"Continental Classroom - A Course in Modern Chemistry" 1960. A production of the National Broadcasting Company and the Learning Resources Institute
in cooperation with the American Association of Colleges for Teacher Education and
the American Chemical Society.
Demonstrating Symmetry. (2:37)
Transcript
Linus Pauling: I mentioned that the amino acids are all left-handed amino acids, except glycine,
they don't have any symmetry elements. Now, the residues in the polypeptide chain
are very much like one another. They differ only in the side chain of atoms that are
attached to the central carbon atom and we might well expect that in a stable structure,
each residue would be related to the next one in the same way. The mathematicians
have proved and interesting theorem. It is this: If one has an asymmetrical object
such as this fish. This is a flounder, a flatfish, its left hand is different from
its right, not like ordinary fish that have a median plane of symmetry. If one has
an asymmetric object which is related to a second equivalent object by some translation,
some general operation in space, and that is related to the third in a certain way,
the same way, and that to the fourth in the same way, the automatically there is built
up a helix. As this operation is repeated, a helix is generated. Here I have the result
of operating on a flatfish to convert it into a second flatfish, also a left-handed
flatfish, and then into a third one, a fourth one, and in this way this helix is built
up. The most general structure that can be made by the operation on an asymmetric
object of the same operation over and over again is a helical structure. Well, this
suggests to us that the amino acid residues and the polypeptide chains may be related
to one another in a helical fashion. And, if I take this model and just start doing
the same thing over and over to successive residues, twisting always in the same direction
around and around like that, I get very quickly the structure that is shown here.
This is a little helix in which each of the residues is attached to a residue some
distance removed from it along the chain by an N-H-O hydrogen bond. This structure,
this helical structure, is called the alpha helix and it is a very important structure
in proteins. This is the primary way in which peptide chains are folded in proteins.
Clip
Creator: Linus Pauling Clip ID: 1960v.37-07
Full Work
Creator: Linus Pauling Associated: John F. Baxter, National Broadcasting Company
