DNA Theory & Quantum Mechanics?

[merjer]

Hi everyone, I’m trying to write (for a freshman science for huamnities students course ) a book report on James Waton’s (the dude who discovered the structure of DNA) The Double Helix. In the book there is a statement that I found interesting but rather muddling. To avoid confusion, I’ll quote it.

“Here there was no doubt that the keto form, not the enol, was present. Moreover, he felt sure that the quantum-mechanical arguments which showed why diketopiperazine has the keto form should also hold for guanine and thymine.”

I haven’t the foggiest idea why quantum-mechanical theory might substantiate a theory about DNA?

[Squink]

@merjer

I haven’t the foggiest idea why quantum-mechanical theory might substantiate a theory about DNA?

They were probably doing some primitive molecular orbital type calculations. There’s a bit on Corey and Pauling’s structural studies on diketopiperazine here, but to get much more you’ll probably have to go digging in old literature.

[Colibri]

I agree. Quantum mechanics would determine the form of the electron clouds around the molecules and the kind of bonds between atoms within them. This in turn would determine which of two alternative forms of the bases would be present.

Here’s a bit more on keto and enol forms

Here’s a detailed analysis of Watson and Crick’s paper which includes this explanation:

“Keto” and “enol” are two possible tautomeric forms of a carbon in the bases. A carbon in the “keto” form is a carbon with a double bond to oxygen (as in “ketone”), and two single bonds to adjacent members of the ring. A carbon in the “enol” form has a single bond to an alcohol group (-OH) and a double-bond to a nitrogen and a single bond to an adjacent carbon.

Hydrogen bonding is a key attribute of the Watson-Crick model. The opportunity for hydrogen bonding depends on whether the bases are in the “keto” or the “enol” form. Watson and Crick found that the AT and GC pairing make sense only if the bases are in the “keto” tautomeric form.

[edwino]

I saw Watson talk and he talked a bit about this. Quantum mechanics may not be the right term, although it is a minor difference. I believe in that state, the keto and enol forms are not only tautomeric but in equilibrium and resonance structures of each other. At least that’s how he implied it – I don’t remember enough orgo and biochem to know better.

The predominance of the particular resonance structure would be determined by the chemical environment of the phosphate backbone, the sugar moeity, the inside of the double helix, and their relationships to each other in 3D, as the distributions of the electrons would be altered. Hence quantum mechanics, to a degree.

Watson and Crick used this to figured out how to make sense of Erwin Chargaff’s data on base composition (G=C, A=T in percentage in DNA), in that the G-C and A-T bindings were permissible (3 H-bonds for G-C, 2 for A-T). With Rosalind Franklin’s spec data, they could build this into a two-helix (not three or four) model with the bases on the inside (not on the outside as had also been predicted).

[Smeghead]

If I recall correctly, it was another scientist working in the same laboratory that mentioned they were originally using the wrong form, based on work he’d been doing. I forget his name, though.