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Comment by jeangenie | Hacker News Reader

I have a silly question: how is it determined which side is up and down?

The direction of light, from the perspective of the viewer, is the base. Its not about up and down, but rather left or right ..

It's still a completely arbitrary mapping.

Well actually, I suspect that it isn't arbitrary at all: historically, people have considered left-handed as deviant, strange, sinister, so obviously the good molecules are right-handed.

(Disclaimer: I'm a slightly bitter left-handed designer who has seen one too many "more intuitive" designs that are optimised for right-handedpeople)

The "left" or "right" refer to the direction that the chemical (usually in solution) rotates the plane of polarized light passing through it, from the point of view of an observer that the light is traveling towards.

An experimental setup to demonstrate this is a light source, a linear polarizer oriented in some direction (say along the x-axis for definiteness), followed by the sample container, followed by another linear polarizer, followed by a detector.

With nothing in the sample container, the second polarizer is rotated around the light beam axis until no light passes through. This will be very close to a right angle with the first one (try it with polarizing sunglasses :) ). The sample is then added, and the change in angle of the second polarizer required to again allow no light through is observed.

Also, try reading the article. Naturally-produced RNA, etc are all left-handed. :)

Looking down on a right-handed spiral stair case you have to go counter-clockwise to go down, or clockwise to go up.

Rotating the spiral along it's axis changes nothing.

Rotating the spiral 180 deg around any axis perpendicular to its own, will not change the above.

Only when you reflect the spiral does it change handedness.

I don't follow your question. A right-handed helix remains so regardless of its orientation, just like how I wouldn't become left-handed if I were suspended upside-down.

How can you distinguish between two chiral molecules and two identical molecules one of which has been rotated 180 degrees with respect to the other?

There's no way to rotate a clockwise screw that makes it a counter-clockwise screw. Try it.

That's the definition of chirality. If you have a tetrahedral orientation of four different atoms around a center atom (e.g. carbon), there will exist a "right handed" and "left handed" version.

You can rotate the "left handed" version any way you want, but you won't be able to superimposed it on the "right handed" version.

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