Sunday, April 5, 2015

Pauling's Left-Handed alpha-Helix #chempaperaday 209

Left-handed alpha-helices are not common. In fact, I was able to find only one example with my limited search attempts although I am sure there are many more. There are stereochemical issues, bond distance and dihedral angle restrictions if nature or  you want to make one of them. So, when I first saw the title, I did not want to believe that Pauling proposed a left-handed alpha-helix. I thought he just made a mistake in one of his books or papers. If you read the paper, you will see that this is not the case. For some reason unknown to us, he drew this helix. As the paper explains, in fact he made an arbitrary assignment to R groups when he first reported "the structure of proteins."

But, as the author explains in detail, he should have known the absolute configuration of the amino acids. In fact, his colleagues at his own department were doing research on the subject. So, he should definitely have known much more about his "incorrect" assignment. You can read the article and learn more about the issue. My own understanding is that he was not in close communication with his faculty members in those years due to the political troubles he had had. Maybe the people who really knew the absolute configuration did not correct him on purpose, or he did not want to be corrected by one of those people who were not on his side when he needed support. But, I don't think he was simply not "interested in the problem of absolute configuration." I think this is contrary what we know about his scientific curiosity and genius.

The author is Jack D. Dunitz who is a giant in his field and you may know his name if you can remember Burgi-Dunitz angle.

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