Tautomer-Selective Spectroscopy of Nucleobases, Isolated in the Gas Phase

In his book The Double Helix, James Watson describes an early proposal for DNA structure in which equal bases pair with each other. He writes: ‘‘My scheme was torn to shreds the following noon. Against me was the awkward chemical fact that I had chosen the wrong tautomeric forms for guanine and thymine.’’ This fact was pointed out to him by the American crystallographer Jerry Donahue who ‘‘ . . . protested that the idea would not work. The tautomeric forms I had copied out of Davidson’s book were, in Jerry’s opinion, incorrectly assigned. My immediate retort that several other texts also pictured guanine and thymine in the enol form cut no ice with Jerry. Happily he let out that for years organic chemists have been arbitrarily favoring particular tautomeric forms over the alternatives on only the flimsiest of grounds’’ [1]. Without the correct keto form of these bases, Watson and Crick could not have arrived at their successful structure of DNA. In fact, for each of the bases, the keto tautomer is the form normally occurring in DNA. The rare imino and enol forms can lead to alternate base-pair combinations and thus cause mutations in replication. This type of mutation is called a tautomeric shift [2, 3]. Possible mismatches include iminoC–A, enolT–G, C–iminoA, and T–enolG. Tautomerization can occur in DNA by single or double proton transfer. In solution equilibrium, generally the keto form dominates, complicating efforts to study the properties of the individual forms. One approach to studying tautomeric properties is double-resonant gas-phase spectroscopy, as this technique offers isomeric selectivity. This chapter summarizes the results from such investigations.