Surface-Induced Keto-Enol Tautomerization of DNA Base Molecules and Consequent [4+2]-like Cycloaddition on Si(111)7x7

Adsorption and film growth of deoxyribonucleic acid (DNA) base molecules (cytosine, guanine, thymine, and adenine) on Si(111)7x7 have been studied by combining X-ray photoelectron spectroscopy (XPS) with ab initio calculations based on the density functional theory (DFT). Multiple tautomeric forms and keto-enol tautomerization are revealed by the O 1s, N 1s, and C 1s XPS spectra of the O-containing DNA bases: cytosine, guanine, and thymine. While the carbonyl groupcontaining keto tautomer is more stable in a thick film and in powder, the hydroxyl groupcontaining enol tautomer is found at the interface. The keto-enol tautomerization, as induced by the reactive Si(111)7x7 surface, leads to the formation of a conjugated aromatic six-membered ring with a delocalized pi electron system and to the consequent [4 + 2]-like cycloaddition between the enol tautomer and the 7x7 surface. The DFT calculation suggests that the enol tautomer exhibits a kinetic advantage over the keto one for the [4 + 2]-like cycloaddition. Among the several plausible pathways for the cycloaddition provided by the enol tautomer, the experimentally determined one involves a ring N and ring C atom (a polar pair), rather than two ring C atoms (a nonpolar pair), to better match the polar Si adatom-restatom pair of the 7x7 surface. Furthermore, the reacted ring C atom does not have any attached terminal functional group (e.g., -NH2 and -OH). Further deposition leads to continuous film growth in the keto tautomeric form for cytosine and guanine. For the only O-free DNA base molecule, adenine, dative bonding N -> Si, rather than the [4 + 2]-like cycloaddition, is observed on the 7x7 surface. Of the four DNA base molecules, adenine is also the only one with its aromaticity maintained when adsorbed on the Si(111)7x7 surface. A reactive surface like the 7x7 surface could therefore provide a new control to trigger tautomerization that is often associated with genetic mutation.