, then you could say it was a coin flip, left or right, that saw all life as we know it today being left handed. but if there was a chiral preference from the start, then that chirality is just going to take over once organisms start to become enantio-selective thanks to a greater abundance of raw materials.
Apparently someone found a preference for L amino acids in samples collected from meteorites. I'm a little fuzzy on that detail however. The idea that there is some pre-biotic preference for L amino acids on earth has been kicking around for a while however, only no one has a good answer for where it came from.

Also; How do neutrinos turn nitrogen into carbon? Is there a real mechanism for that or did someone pull that out of their ass?

[Gil Hamilton]

This article is baloney. Spin is a property of the nucleus of the atom. Chirality is a geometry bond issue, which has to do with electrons. Spin does NOT determine chirality.

In biological systems, what determines the chirality of any molecule produced is the chirality of the protein that assembled it. Think of it like if you made a fake hand in a mold. While your left and right hand are identical and there is no chemical difference, the mold is only going to produce hands that match the cavity inside of the mold. For example, the chemical limonene is produced both in pinecones and in citris, but pinecones contain entirely S-limonene and citris is all R-limonene, because two different chiral things made them. Depending on the synthesis, all things being equal, you should get half and half, but all things are NOT equal in a biological system (in fact, your nose can differentiate between S-limonene and R-limonene, despite the fact that they are only different in optical properties, by the fact that the receptors in your nose are also chiral... so pine scent and lemon scent smell different despite basically being the same chemical).

The mystery is how we STARTED from all L-amino acids, but that is less mysterious that you'd think. It really depends on the synthesis route. The common synthesis routes employed by organic chemists who decide, for what ever reason, that they want to gin up a big ole bottle of lysine or something typically are insensitive to chirality. However, chances are natural amino acids aren't made by reagents being mixed, they are probably the product of surface catalysis originally before things got self-propetuating. Inorganic catalysts, very often, are sensitive to chirality. At the beginning, animo acids may have formed on surfaces that due to their synthesis only popped out as L-amino acids.

That's a possibility, but supernovae blasting us with neutrinos? Silly stuff

[starslayer]

Also; How do neutrinos turn nitrogen into carbon? Is there a real mechanism for that or did someone pull that out of their ass?

An anti-electron neutrino hitting a proton can change it into a neutron and a positron, so yes, a neutrino can change nitrogen into carbon.