Sciency Words: Chirality

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Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


Imagine that we’ve finally placed a lander on the surface of Europa. The lander takes samples and detects amino acids, an essential ingredient for life. That raises a few eyebrows, but amino acids are not exactly uncommon in space. Then scientists realize that Europa’s amino acids all share the same chirality.

That’s the part where everybody freaks out.

Chirality, which is a noun, and chiral, which is an adjective, come from a Greek word meaning hand. You’ll see why in a moment.

Amino acids are made of several parts. By definition, they must include an amino group (one nitrogen atom and two hydrogens).

Mr08 Amino Group

By definition, they must also include a carboxyl group (two oxygens, a carbon, and a hydrogen).

Mr08 Carboxyl Group

They also need to have at least one carbon atom positioned between them, for structural purposes.

Mr08 Alpha Carbon

An amino acid with only one carbon separating the amino and carboxyl groups is called an alpha amino acid. If there were two carbons, it would be a beta amino acid, and so on.

But notice: that central carbon atom still has two available bonding sites.

Mr08 Missing Side Chain 1

So amino acids have one more crucial component called a side chain. It’s these side chains that give each type of amino acid its unique flavor (literally—amino acids taste different from one another, or so I’m told).

But which side do we attach the side chain to?

Mr08 Missing Side Chain 2

Left side it is! And we’ll attach a single hydrogen atom to the right.

Very early in the development of life on Earth, organisms started manufacturing amino acids out of simpler chemicals, and they always made the “left-handed” kind. Why? Coin toss. It could just have easily gone the other way, as far as we know.

To this day, our DNA continues to code for left-handed amino-acids only. As a result, there are more left-handed than right-handed amino acids present on Earth. If we ever find a similar disparity elsewhere in the universe—whether left or right-handed—that would be compelling evidence for the existence of alien life.

By the way: there’s a common misconception about chirality that you sometimes find in science fiction. Supposedly, humans cannot eat foods made from right-handed amino acids, and aliens with right-handed biochemistries cannot eat our left-handed foods. This is not necessarily true. In fact, humans do consume right-handed amino acids. Some of them are useful to our bodies, just not in the construction of proteins, and they’re not coded for by our DNA.

Of course, there are plenty of other reasons humans and aliens probably shouldn’t share food.

Mr08 Alien Food

So the chirality of amino acids might not be your top concern.

7 thoughts on “Sciency Words: Chirality

  1. I had definitely believed that “can’t eat other-handed amino acids” thing for a long while. Not that we couldn’t physically eat it, but that we couldn’t digest the amino acids. I guess I was wrong, I wasn’t aware we made use of some of them. Cool to know!


  2. I find the coin toss points very interesting. Each one is a junction point where alien life might have taken a different turn. Of course, we can’t rule out that Earth life didn’t itself try to take those other turns with some portion of them turning into an evolutionary dead-end.

    Liked by 1 person

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