From The Mailbag
Continuing the discussion on evolution onegoodmove reader Lara writes:
"I was listening to some of Feynman's lectures over the weekend and I chanced upon this passage in which he discusses left-right asymmetry in biological molecules and ties it to evolution. The argument is not new but it is delivered in Feynman's inimitable style."
Lara has also provided an audio mp3 (7'0 1.6MB) . The excerpt is taken from the lecture that forms the basis for the last chapter (Symmetry in Physical Laws) in Volume 1 of the Feynman Lectures on Physics. This is great stuff give it a listen.
Comments
Fascinating!
Lara, thank you for sending this in. Did he say why he chose alanine to study?
No, Cara, he didn't. Alanine (where the side-chain R = CH3) is one of the simpler amino acids. Glycine (where R = H) is simpler yet but is not optically active. Feynman probably also had ball-and-stick models of L-alanine and D-alanine handy—they are depicted in the book chapter mentioned above.
Incidentally, in the lecture, Feynman starts to say that Life uses only L-alanine but then corrects himself, adding the caveat, "Here and there in the living creatures there's a special use for this molecule [D-alanine] but [it's] very rare . . ." It turns out that D-alanine is found in the cell walls of prokaryotic bacteria. Penicillin works by mimicking D-alanyl-D-alanine and irreversibly binding to the transpeptidase enzyme responsible for building the bacterial cell walls, thereby inhibiting the enzyme's action. Eukaryotic cells, e.g., mammalian cells, which do not contain D-alanyl-D-alanine in their cell walls, are not affected.